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+unwrap_used = "deny"
+expect_used = "deny"
+missing_panics_doc = "allow"
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+
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diff --git a/LICENSE b/LICENSE
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index 0000000..af68f86
--- /dev/null
+++ b/LICENSE
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+Everything in this repository is licensed under AGPLv3 or later, unless a file says otherwise at its top.
+
+Full licence text: LICENSE-AGPL-3.0-or-later
diff --git a/LICENSE-AGPL-3.0-or-later b/LICENSE-AGPL-3.0-or-later
new file mode 100644
index 0000000..be3f7b2
--- /dev/null
+++ b/LICENSE-AGPL-3.0-or-later
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+                    GNU AFFERO GENERAL PUBLIC LICENSE
+                       Version 3, 19 November 2007
+
+ Copyright (C) 2007 Free Software Foundation, Inc. <https://fsf.org/>
+ Everyone is permitted to copy and distribute verbatim copies
+ of this license document, but changing it is not allowed.
+
+                            Preamble
+
+  The GNU Affero General Public License is a free, copyleft license for
+software and other kinds of works, specifically designed to ensure
+cooperation with the community in the case of network server software.
+
+  The licenses for most software and other practical works are designed
+to take away your freedom to share and change the works.  By contrast,
+our General Public Licenses are intended to guarantee your freedom to
+share and change all versions of a program--to make sure it remains free
+software for all its users.
+
+  When we speak of free software, we are referring to freedom, not
+price.  Our General Public Licenses are designed to make sure that you
+have the freedom to distribute copies of free software (and charge for
+them if you wish), that you receive source code or can get it if you
+want it, that you can change the software or use pieces of it in new
+free programs, and that you know you can do these things.
+
+  Developers that use our General Public Licenses protect your rights
+with two steps: (1) assert copyright on the software, and (2) offer
+you this License which gives you legal permission to copy, distribute
+and/or modify the software.
+
+  A secondary benefit of defending all users' freedom is that
+improvements made in alternate versions of the program, if they
+receive widespread use, become available for other developers to
+incorporate.  Many developers of free software are heartened and
+encouraged by the resulting cooperation.  However, in the case of
+software used on network servers, this result may fail to come about.
+The GNU General Public License permits making a modified version and
+letting the public access it on a server without ever releasing its
+source code to the public.
+
+  The GNU Affero General Public License is designed specifically to
+ensure that, in such cases, the modified source code becomes available
+to the community.  It requires the operator of a network server to
+provide the source code of the modified version running there to the
+users of that server.  Therefore, public use of a modified version, on
+a publicly accessible server, gives the public access to the source
+code of the modified version.
+
+  An older license, called the Affero General Public License and
+published by Affero, was designed to accomplish similar goals.  This is
+a different license, not a version of the Affero GPL, but Affero has
+released a new version of the Affero GPL which permits relicensing under
+this license.
+
+  The precise terms and conditions for copying, distribution and
+modification follow.
+
+                       TERMS AND CONDITIONS
+
+  0. Definitions.
+
+  "This License" refers to version 3 of the GNU Affero General Public License.
+
+  "Copyright" also means copyright-like laws that apply to other kinds of
+works, such as semiconductor masks.
+
+  "The Program" refers to any copyrightable work licensed under this
+License.  Each licensee is addressed as "you".  "Licensees" and
+"recipients" may be individuals or organizations.
+
+  To "modify" a work means to copy from or adapt all or part of the work
+in a fashion requiring copyright permission, other than the making of an
+exact copy.  The resulting work is called a "modified version" of the
+earlier work or a work "based on" the earlier work.
+
+  A "covered work" means either the unmodified Program or a work based
+on the Program.
+
+  To "propagate" a work means to do anything with it that, without
+permission, would make you directly or secondarily liable for
+infringement under applicable copyright law, except executing it on a
+computer or modifying a private copy.  Propagation includes copying,
+distribution (with or without modification), making available to the
+public, and in some countries other activities as well.
+
+  To "convey" a work means any kind of propagation that enables other
+parties to make or receive copies.  Mere interaction with a user through
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+                     END OF TERMS AND CONDITIONS
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+            How to Apply These Terms to Your New Programs
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+  If you develop a new program, and you want it to be of the greatest
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+    This program is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
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+    You should have received a copy of the GNU Affero General Public License
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+Also add information on how to contact you by electronic and paper mail.
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+  If your software can interact with users remotely through a computer
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+For more information on this, and how to apply and follow the GNU AGPL, see
+<https://www.gnu.org/licenses/>.
diff --git a/crates/bone-app/Cargo.toml b/crates/bone-app/Cargo.toml
new file mode 100644
index 0000000..d09066e
--- /dev/null
+++ b/crates/bone-app/Cargo.toml
@@ -0,0 +1,21 @@
+[package]
+name = "bone-app"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+rust-version.workspace = true
+
+[dependencies]
+bone-types = { workspace = true }
+bone-document = { workspace = true }
+bone-render = { workspace = true }
+
+pollster = { workspace = true }
+thiserror = { workspace = true }
+tracing = { workspace = true }
+tracing-subscriber = { workspace = true }
+uom = { workspace = true }
+winit = { workspace = true }
+
+[lints]
+workspace = true
diff --git a/crates/bone-app/src/main.rs b/crates/bone-app/src/main.rs
new file mode 100644
index 0000000..5180fa3
--- /dev/null
+++ b/crates/bone-app/src/main.rs
@@ -0,0 +1,409 @@
+use std::sync::Arc;
+
+use bone_document::{EditOutcome, Sketch, SketchEdit, SketchEntity};
+use bone_render::{
+    Camera2, PixelsPerMm, SketchRenderer, SketchScene, Style, SurfaceContext, ViewportExtent,
+    ViewportPx,
+};
+use bone_types::{
+    Length, Point2, Point3, SketchEntityId, SketchPlaneBasis, Tolerance, UnitVec3, Vec2,
+};
+use tracing_subscriber::EnvFilter;
+use uom::si::length::millimeter;
+use winit::{
+    application::ApplicationHandler,
+    dpi::{PhysicalPosition, PhysicalSize},
+    event::{ElementState, KeyEvent, MouseButton, MouseScrollDelta, WindowEvent},
+    event_loop::{ActiveEventLoop, ControlFlow, EventLoop},
+    keyboard::{KeyCode, ModifiersState, PhysicalKey},
+    window::{Window, WindowId},
+};
+
+#[derive(Debug, thiserror::Error)]
+enum AppError {
+    #[error("event loop: {0}")]
+    EventLoop(#[from] winit::error::EventLoopError),
+}
+
+const DEFAULT_LOG_FILTER: &str = "bone_app=info,bone_render=info,bone_document=info,bone_kernel=info,bone_types=info,wgpu_core=warn,wgpu_hal=warn";
+const ZOOM_STEP_PER_LINE: f64 = 1.1;
+const ZOOM_STEP_PER_PIXEL: f64 = 1.0025;
+const ZOOM_KEY_STEP: f64 = 1.25;
+const ZOOM_MIN: f64 = 0.01;
+const ZOOM_MAX: f64 = 1.0e5;
+const INITIAL_ZOOM_PX_PER_MM: f64 = 12.0;
+const PAN_STEP_PX: f64 = 40.0;
+const PAN_FAST_MULTIPLIER: f64 = 5.0;
+const ZOOM_FIT_MARGIN: f64 = 0.9;
+
+struct RenderState {
+    surface: SurfaceContext,
+    renderer: SketchRenderer,
+    scene: SketchScene,
+    camera: Camera2,
+    style: Style,
+}
+
+#[derive(Default)]
+struct InputState {
+    cursor_px: Option<PhysicalPosition<f64>>,
+    left_down: bool,
+    middle_down: bool,
+    modifiers: ModifiersState,
+}
+
+impl InputState {
+    fn panning(&self) -> bool {
+        self.middle_down || (self.left_down && self.modifiers.shift_key())
+    }
+
+    fn pan_step_px(&self) -> f64 {
+        if self.modifiers.shift_key() {
+            PAN_STEP_PX * PAN_FAST_MULTIPLIER
+        } else {
+            PAN_STEP_PX
+        }
+    }
+}
+
+struct App {
+    window: Option<Arc<Window>>,
+    render: Option<RenderState>,
+    input: InputState,
+}
+
+fn plane_xy() -> SketchPlaneBasis {
+    let Ok(basis) = SketchPlaneBasis::new(
+        Point3::origin(),
+        UnitVec3::x_axis(),
+        UnitVec3::y_axis(),
+        Tolerance::new(1e-9),
+    ) else {
+        unreachable!("canonical XY axes are orthonormal");
+    };
+    basis
+}
+
+fn add_point(sketch: Sketch, x: f64, y: f64) -> (Sketch, SketchEntityId) {
+    let Ok((next, EditOutcome::Entity(id))) = sketch.apply(SketchEdit::AddEntity(
+        SketchEntity::point(Point2::from_mm(x, y)),
+    )) else {
+        unreachable!("AddEntity(Point) on fresh sketch yields Entity outcome");
+    };
+    (next, id)
+}
+
+fn add_line(sketch: Sketch, a: SketchEntityId, b: SketchEntityId) -> Sketch {
+    let Ok((next, _)) = sketch.apply(SketchEdit::AddEntity(SketchEntity::line(a, b, false))) else {
+        unreachable!("AddEntity(Line) referencing freshly added Points succeeds");
+    };
+    next
+}
+
+fn add_circle(sketch: Sketch, center: SketchEntityId, radius_mm: f64) -> Sketch {
+    let Ok((next, _)) = sketch.apply(SketchEdit::AddEntity(SketchEntity::circle(
+        center,
+        Length::new::<millimeter>(radius_mm),
+        false,
+    ))) else {
+        unreachable!("AddEntity(Circle) referencing freshly added center succeeds");
+    };
+    next
+}
+
+fn add_arc(
+    sketch: Sketch,
+    center: SketchEntityId,
+    start: SketchEntityId,
+    end: SketchEntityId,
+) -> Sketch {
+    let Ok((next, _)) = sketch.apply(SketchEdit::AddEntity(SketchEntity::arc(
+        center, start, end, false,
+    ))) else {
+        unreachable!("AddEntity(Arc) referencing freshly added points succeeds");
+    };
+    next
+}
+
+fn default_sketch() -> Sketch {
+    let sketch = Sketch::new(plane_xy());
+    let (sketch, p0) = add_point(sketch, -20.0, -12.5);
+    let (sketch, p1) = add_point(sketch, 20.0, -12.5);
+    let (sketch, p2) = add_point(sketch, 20.0, 12.5);
+    let (sketch, p3) = add_point(sketch, -20.0, 12.5);
+    let sketch = add_line(sketch, p0, p1);
+    let sketch = add_line(sketch, p1, p2);
+    let sketch = add_line(sketch, p2, p3);
+    let sketch = add_line(sketch, p3, p0);
+    let (sketch, origin) = add_point(sketch, 0.0, 0.0);
+    let sketch = add_circle(sketch, origin, 5.0);
+    let (sketch, arc_center) = add_point(sketch, -12.0, 0.0);
+    let (sketch, arc_start) = add_point(sketch, -8.0, 0.0);
+    let (sketch, arc_end) = add_point(sketch, -12.0, 4.0);
+    add_arc(sketch, arc_center, arc_start, arc_end)
+}
+
+fn viewport_extent(size: PhysicalSize<u32>) -> ViewportExtent {
+    ViewportExtent::new(
+        ViewportPx::new(size.width.max(1)),
+        ViewportPx::new(size.height.max(1)),
+    )
+}
+
+fn zoom_factor(delta: MouseScrollDelta) -> f64 {
+    match delta {
+        MouseScrollDelta::LineDelta(_, y) => ZOOM_STEP_PER_LINE.powf(f64::from(y)),
+        MouseScrollDelta::PixelDelta(p) => ZOOM_STEP_PER_PIXEL.powf(p.y),
+    }
+}
+
+fn zoom_about(camera: Camera2, cursor: Option<PhysicalPosition<f64>>, factor: f64) -> Camera2 {
+    if !factor.is_finite() || factor <= 0.0 {
+        return camera;
+    }
+    let zoom_before = camera.zoom().value();
+    let zoom_after = (zoom_before * factor).clamp(ZOOM_MIN, ZOOM_MAX);
+    if !zoom_after.is_finite() {
+        return camera;
+    }
+    let extent = camera.extent();
+    let w = f64::from(extent.width().value());
+    let h = f64::from(extent.height().value());
+    let (pan_x, pan_y) = camera.pan_mm().coords_mm();
+    let (cursor_x, cursor_y) = match cursor {
+        Some(c) => (c.x, c.y),
+        None => (w * 0.5, h * 0.5),
+    };
+    let horizontal_px = cursor_x - w * 0.5;
+    let vertical_px = h * 0.5 - cursor_y;
+    let world_x = pan_x + horizontal_px / zoom_before;
+    let world_y = pan_y + vertical_px / zoom_before;
+    let new_pan_x = world_x - horizontal_px / zoom_after;
+    let new_pan_y = world_y - vertical_px / zoom_after;
+    camera
+        .with_zoom(PixelsPerMm::new(zoom_after))
+        .with_pan(Vec2::from_mm(new_pan_x, new_pan_y))
+}
+
+fn pan_by_px(camera: Camera2, horizontal_px: f64, vertical_px: f64) -> Camera2 {
+    let mm_per_px = camera.world_mm_per_pixel();
+    let (pan_x, pan_y) = camera.pan_mm().coords_mm();
+    camera.with_pan(Vec2::from_mm(
+        pan_x - horizontal_px * mm_per_px,
+        pan_y + vertical_px * mm_per_px,
+    ))
+}
+
+fn zoom_fit(camera: Camera2, scene: &SketchScene) -> Camera2 {
+    let Some(aabb) = scene.aabb() else {
+        return camera;
+    };
+    let (mnx, mny) = aabb.min().coords_mm();
+    let (mxx, mxy) = aabb.max().coords_mm();
+    let center = Vec2::from_mm((mnx + mxx) * 0.5, (mny + mxy) * 0.5);
+    let world_w = mxx - mnx;
+    let world_h = mxy - mny;
+    if world_w <= 0.0 && world_h <= 0.0 {
+        return camera.with_pan(center);
+    }
+    let extent = camera.extent();
+    let w_px = f64::from(extent.width().value());
+    let h_px = f64::from(extent.height().value());
+    let axis_zoom = |pixels: f64, world: f64| {
+        if world > 0.0 {
+            pixels / world
+        } else {
+            f64::INFINITY
+        }
+    };
+    let raw_zoom = axis_zoom(w_px, world_w).min(axis_zoom(h_px, world_h)) * ZOOM_FIT_MARGIN;
+    let clamped = raw_zoom.clamp(ZOOM_MIN, ZOOM_MAX);
+    camera.with_zoom(PixelsPerMm::new(clamped)).with_pan(center)
+}
+
+enum KeyAction {
+    Exit,
+    Camera(Camera2),
+}
+
+fn keyboard_action(code: KeyCode, input: &InputState, state: &RenderState) -> Option<KeyAction> {
+    match (code, input.modifiers.control_key()) {
+        (KeyCode::KeyQ, true) => Some(KeyAction::Exit),
+        _ => keyboard_camera(code, input, state).map(KeyAction::Camera),
+    }
+}
+
+fn keyboard_camera(code: KeyCode, input: &InputState, state: &RenderState) -> Option<Camera2> {
+    let camera = state.camera;
+    let step = input.pan_step_px();
+    let shift = input.modifiers.shift_key();
+    match code {
+        KeyCode::ArrowLeft => Some(pan_by_px(camera, step, 0.0)),
+        KeyCode::ArrowRight => Some(pan_by_px(camera, -step, 0.0)),
+        KeyCode::ArrowUp => Some(pan_by_px(camera, 0.0, step)),
+        KeyCode::ArrowDown => Some(pan_by_px(camera, 0.0, -step)),
+        KeyCode::KeyF => Some(zoom_fit(camera, &state.scene)),
+        KeyCode::KeyZ => Some(zoom_about(
+            camera,
+            input.cursor_px,
+            if shift {
+                1.0 / ZOOM_KEY_STEP
+            } else {
+                ZOOM_KEY_STEP
+            },
+        )),
+        KeyCode::Equal => Some(zoom_about(camera, input.cursor_px, ZOOM_KEY_STEP)),
+        KeyCode::Minus => Some(zoom_about(camera, input.cursor_px, 1.0 / ZOOM_KEY_STEP)),
+        _ => None,
+    }
+}
+
+impl ApplicationHandler for App {
+    fn resumed(&mut self, event_loop: &ActiveEventLoop) {
+        if self.window.is_some() {
+            return;
+        }
+        let attrs = Window::default_attributes().with_title("bone");
+        let window = match event_loop.create_window(attrs) {
+            Ok(w) => Arc::new(w),
+            Err(e) => {
+                tracing::error!(error = %e, "create_window failed");
+                event_loop.exit();
+                return;
+            }
+        };
+        let extent = viewport_extent(window.inner_size());
+        let surface = match pollster::block_on(SurfaceContext::new(window.clone(), extent)) {
+            Ok(s) => s,
+            Err(e) => {
+                tracing::error!(error = %e, "SurfaceContext::new failed");
+                event_loop.exit();
+                return;
+            }
+        };
+        let renderer = SketchRenderer::new(surface.gpu(), surface.color_format());
+        let sketch = default_sketch();
+        let scene = match SketchScene::extract(&sketch) {
+            Ok(s) => s,
+            Err(e) => {
+                tracing::error!(error = %e, "scene extract failed");
+                event_loop.exit();
+                return;
+            }
+        };
+        let camera = Camera2::new(extent).with_zoom(PixelsPerMm::new(INITIAL_ZOOM_PX_PER_MM));
+        let style = Style::default();
+        window.request_redraw();
+        self.window = Some(window);
+        self.render = Some(RenderState {
+            surface,
+            renderer,
+            scene,
+            camera,
+            style,
+        });
+    }
+
+    fn window_event(&mut self, event_loop: &ActiveEventLoop, _id: WindowId, event: WindowEvent) {
+        let (Some(state), Some(window)) = (self.render.as_mut(), self.window.as_ref()) else {
+            return;
+        };
+        match event {
+            WindowEvent::CloseRequested => event_loop.exit(),
+            WindowEvent::Resized(size) => {
+                let extent = viewport_extent(size);
+                state.surface.resize(extent);
+                state.camera = state.camera.with_extent(extent);
+                window.request_redraw();
+            }
+            WindowEvent::RedrawRequested => {
+                let surface = &mut state.surface;
+                let renderer = &state.renderer;
+                let scene = &state.scene;
+                let camera = state.camera;
+                let style = &state.style;
+                surface.render(
+                    |encoder, color, pick| {
+                        renderer.encode_passes(encoder, color, pick, scene, camera, style);
+                    },
+                    || window.pre_present_notify(),
+                );
+            }
+            WindowEvent::Focused(false) => {
+                self.input.cursor_px = None;
+                self.input.left_down = false;
+                self.input.middle_down = false;
+                self.input.modifiers = ModifiersState::empty();
+            }
+            WindowEvent::ModifiersChanged(mods) => {
+                self.input.modifiers = mods.state();
+            }
+            WindowEvent::CursorMoved { position, .. } => {
+                let prev = self.input.cursor_px;
+                self.input.cursor_px = Some(position);
+                if self.input.panning()
+                    && let Some(p) = prev
+                {
+                    state.camera = pan_by_px(state.camera, position.x - p.x, position.y - p.y);
+                    window.request_redraw();
+                }
+            }
+            WindowEvent::CursorLeft { .. } => {
+                self.input.cursor_px = None;
+            }
+            WindowEvent::MouseInput {
+                state: btn_state,
+                button: MouseButton::Left,
+                ..
+            } => {
+                self.input.left_down = btn_state == ElementState::Pressed;
+            }
+            WindowEvent::MouseInput {
+                state: btn_state,
+                button: MouseButton::Middle,
+                ..
+            } => {
+                self.input.middle_down = btn_state == ElementState::Pressed;
+            }
+            WindowEvent::MouseWheel { delta, .. } => {
+                state.camera = zoom_about(state.camera, self.input.cursor_px, zoom_factor(delta));
+                window.request_redraw();
+            }
+            WindowEvent::KeyboardInput {
+                event:
+                    KeyEvent {
+                        physical_key: PhysicalKey::Code(code),
+                        state: ElementState::Pressed,
+                        ..
+                    },
+                ..
+            } => match keyboard_action(code, &self.input, state) {
+                Some(KeyAction::Exit) => event_loop.exit(),
+                Some(KeyAction::Camera(next)) => {
+                    state.camera = next;
+                    window.request_redraw();
+                }
+                None => {}
+            },
+            _ => {}
+        }
+    }
+}
+
+fn main() -> Result<(), AppError> {
+    tracing_subscriber::fmt()
+        .with_env_filter(
+            EnvFilter::try_from_default_env()
+                .unwrap_or_else(|_| EnvFilter::new(DEFAULT_LOG_FILTER)),
+        )
+        .init();
+    let event_loop = EventLoop::new()?;
+    event_loop.set_control_flow(ControlFlow::Wait);
+    let mut app = App {
+        window: None,
+        render: None,
+        input: InputState::default(),
+    };
+    event_loop.run_app(&mut app)?;
+    Ok(())
+}
diff --git a/crates/bone-document/Cargo.toml b/crates/bone-document/Cargo.toml
new file mode 100644
index 0000000..e20a743
--- /dev/null
+++ b/crates/bone-document/Cargo.toml
@@ -0,0 +1,25 @@
+[package]
+name = "bone-document"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+rust-version.workspace = true
+
+[dependencies]
+bone-types = { workspace = true }
+bone-solver = { workspace = true }
+blake3 = { workspace = true }
+ron = { workspace = true }
+serde = { workspace = true }
+slotmap = { workspace = true }
+thiserror = { workspace = true }
+tracing = { workspace = true }
+
+[dev-dependencies]
+insta = { workspace = true }
+nalgebra = { workspace = true }
+proptest = { workspace = true }
+tempfile = { workspace = true }
+
+[lints]
+workspace = true
diff --git a/crates/bone-document/src/document/feature_tree.rs b/crates/bone-document/src/document/feature_tree.rs
new file mode 100644
index 0000000..a2629ee
--- /dev/null
+++ b/crates/bone-document/src/document/feature_tree.rs
@@ -0,0 +1,105 @@
+use bone_types::{FeatureId, SketchId};
+use serde::{Deserialize, Serialize};
+use slotmap::{Key, KeyData};
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, Serialize, Deserialize)]
+pub enum PrincipalPlane {
+    Xy,
+    Yz,
+    Zx,
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, Serialize, Deserialize)]
+pub enum FeatureNode {
+    Origin,
+    PrincipalPlane(PrincipalPlane),
+    Sketch(SketchId),
+}
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(deny_unknown_fields)]
+struct FeatureEntry {
+    id: FeatureId,
+    node: FeatureNode,
+}
+
+#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
+#[serde(deny_unknown_fields)]
+pub struct FeatureTree {
+    entries: Vec<FeatureEntry>,
+}
+
+impl FeatureTree {
+    #[must_use]
+    pub fn seeded() -> Self {
+        let seeds = [
+            FeatureNode::Origin,
+            FeatureNode::PrincipalPlane(PrincipalPlane::Xy),
+            FeatureNode::PrincipalPlane(PrincipalPlane::Yz),
+            FeatureNode::PrincipalPlane(PrincipalPlane::Zx),
+        ];
+        let entries = seeds
+            .into_iter()
+            .zip(1u32..)
+            .map(|(node, idx)| FeatureEntry {
+                id: feature_id_from_idx(idx),
+                node,
+            })
+            .collect();
+        Self { entries }
+    }
+
+    #[must_use]
+    pub fn node(&self, id: FeatureId) -> Option<FeatureNode> {
+        self.entries.iter().find(|e| e.id == id).map(|e| e.node)
+    }
+
+    pub fn iter(&self) -> impl Iterator<Item = (FeatureId, FeatureNode)> + '_ {
+        self.entries.iter().map(|e| (e.id, e.node))
+    }
+
+    #[must_use]
+    pub fn feature_of_sketch(&self, sketch: SketchId) -> Option<FeatureId> {
+        self.entries
+            .iter()
+            .find(|e| matches!(e.node, FeatureNode::Sketch(s) if s == sketch))
+            .map(|e| e.id)
+    }
+
+    pub fn push_sketch(&mut self, sketch: SketchId) -> FeatureId {
+        if let Some(existing) = self.feature_of_sketch(sketch) {
+            return existing;
+        }
+        let id = self.allocate();
+        self.entries.push(FeatureEntry {
+            id,
+            node: FeatureNode::Sketch(sketch),
+        });
+        id
+    }
+
+    pub fn remove_sketch(&mut self, sketch: SketchId) -> Option<FeatureId> {
+        let id = self.feature_of_sketch(sketch)?;
+        self.entries.retain(|e| e.id != id);
+        Some(id)
+    }
+
+    fn allocate(&self) -> FeatureId {
+        let highest = self.entries.iter().map(|e| idx_of(e.id)).max().unwrap_or(0);
+        let Some(next) = highest.checked_add(1) else {
+            panic!("FeatureTree exhausted 32-bit feature id space");
+        };
+        feature_id_from_idx(next)
+    }
+}
+
+fn feature_id_from_idx(idx: u32) -> FeatureId {
+    FeatureId::from(KeyData::from_ffi((1u64 << 32) | u64::from(idx)))
+}
+
+fn idx_of(id: FeatureId) -> u32 {
+    let Ok(idx) = u32::try_from(id.data().as_ffi() & 0xFFFF_FFFF) else {
+        panic!("lower 32 bits of ffi key fit in u32");
+    };
+    idx
+}
diff --git a/crates/bone-document/src/document/mod.rs b/crates/bone-document/src/document/mod.rs
new file mode 100644
index 0000000..0283440
--- /dev/null
+++ b/crates/bone-document/src/document/mod.rs
@@ -0,0 +1,271 @@
+use std::collections::BTreeMap;
+
+use bone_types::{DocumentId, FeatureId, SchemaHeader, SketchId};
+use serde::{Deserialize, Serialize};
+
+use crate::Sketch;
+
+pub mod feature_tree;
+
+pub use feature_tree::{FeatureNode, FeatureTree, PrincipalPlane};
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, Serialize, Deserialize)]
+#[serde(deny_unknown_fields)]
+pub enum UnitsPreference {
+    Millimetre,
+    Centimetre,
+    Metre,
+    Inch,
+    Foot,
+}
+
+impl UnitsPreference {
+    pub const DEFAULT: Self = Self::Millimetre;
+}
+
+#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
+#[serde(deny_unknown_fields)]
+pub struct SketchRegistryEntry {
+    pub label: String,
+    pub filename: String,
+}
+
+#[derive(Clone, Debug, PartialEq, Default, Serialize, Deserialize)]
+#[serde(deny_unknown_fields)]
+pub struct SketchRegistry {
+    order: Vec<SketchId>,
+    entries: BTreeMap<SketchId, SketchRegistryEntry>,
+}
+
+impl SketchRegistry {
+    #[must_use]
+    pub fn new() -> Self {
+        Self::default()
+    }
+
+    #[must_use]
+    pub fn order(&self) -> &[SketchId] {
+        &self.order
+    }
+
+    #[must_use]
+    pub fn entry(&self, id: SketchId) -> Option<&SketchRegistryEntry> {
+        self.entries.get(&id)
+    }
+
+    pub fn iter(&self) -> impl Iterator<Item = (SketchId, &SketchRegistryEntry)> + '_ {
+        self.order
+            .iter()
+            .filter_map(|id| self.entries.get(id).map(|e| (*id, e)))
+    }
+
+    pub(crate) fn insert(&mut self, id: SketchId, entry: SketchRegistryEntry) {
+        if !self.entries.contains_key(&id) {
+            self.order.push(id);
+        }
+        self.entries.insert(id, entry);
+    }
+
+    pub(crate) fn remove(&mut self, id: SketchId) -> Option<SketchRegistryEntry> {
+        self.order.retain(|other| *other != id);
+        self.entries.remove(&id)
+    }
+}
+
+#[derive(Clone, Debug, Default, PartialEq, Serialize, Deserialize)]
+#[serde(deny_unknown_fields)]
+pub struct DocumentParameters {
+    order: Vec<String>,
+    entries: BTreeMap<String, f64>,
+}
+
+impl DocumentParameters {
+    #[must_use]
+    pub fn new() -> Self {
+        Self::default()
+    }
+
+    #[must_use]
+    pub fn order(&self) -> &[String] {
+        &self.order
+    }
+
+    #[must_use]
+    pub fn get(&self, name: &str) -> Option<f64> {
+        self.entries.get(name).copied()
+    }
+
+    pub fn insert(&mut self, name: String, value: f64) {
+        if !self.entries.contains_key(&name) {
+            self.order.push(name.clone());
+        }
+        self.entries.insert(name, value);
+    }
+
+    pub fn remove(&mut self, name: &str) -> Option<f64> {
+        self.order.retain(|n| n != name);
+        self.entries.remove(name)
+    }
+}
+
+#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
+#[serde(deny_unknown_fields)]
+pub struct DocumentHeader {
+    pub schema: SchemaHeader,
+    pub id: DocumentId,
+    pub name: String,
+    pub units: UnitsPreference,
+    pub parameters: DocumentParameters,
+    pub feature_tree: FeatureTree,
+    pub sketches: SketchRegistry,
+}
+
+impl DocumentHeader {
+    #[must_use]
+    pub fn new(id: DocumentId, name: String) -> Self {
+        Self {
+            schema: SchemaHeader::bone_document(),
+            id,
+            name,
+            units: UnitsPreference::DEFAULT,
+            parameters: DocumentParameters::new(),
+            feature_tree: FeatureTree::seeded(),
+            sketches: SketchRegistry::new(),
+        }
+    }
+}
+
+#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
+#[serde(deny_unknown_fields)]
+pub struct SketchFile {
+    pub schema: SchemaHeader,
+    pub sketch: Sketch,
+}
+
+impl SketchFile {
+    #[must_use]
+    pub fn new(sketch: Sketch) -> Self {
+        Self {
+            schema: SchemaHeader::bone_document(),
+            sketch,
+        }
+    }
+}
+
+#[derive(Clone, Debug, PartialEq)]
+pub struct Document {
+    header: DocumentHeader,
+    sketches: BTreeMap<SketchId, Sketch>,
+}
+
+impl Document {
+    #[must_use]
+    pub fn new(id: DocumentId, name: String) -> Self {
+        Self {
+            header: DocumentHeader::new(id, name),
+            sketches: BTreeMap::new(),
+        }
+    }
+
+    pub(crate) fn from_parts(header: DocumentHeader, sketches: BTreeMap<SketchId, Sketch>) -> Self {
+        Self { header, sketches }
+    }
+
+    #[must_use]
+    pub fn header(&self) -> &DocumentHeader {
+        &self.header
+    }
+
+    #[must_use]
+    pub fn id(&self) -> DocumentId {
+        self.header.id
+    }
+
+    #[must_use]
+    pub fn name(&self) -> &str {
+        &self.header.name
+    }
+
+    #[must_use]
+    pub fn units(&self) -> UnitsPreference {
+        self.header.units
+    }
+
+    #[must_use]
+    pub fn feature_tree(&self) -> &FeatureTree {
+        &self.header.feature_tree
+    }
+
+    #[must_use]
+    pub fn parameters(&self) -> &DocumentParameters {
+        &self.header.parameters
+    }
+
+    #[must_use]
+    pub fn registry(&self) -> &SketchRegistry {
+        &self.header.sketches
+    }
+
+    #[must_use]
+    pub fn sketch(&self, id: SketchId) -> Option<&Sketch> {
+        self.sketches.get(&id)
+    }
+
+    pub fn sketches(&self) -> impl Iterator<Item = (SketchId, &Sketch)> + '_ {
+        self.header
+            .sketches
+            .order()
+            .iter()
+            .filter_map(|id| self.sketches.get(id).map(|s| (*id, s)))
+    }
+
+    #[must_use]
+    pub fn sketches_map(&self) -> &BTreeMap<SketchId, Sketch> {
+        &self.sketches
+    }
+
+    pub fn set_name(&mut self, name: String) {
+        self.header.name = name;
+    }
+
+    pub fn set_units(&mut self, units: UnitsPreference) {
+        self.header.units = units;
+    }
+
+    pub fn insert_sketch(&mut self, id: SketchId, label: String, sketch: Sketch) {
+        let filename = sketch_filename(id);
+        self.header
+            .sketches
+            .insert(id, SketchRegistryEntry { label, filename });
+        self.header.feature_tree.push_sketch(id);
+        self.sketches.insert(id, sketch);
+    }
+
+    pub fn replace_sketch(&mut self, id: SketchId, sketch: Sketch) -> Option<Sketch> {
+        self.sketches.insert(id, sketch)
+    }
+
+    pub fn remove_sketch(&mut self, id: SketchId) -> Option<Sketch> {
+        self.header.sketches.remove(id);
+        self.header.feature_tree.remove_sketch(id);
+        self.sketches.remove(&id)
+    }
+
+    pub fn set_parameter(&mut self, name: String, value: f64) {
+        self.header.parameters.insert(name, value);
+    }
+
+    #[must_use]
+    pub fn sketch_of_feature(&self, feature: FeatureId) -> Option<&Sketch> {
+        match self.header.feature_tree.node(feature)? {
+            FeatureNode::Sketch(id) => self.sketches.get(&id),
+            FeatureNode::Origin | FeatureNode::PrincipalPlane(_) => None,
+        }
+    }
+}
+
+#[must_use]
+pub fn sketch_filename(id: SketchId) -> String {
+    use slotmap::Key;
+    format!("{:016x}.ron", id.data().as_ffi())
+}
diff --git a/crates/bone-document/src/evaluator.rs b/crates/bone-document/src/evaluator.rs
new file mode 100644
index 0000000..85ae626
--- /dev/null
+++ b/crates/bone-document/src/evaluator.rs
@@ -0,0 +1,81 @@
+use std::collections::{BTreeMap, BTreeSet, btree_map::Entry};
+
+use bone_solver::SolverError;
+use bone_types::FeatureId;
+
+use crate::Sketch;
+
+#[derive(Clone, Debug, PartialEq)]
+pub enum EvaluatedSketch {
+    Solved(Sketch),
+    Failed(SolverError),
+}
+
+#[must_use]
+pub fn evaluate_sketch(input: &Sketch) -> EvaluatedSketch {
+    match input.solve() {
+        Ok(solved) => EvaluatedSketch::Solved(solved),
+        Err(error) => EvaluatedSketch::Failed(error),
+    }
+}
+
+#[derive(Clone, Debug, Default)]
+pub struct FeatureCache {
+    entries: BTreeMap<FeatureId, CachedSketch>,
+}
+
+#[derive(Clone, Debug)]
+struct CachedSketch {
+    input: Sketch,
+    output: EvaluatedSketch,
+}
+
+impl FeatureCache {
+    #[must_use]
+    pub fn new() -> Self {
+        Self::default()
+    }
+
+    pub fn evaluate(&mut self, feature: FeatureId, input: &Sketch) -> &EvaluatedSketch {
+        let cached = match self.entries.entry(feature) {
+            Entry::Occupied(mut slot) => {
+                if &slot.get().input != input {
+                    *slot.get_mut() = CachedSketch {
+                        input: input.clone(),
+                        output: evaluate_sketch(input),
+                    };
+                }
+                slot.into_mut()
+            }
+            Entry::Vacant(slot) => slot.insert(CachedSketch {
+                input: input.clone(),
+                output: evaluate_sketch(input),
+            }),
+        };
+        &cached.output
+    }
+
+    #[must_use]
+    pub fn lookup(&self, feature: FeatureId) -> Option<&EvaluatedSketch> {
+        self.entries.get(&feature).map(|cached| &cached.output)
+    }
+
+    pub fn invalidate(&mut self, feature: FeatureId) -> bool {
+        self.entries.remove(&feature).is_some()
+    }
+
+    pub fn retain(&mut self, live: impl IntoIterator<Item = FeatureId>) {
+        let keep: BTreeSet<FeatureId> = live.into_iter().collect();
+        self.entries.retain(|id, _| keep.contains(id));
+    }
+
+    #[must_use]
+    pub fn len(&self) -> usize {
+        self.entries.len()
+    }
+
+    #[must_use]
+    pub fn is_empty(&self) -> bool {
+        self.entries.is_empty()
+    }
+}
diff --git a/crates/bone-document/src/io/blob.rs b/crates/bone-document/src/io/blob.rs
new file mode 100644
index 0000000..e28964b
--- /dev/null
+++ b/crates/bone-document/src/io/blob.rs
@@ -0,0 +1,167 @@
+use std::path::PathBuf;
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
+pub struct BlobHash(blake3::Hash);
+
+impl BlobHash {
+    #[must_use]
+    pub fn of(bytes: &[u8]) -> Self {
+        Self(blake3::hash(bytes))
+    }
+
+    #[must_use]
+    pub const fn from_bytes(bytes: [u8; 32]) -> Self {
+        Self(blake3::Hash::from_bytes(bytes))
+    }
+
+    #[must_use]
+    pub fn bytes(self) -> [u8; 32] {
+        *self.0.as_bytes()
+    }
+
+    #[must_use]
+    pub fn full_hex(self) -> String {
+        self.0.to_hex().to_string()
+    }
+
+    #[must_use]
+    pub fn truncated_128_hex(self) -> String {
+        self.0.to_hex()[..32].to_string()
+    }
+
+    #[must_use]
+    pub fn relative_path(self, kind: BlobKind) -> PathBuf {
+        let hex = self.truncated_128_hex();
+        let (aa, rest) = hex.split_at(2);
+        PathBuf::from(aa).join(format!("{rest}.{ext}", ext = kind.as_str()))
+    }
+}
+
+impl core::fmt::Display for BlobHash {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        write!(f, "{}", self.0.to_hex())
+    }
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
+pub struct BlobKind(&'static str);
+
+impl BlobKind {
+    pub const STEP: Self = Self("step");
+    pub const TESS: Self = Self("tess");
+    pub const THUMB: Self = Self("thumb");
+
+    #[must_use]
+    pub fn new(ext: &'static str) -> Self {
+        assert!(
+            !ext.is_empty()
+                && ext
+                    .bytes()
+                    .all(|b| b.is_ascii_lowercase() || b.is_ascii_digit()),
+            "BlobKind extension must be non-empty lowercase ASCII alphanumeric",
+        );
+        Self(ext)
+    }
+
+    #[must_use]
+    pub const fn as_str(self) -> &'static str {
+        self.0
+    }
+}
+
+impl core::fmt::Display for BlobKind {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        f.write_str(self.0)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn of_matches_blake3_reference() {
+        let hash = BlobHash::of(b"");
+        assert_eq!(
+            hash.full_hex(),
+            "af1349b9f5f9a1a6a0404dea36dcc9499bcb25c9adc112b7cc9a93cae41f3262"
+        );
+    }
+
+    #[test]
+    fn truncated_is_first_32_hex_chars() {
+        let hash = BlobHash::of(b"example");
+        let full = hash.full_hex();
+        let short = hash.truncated_128_hex();
+        assert_eq!(short.len(), 32);
+        assert_eq!(short, full[..32]);
+    }
+
+    #[test]
+    fn relative_path_is_fanout_then_rest_dot_kind() {
+        let hash = BlobHash::of(b"example");
+        let path = hash.relative_path(BlobKind::STEP);
+        let parts: Vec<String> = path
+            .components()
+            .map(|c| c.as_os_str().to_string_lossy().into_owned())
+            .collect();
+        let Some([fanout, file]) = parts
+            .get(..2)
+            .and_then(|s| <&[String; 2]>::try_from(s).ok())
+        else {
+            panic!("two components, got {parts:?}");
+        };
+        assert_eq!(fanout.len(), 2);
+        let Some(stem) = file.strip_suffix(".step") else {
+            panic!("ends with .step: {file}");
+        };
+        assert_eq!(stem.len(), 30);
+        let hex = hash.truncated_128_hex();
+        assert_eq!(fanout, &hex[..2]);
+        assert_eq!(stem, &hex[2..]);
+    }
+
+    #[test]
+    fn from_bytes_roundtrips_through_bytes() {
+        let bytes = [7u8; 32];
+        let hash = BlobHash::from_bytes(bytes);
+        assert_eq!(hash.bytes(), bytes);
+    }
+
+    #[test]
+    fn blob_kind_presets_match_adr_examples() {
+        assert_eq!(BlobKind::STEP.as_str(), "step");
+        assert_eq!(BlobKind::TESS.as_str(), "tess");
+        assert_eq!(BlobKind::THUMB.as_str(), "thumb");
+    }
+
+    #[test]
+    fn blob_kind_new_accepts_lowercase_alphanumeric() {
+        assert_eq!(BlobKind::new("gltf").as_str(), "gltf");
+        assert_eq!(BlobKind::new("v1").as_str(), "v1");
+    }
+
+    #[test]
+    #[should_panic = "BlobKind extension"]
+    fn blob_kind_new_rejects_empty() {
+        let _ = BlobKind::new("");
+    }
+
+    #[test]
+    #[should_panic = "BlobKind extension"]
+    fn blob_kind_new_rejects_uppercase() {
+        let _ = BlobKind::new("STEP");
+    }
+
+    #[test]
+    #[should_panic = "BlobKind extension"]
+    fn blob_kind_new_rejects_path_separator() {
+        let _ = BlobKind::new("bad/ext");
+    }
+
+    #[test]
+    #[should_panic = "BlobKind extension"]
+    fn blob_kind_new_rejects_dot() {
+        let _ = BlobKind::new("step.bak");
+    }
+}
diff --git a/crates/bone-document/src/io/folder.rs b/crates/bone-document/src/io/folder.rs
new file mode 100644
index 0000000..b6e7713
--- /dev/null
+++ b/crates/bone-document/src/io/folder.rs
@@ -0,0 +1,412 @@
+use std::collections::{BTreeMap, BTreeSet};
+use std::io::Write;
+use std::path::{Path, PathBuf};
+use std::{fs, io};
+
+use bone_types::{SchemaHeader, SchemaVersion, SketchId};
+
+use crate::document::{Document, DocumentHeader, FeatureNode, SketchFile, sketch_filename};
+use crate::io::blob::{BlobHash, BlobKind};
+use crate::io::ron_io::{RonError, from_str, to_string};
+use crate::sketch::SketchEditError;
+
+pub const DOCUMENT_FILE: &str = "document.ron";
+pub const SKETCHES_DIR: &str = "sketches";
+pub const BLOBS_DIR: &str = "blobs";
+pub const CACHES_DIR: &str = "caches";
+
+const ROOT_GITIGNORE: &str = "caches/\n";
+const ROOT_GITATTRIBUTES: &str = "* text=auto eol=lf\n*.boneblob binary\n";
+const CACHES_GITIGNORE: &str = "*\n!.gitignore\n!CACHEDIR.TAG\n";
+const CACHEDIR_TAG: &str = concat!(
+    "Signature: 8a477f597d28d172789f06886806bc55\n",
+    "# This file is a cache directory tag automatically created by bone.\n",
+    "# For information about cache directory tags see https://bford.info/cachedir/\n",
+);
+
+#[derive(Debug, thiserror::Error)]
+#[error(transparent)]
+pub struct FolderError(Box<FolderErrorKind>);
+
+impl FolderError {
+    #[must_use]
+    pub fn kind(&self) -> &FolderErrorKind {
+        &self.0
+    }
+
+    #[must_use]
+    pub fn into_kind(self) -> FolderErrorKind {
+        *self.0
+    }
+}
+
+impl From<FolderErrorKind> for FolderError {
+    fn from(kind: FolderErrorKind) -> Self {
+        Self(Box::new(kind))
+    }
+}
+
+impl FolderErrorKind {
+    fn wrap(self) -> FolderError {
+        FolderError(Box::new(self))
+    }
+}
+
+#[derive(Debug, thiserror::Error)]
+pub enum FolderErrorKind {
+    #[error("io at {path}: {source}")]
+    Io {
+        path: PathBuf,
+        #[source]
+        source: io::Error,
+    },
+    #[error("ron at {path}: {source}")]
+    Ron {
+        path: PathBuf,
+        #[source]
+        source: RonError,
+    },
+    #[error("unknown schema {found} (expected name {expected_name})")]
+    UnknownSchema {
+        found: String,
+        expected_name: &'static str,
+    },
+    #[error("schema {name} major v{found} is unsupported (this build supports v{supported})")]
+    UnsupportedMajor {
+        name: String,
+        found: SchemaVersion,
+        supported: SchemaVersion,
+    },
+    #[error("registry references sketch {id:?} with no file on disk")]
+    MissingSketchFile { id: SketchId },
+    #[error("integrity of {path}: {source}")]
+    SketchIntegrity {
+        path: PathBuf,
+        #[source]
+        source: SketchEditError,
+    },
+    #[error("feature tree references sketch {id:?} not in registry")]
+    DanglingTreeSketch { id: SketchId },
+    #[error("registry has sketch {id:?} absent from feature tree")]
+    OrphanRegistered { id: SketchId },
+}
+
+#[derive(Clone, Debug, PartialEq, Eq, Hash)]
+pub struct DocumentFolder {
+    path: PathBuf,
+}
+
+impl DocumentFolder {
+    #[must_use]
+    pub fn new(path: impl Into<PathBuf>) -> Self {
+        Self { path: path.into() }
+    }
+
+    #[must_use]
+    pub fn path(&self) -> &Path {
+        &self.path
+    }
+
+    #[must_use]
+    pub fn document_file(&self) -> PathBuf {
+        self.path.join(DOCUMENT_FILE)
+    }
+
+    #[must_use]
+    pub fn sketches_dir(&self) -> PathBuf {
+        self.path.join(SKETCHES_DIR)
+    }
+
+    #[must_use]
+    pub fn blobs_dir(&self) -> PathBuf {
+        self.path.join(BLOBS_DIR)
+    }
+
+    #[must_use]
+    pub fn caches_dir(&self) -> PathBuf {
+        self.path.join(CACHES_DIR)
+    }
+
+    #[must_use]
+    pub fn sketch_path(&self, id: SketchId) -> PathBuf {
+        self.sketches_dir().join(sketch_filename(id))
+    }
+
+    #[must_use]
+    pub fn blob_path(&self, hash: BlobHash, kind: BlobKind) -> PathBuf {
+        self.blobs_dir().join(hash.relative_path(kind))
+    }
+}
+
+pub fn save(document: &Document, folder: &DocumentFolder) -> Result<(), FolderError> {
+    ensure_dir(folder.path())?;
+    ensure_dir(&folder.sketches_dir())?;
+    ensure_dir(&folder.blobs_dir())?;
+    ensure_dir(&folder.caches_dir())?;
+
+    write_if_different(&folder.path().join(".gitignore"), ROOT_GITIGNORE)?;
+    write_if_different(&folder.path().join(".gitattributes"), ROOT_GITATTRIBUTES)?;
+    write_if_different(&folder.caches_dir().join("CACHEDIR.TAG"), CACHEDIR_TAG)?;
+    write_if_different(&folder.caches_dir().join(".gitignore"), CACHES_GITIGNORE)?;
+
+    document
+        .sketches()
+        .try_for_each(|(id, sketch)| -> Result<(), FolderError> {
+            let file = SketchFile::new(sketch.clone());
+            let ron = to_ron(&folder.sketch_path(id), &file)?;
+            atomic_write(&folder.sketch_path(id), &ron)
+        })?;
+
+    let document_ron = to_ron(&folder.document_file(), document.header())?;
+    atomic_write(&folder.document_file(), &document_ron)?;
+
+    let registry_ids: BTreeSet<SketchId> = document.registry().order().iter().copied().collect();
+    remove_stale_sketches(&folder.sketches_dir(), &registry_ids)?;
+
+    Ok(())
+}
+
+pub fn load(folder: &DocumentFolder) -> Result<Document, FolderError> {
+    let header_path = folder.document_file();
+    let header_text = read_to_string(&header_path)?;
+    let header: DocumentHeader = from_ron(&header_path, &header_text)?;
+    check_schema(&header.schema)?;
+    validate_header(&header)?;
+
+    let sketches =
+        header
+            .sketches
+            .order()
+            .iter()
+            .copied()
+            .try_fold(BTreeMap::new(), |mut acc, id| {
+                let path = folder.sketch_path(id);
+                let text = read_to_string(&path).map_err(|e| match e.into_kind() {
+                    FolderErrorKind::Io { source, .. }
+                        if source.kind() == io::ErrorKind::NotFound =>
+                    {
+                        FolderErrorKind::MissingSketchFile { id }.wrap()
+                    }
+                    other => other.wrap(),
+                })?;
+                let file: SketchFile = from_ron(&path, &text)?;
+                check_schema(&file.schema)?;
+                file.sketch.validate().map_err(|source| {
+                    FolderErrorKind::SketchIntegrity {
+                        path: path.clone(),
+                        source,
+                    }
+                    .wrap()
+                })?;
+                acc.insert(id, file.sketch);
+                Ok::<_, FolderError>(acc)
+            })?;
+
+    Ok(Document::from_parts(header, sketches))
+}
+
+fn validate_header(header: &DocumentHeader) -> Result<(), FolderError> {
+    let registered: BTreeSet<SketchId> = header.sketches.order().iter().copied().collect();
+    let tree_sketches: BTreeSet<SketchId> = header
+        .feature_tree
+        .iter()
+        .filter_map(|(_, node)| match node {
+            FeatureNode::Sketch(id) => Some(id),
+            FeatureNode::Origin | FeatureNode::PrincipalPlane(_) => None,
+        })
+        .collect();
+    if let Some(&id) = tree_sketches.difference(&registered).next() {
+        return Err(FolderErrorKind::DanglingTreeSketch { id }.wrap());
+    }
+    if let Some(&id) = registered.difference(&tree_sketches).next() {
+        return Err(FolderErrorKind::OrphanRegistered { id }.wrap());
+    }
+    Ok(())
+}
+
+fn check_schema(schema: &SchemaHeader) -> Result<(), FolderError> {
+    if !schema.is_bone_document() {
+        return Err(FolderErrorKind::UnknownSchema {
+            found: schema.name.clone(),
+            expected_name: SchemaHeader::BONE_DOCUMENT_NAME,
+        }
+        .wrap());
+    }
+    let supported = SchemaVersion::new(
+        SchemaHeader::BONE_DOCUMENT_MAJOR,
+        SchemaHeader::BONE_DOCUMENT_MINOR,
+    );
+    if schema.version.major != SchemaHeader::BONE_DOCUMENT_MAJOR {
+        return Err(FolderErrorKind::UnsupportedMajor {
+            name: schema.name.clone(),
+            found: schema.version,
+            supported,
+        }
+        .wrap());
+    }
+    if schema.version.minor > SchemaHeader::BONE_DOCUMENT_MINOR {
+        tracing::info!(
+            name = %schema.name,
+            found = %schema.version,
+            supported = %supported,
+            "accepting newer minor schema version; unknown fields will be rejected by deny_unknown_fields"
+        );
+    }
+    Ok(())
+}
+
+fn ensure_dir(path: &Path) -> Result<(), FolderError> {
+    fs::create_dir_all(path).map_err(|source| {
+        FolderErrorKind::Io {
+            path: path.to_path_buf(),
+            source,
+        }
+        .wrap()
+    })
+}
+
+fn write_if_different(path: &Path, contents: &str) -> Result<(), FolderError> {
+    if let Ok(existing) = fs::read_to_string(path)
+        && existing == contents
+    {
+        return Ok(());
+    }
+    atomic_write(path, contents)
+}
+
+fn atomic_write(path: &Path, contents: &str) -> Result<(), FolderError> {
+    let parent = path.parent().unwrap_or_else(|| Path::new("."));
+    ensure_dir(parent)?;
+    let tmp = tmp_sibling(path);
+    write_and_sync(&tmp, contents)?;
+    fs::rename(&tmp, path).map_err(|source| {
+        let _ = fs::remove_file(&tmp);
+        FolderErrorKind::Io {
+            path: path.to_path_buf(),
+            source,
+        }
+        .wrap()
+    })?;
+    sync_dir(parent)
+}
+
+fn write_and_sync(path: &Path, contents: &str) -> Result<(), FolderError> {
+    let mut file = fs::File::create(path).map_err(|source| {
+        FolderErrorKind::Io {
+            path: path.to_path_buf(),
+            source,
+        }
+        .wrap()
+    })?;
+    file.write_all(contents.as_bytes()).map_err(|source| {
+        FolderErrorKind::Io {
+            path: path.to_path_buf(),
+            source,
+        }
+        .wrap()
+    })?;
+    file.sync_all().map_err(|source| {
+        FolderErrorKind::Io {
+            path: path.to_path_buf(),
+            source,
+        }
+        .wrap()
+    })
+}
+
+#[cfg(unix)]
+fn sync_dir(path: &Path) -> Result<(), FolderError> {
+    fs::File::open(path)
+        .and_then(|f| f.sync_all())
+        .map_err(|source| {
+            FolderErrorKind::Io {
+                path: path.to_path_buf(),
+                source,
+            }
+            .wrap()
+        })
+}
+
+#[cfg(not(unix))]
+fn sync_dir(_: &Path) -> Result<(), FolderError> {
+    Ok(())
+}
+
+fn tmp_sibling(path: &Path) -> PathBuf {
+    let file_name = path
+        .file_name()
+        .map(std::ffi::OsStr::to_os_string)
+        .unwrap_or_default();
+    let mut tmp_name = file_name;
+    tmp_name.push(".tmp");
+    path.with_file_name(tmp_name)
+}
+
+fn read_to_string(path: &Path) -> Result<String, FolderError> {
+    fs::read_to_string(path).map_err(|source| {
+        FolderErrorKind::Io {
+            path: path.to_path_buf(),
+            source,
+        }
+        .wrap()
+    })
+}
+
+fn to_ron<T: serde::Serialize>(path: &Path, value: &T) -> Result<String, FolderError> {
+    to_string(value).map_err(|source| {
+        FolderErrorKind::Ron {
+            path: path.to_path_buf(),
+            source,
+        }
+        .wrap()
+    })
+}
+
+fn from_ron<T: serde::de::DeserializeOwned>(path: &Path, text: &str) -> Result<T, FolderError> {
+    from_str(text).map_err(|source| {
+        FolderErrorKind::Ron {
+            path: path.to_path_buf(),
+            source,
+        }
+        .wrap()
+    })
+}
+
+fn remove_stale_sketches(dir: &Path, live: &BTreeSet<SketchId>) -> Result<(), FolderError> {
+    let entries = match fs::read_dir(dir) {
+        Ok(iter) => iter,
+        Err(ref source) if source.kind() == io::ErrorKind::NotFound => return Ok(()),
+        Err(source) => {
+            return Err(FolderErrorKind::Io {
+                path: dir.to_path_buf(),
+                source,
+            }
+            .into());
+        }
+    };
+    let live_names: BTreeSet<String> = live.iter().copied().map(sketch_filename).collect();
+    let modified = entries.into_iter().try_fold(false, |modified, entry| {
+        let entry = entry.map_err(|source| {
+            FolderErrorKind::Io {
+                path: dir.to_path_buf(),
+                source,
+            }
+            .wrap()
+        })?;
+        let name = entry.file_name().to_string_lossy().into_owned();
+        let looks_like_ron = Path::new(&name)
+            .extension()
+            .is_some_and(|ext| ext.eq_ignore_ascii_case("ron"));
+        if looks_like_ron && !live_names.contains(&name) {
+            let path = entry.path();
+            fs::remove_file(&path).map_err(|source| FolderErrorKind::Io { path, source }.wrap())?;
+            Ok::<_, FolderError>(true)
+        } else {
+            Ok(modified)
+        }
+    })?;
+    if modified {
+        sync_dir(dir)?;
+    }
+    Ok(())
+}
diff --git a/crates/bone-document/src/io/mod.rs b/crates/bone-document/src/io/mod.rs
new file mode 100644
index 0000000..e8de835
--- /dev/null
+++ b/crates/bone-document/src/io/mod.rs
@@ -0,0 +1,7 @@
+pub mod blob;
+pub mod folder;
+pub mod ron_io;
+
+pub use blob::{BlobHash, BlobKind};
+pub use folder::{DocumentFolder, FolderError, FolderErrorKind, load, save};
+pub use ron_io::{RonError, from_str, to_string};
diff --git a/crates/bone-document/src/io/ron_io.rs b/crates/bone-document/src/io/ron_io.rs
new file mode 100644
index 0000000..b870bb6
--- /dev/null
+++ b/crates/bone-document/src/io/ron_io.rs
@@ -0,0 +1,34 @@
+use ron::extensions::Extensions;
+use ron::ser::PrettyConfig;
+use serde::{Serialize, de::DeserializeOwned};
+
+#[derive(Debug, thiserror::Error)]
+pub enum RonError {
+    #[error("serialize: {0}")]
+    Serialize(#[from] ron::Error),
+    #[error("deserialize: {0}")]
+    Deserialize(#[from] ron::error::SpannedError),
+}
+
+#[must_use]
+pub fn pretty_config() -> PrettyConfig {
+    PrettyConfig::new()
+        .struct_names(true)
+        .separate_tuple_members(false)
+        .compact_arrays(true)
+        .depth_limit(usize::MAX)
+        .indentor("    ".to_owned())
+        .extensions(
+            Extensions::UNWRAP_NEWTYPES
+                | Extensions::IMPLICIT_SOME
+                | Extensions::EXPLICIT_STRUCT_NAMES,
+        )
+}
+
+pub fn to_string<T: Serialize>(value: &T) -> Result<String, RonError> {
+    Ok(ron::ser::to_string_pretty(value, pretty_config())?)
+}
+
+pub fn from_str<T: DeserializeOwned>(input: &str) -> Result<T, RonError> {
+    Ok(ron::de::from_str(input)?)
+}
diff --git a/crates/bone-document/src/lib.rs b/crates/bone-document/src/lib.rs
new file mode 100644
index 0000000..0c6357d
--- /dev/null
+++ b/crates/bone-document/src/lib.rs
@@ -0,0 +1,34 @@
+pub mod document;
+pub mod evaluator;
+pub mod io;
+pub mod sketch;
+pub mod undo;
+
+pub use document::{
+    Document, DocumentHeader, DocumentParameters, FeatureNode, FeatureTree, PrincipalPlane,
+    SketchFile, SketchRegistry, SketchRegistryEntry, UnitsPreference, sketch_filename,
+};
+pub use evaluator::{EvaluatedSketch, FeatureCache, evaluate_sketch};
+pub use io::{
+    BlobHash, BlobKind, DocumentFolder, FolderError, FolderErrorKind, RonError, from_str, load,
+    save, to_string,
+};
+pub use sketch::{
+    ArcData, CircleData, DimensionKind, DimensionRefs, DimensionValue, DimensionValueMismatch,
+    EditOutcome, EntityRefs, LineData, PointData, RelationRefs, Sketch, SketchDimension,
+    SketchDofReport, SketchEdit, SketchEditError, SketchEntity, SketchEntityKind, SketchParameter,
+    SketchRelation,
+};
+pub use undo::UndoStack;
+
+#[derive(Debug, thiserror::Error)]
+pub enum DocumentError {
+    #[error(transparent)]
+    SketchEdit(#[from] SketchEditError),
+    #[error(transparent)]
+    Folder(#[from] FolderError),
+    #[error(transparent)]
+    Ron(#[from] RonError),
+}
+
+pub type Result<T, E = DocumentError> = core::result::Result<T, E>;
diff --git a/crates/bone-document/src/sketch/dimension.rs b/crates/bone-document/src/sketch/dimension.rs
new file mode 100644
index 0000000..c4777a7
--- /dev/null
+++ b/crates/bone-document/src/sketch/dimension.rs
@@ -0,0 +1,123 @@
+use bone_types::{Angle, Length, SketchEntityId, dimensioned_serde};
+use serde::{Deserialize, Serialize};
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, Serialize, Deserialize)]
+pub enum DimensionKind {
+    Driving,
+    Driven,
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, Serialize, Deserialize)]
+pub enum DimensionValue {
+    #[serde(with = "dimensioned_serde::length_si")]
+    Length(Length),
+    #[serde(with = "dimensioned_serde::angle_si")]
+    Angle(Angle),
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, Serialize, Deserialize)]
+#[serde(deny_unknown_fields)]
+pub enum SketchDimension {
+    Linear {
+        a: SketchEntityId,
+        b: SketchEntityId,
+        #[serde(with = "dimensioned_serde::length_si")]
+        value: Length,
+        kind: DimensionKind,
+    },
+    Radius {
+        target: SketchEntityId,
+        #[serde(with = "dimensioned_serde::length_si")]
+        value: Length,
+        kind: DimensionKind,
+    },
+    Diameter {
+        target: SketchEntityId,
+        #[serde(with = "dimensioned_serde::length_si")]
+        value: Length,
+        kind: DimensionKind,
+    },
+    Angular {
+        a: SketchEntityId,
+        b: SketchEntityId,
+        #[serde(with = "dimensioned_serde::angle_si")]
+        value: Angle,
+        kind: DimensionKind,
+    },
+}
+
+impl SketchDimension {
+    #[must_use]
+    pub const fn kind(&self) -> DimensionKind {
+        match *self {
+            Self::Linear { kind, .. }
+            | Self::Radius { kind, .. }
+            | Self::Diameter { kind, .. }
+            | Self::Angular { kind, .. } => kind,
+        }
+    }
+
+    #[must_use]
+    pub const fn value(&self) -> DimensionValue {
+        match *self {
+            Self::Linear { value, .. }
+            | Self::Radius { value, .. }
+            | Self::Diameter { value, .. } => DimensionValue::Length(value),
+            Self::Angular { value, .. } => DimensionValue::Angle(value),
+        }
+    }
+
+    #[must_use]
+    pub const fn references(&self) -> DimensionRefs {
+        match *self {
+            Self::Linear { a, b, .. } | Self::Angular { a, b, .. } => {
+                DimensionRefs([Some(a), Some(b)])
+            }
+            Self::Radius { target, .. } | Self::Diameter { target, .. } => {
+                DimensionRefs([Some(target), None])
+            }
+        }
+    }
+
+    pub fn with_value(self, value: DimensionValue) -> Result<Self, DimensionValueMismatch> {
+        match (self, value) {
+            (Self::Linear { a, b, kind, .. }, DimensionValue::Length(value)) => {
+                Ok(Self::Linear { a, b, value, kind })
+            }
+            (Self::Radius { target, kind, .. }, DimensionValue::Length(value)) => {
+                Ok(Self::Radius {
+                    target,
+                    value,
+                    kind,
+                })
+            }
+            (Self::Diameter { target, kind, .. }, DimensionValue::Length(value)) => {
+                Ok(Self::Diameter {
+                    target,
+                    value,
+                    kind,
+                })
+            }
+            (Self::Angular { a, b, kind, .. }, DimensionValue::Angle(value)) => {
+                Ok(Self::Angular { a, b, value, kind })
+            }
+            _ => Err(DimensionValueMismatch),
+        }
+    }
+}
+
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub struct DimensionRefs([Option<SketchEntityId>; 2]);
+
+impl IntoIterator for DimensionRefs {
+    type Item = SketchEntityId;
+    type IntoIter = core::iter::Flatten<core::array::IntoIter<Option<SketchEntityId>, 2>>;
+
+    fn into_iter(self) -> Self::IntoIter {
+        self.0.into_iter().flatten()
+    }
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq, thiserror::Error)]
+#[error("dimension value kind does not match variant")]
+pub struct DimensionValueMismatch;
diff --git a/crates/bone-document/src/sketch/edit.rs b/crates/bone-document/src/sketch/edit.rs
new file mode 100644
index 0000000..0398233
--- /dev/null
+++ b/crates/bone-document/src/sketch/edit.rs
@@ -0,0 +1,35 @@
+use bone_types::{SketchDimensionId, SketchEntityId, SketchParameterId, SketchRelationId};
+
+use super::dimension::{DimensionValue, SketchDimension};
+use super::entity::SketchEntity;
+use super::parameter::SketchParameter;
+use super::relation::SketchRelation;
+
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub enum SketchEdit {
+    AddEntity(SketchEntity),
+    AddRelation(SketchRelation),
+    AddDimension(SketchDimension),
+    AddParameter(SketchParameter),
+    DeleteEntity(SketchEntityId),
+    DeleteRelation(SketchRelationId),
+    DeleteDimension(SketchDimensionId),
+    DeleteParameter(SketchParameterId),
+    SetConstruction {
+        id: SketchEntityId,
+        for_construction: bool,
+    },
+    UpdateDimensionValue {
+        id: SketchDimensionId,
+        value: DimensionValue,
+    },
+}
+
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub enum EditOutcome {
+    Entity(SketchEntityId),
+    Relation(SketchRelationId),
+    Dimension(SketchDimensionId),
+    Parameter(SketchParameterId),
+    None,
+}
diff --git a/crates/bone-document/src/sketch/entity.rs b/crates/bone-document/src/sketch/entity.rs
new file mode 100644
index 0000000..fde4095
--- /dev/null
+++ b/crates/bone-document/src/sketch/entity.rs
@@ -0,0 +1,254 @@
+use bone_types::{Length, Point2, SketchEntityId, dimensioned_serde};
+use serde::{Deserialize, Serialize};
+
+#[derive(Copy, Clone, Debug, PartialEq, Serialize, Deserialize)]
+#[serde(deny_unknown_fields)]
+pub struct PointData {
+    at: Point2,
+}
+
+impl PointData {
+    #[must_use]
+    pub const fn new(at: Point2) -> Self {
+        Self { at }
+    }
+
+    #[must_use]
+    pub const fn at(self) -> Point2 {
+        self.at
+    }
+
+    #[must_use]
+    pub const fn for_construction(self) -> bool {
+        true
+    }
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, Serialize, Deserialize)]
+#[serde(deny_unknown_fields)]
+pub struct LineData {
+    a: SketchEntityId,
+    b: SketchEntityId,
+    for_construction: bool,
+}
+
+impl LineData {
+    #[must_use]
+    pub const fn new(a: SketchEntityId, b: SketchEntityId, for_construction: bool) -> Self {
+        Self {
+            a,
+            b,
+            for_construction,
+        }
+    }
+
+    #[must_use]
+    pub const fn a(self) -> SketchEntityId {
+        self.a
+    }
+
+    #[must_use]
+    pub const fn b(self) -> SketchEntityId {
+        self.b
+    }
+
+    #[must_use]
+    pub const fn for_construction(self) -> bool {
+        self.for_construction
+    }
+
+    #[must_use]
+    pub const fn with_construction(self, for_construction: bool) -> Self {
+        Self {
+            for_construction,
+            ..self
+        }
+    }
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, Serialize, Deserialize)]
+#[serde(deny_unknown_fields)]
+pub struct ArcData {
+    center: SketchEntityId,
+    start: SketchEntityId,
+    end: SketchEntityId,
+    for_construction: bool,
+}
+
+impl ArcData {
+    #[must_use]
+    pub const fn new(
+        center: SketchEntityId,
+        start: SketchEntityId,
+        end: SketchEntityId,
+        for_construction: bool,
+    ) -> Self {
+        Self {
+            center,
+            start,
+            end,
+            for_construction,
+        }
+    }
+
+    #[must_use]
+    pub const fn center(self) -> SketchEntityId {
+        self.center
+    }
+
+    #[must_use]
+    pub const fn start(self) -> SketchEntityId {
+        self.start
+    }
+
+    #[must_use]
+    pub const fn end(self) -> SketchEntityId {
+        self.end
+    }
+
+    #[must_use]
+    pub const fn for_construction(self) -> bool {
+        self.for_construction
+    }
+
+    #[must_use]
+    pub const fn with_construction(self, for_construction: bool) -> Self {
+        Self {
+            for_construction,
+            ..self
+        }
+    }
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, Serialize, Deserialize)]
+#[serde(deny_unknown_fields)]
+pub struct CircleData {
+    center: SketchEntityId,
+    #[serde(with = "dimensioned_serde::length_si")]
+    radius: Length,
+    for_construction: bool,
+}
+
+impl CircleData {
+    #[must_use]
+    pub const fn new(center: SketchEntityId, radius: Length, for_construction: bool) -> Self {
+        Self {
+            center,
+            radius,
+            for_construction,
+        }
+    }
+
+    #[must_use]
+    pub const fn center(self) -> SketchEntityId {
+        self.center
+    }
+
+    #[must_use]
+    pub const fn radius(self) -> Length {
+        self.radius
+    }
+
+    #[must_use]
+    pub const fn for_construction(self) -> bool {
+        self.for_construction
+    }
+
+    #[must_use]
+    pub const fn with_construction(self, for_construction: bool) -> Self {
+        Self {
+            for_construction,
+            ..self
+        }
+    }
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, Serialize, Deserialize)]
+pub enum SketchEntityKind {
+    Point,
+    Line,
+    Arc,
+    Circle,
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, Serialize, Deserialize)]
+pub enum SketchEntity {
+    Point(PointData),
+    Line(LineData),
+    Arc(ArcData),
+    Circle(CircleData),
+}
+
+impl SketchEntity {
+    #[must_use]
+    pub const fn point(at: Point2) -> Self {
+        Self::Point(PointData::new(at))
+    }
+
+    #[must_use]
+    pub const fn line(a: SketchEntityId, b: SketchEntityId, for_construction: bool) -> Self {
+        Self::Line(LineData::new(a, b, for_construction))
+    }
+
+    #[must_use]
+    pub const fn arc(
+        center: SketchEntityId,
+        start: SketchEntityId,
+        end: SketchEntityId,
+        for_construction: bool,
+    ) -> Self {
+        Self::Arc(ArcData::new(center, start, end, for_construction))
+    }
+
+    #[must_use]
+    pub const fn circle(center: SketchEntityId, radius: Length, for_construction: bool) -> Self {
+        Self::Circle(CircleData::new(center, radius, for_construction))
+    }
+
+    #[must_use]
+    pub const fn for_construction(&self) -> bool {
+        match *self {
+            Self::Point(p) => p.for_construction(),
+            Self::Line(l) => l.for_construction(),
+            Self::Arc(a) => a.for_construction(),
+            Self::Circle(c) => c.for_construction(),
+        }
+    }
+
+    #[must_use]
+    pub const fn is_point(&self) -> bool {
+        matches!(self, Self::Point(_))
+    }
+
+    #[must_use]
+    pub const fn kind(&self) -> SketchEntityKind {
+        match *self {
+            Self::Point(_) => SketchEntityKind::Point,
+            Self::Line(_) => SketchEntityKind::Line,
+            Self::Arc(_) => SketchEntityKind::Arc,
+            Self::Circle(_) => SketchEntityKind::Circle,
+        }
+    }
+
+    #[must_use]
+    pub const fn references(&self) -> EntityRefs {
+        match *self {
+            Self::Point(_) => EntityRefs([None, None, None]),
+            Self::Line(l) => EntityRefs([Some(l.a()), Some(l.b()), None]),
+            Self::Arc(a) => EntityRefs([Some(a.center()), Some(a.start()), Some(a.end())]),
+            Self::Circle(c) => EntityRefs([Some(c.center()), None, None]),
+        }
+    }
+}
+
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub struct EntityRefs([Option<SketchEntityId>; 3]);
+
+impl IntoIterator for EntityRefs {
+    type Item = SketchEntityId;
+    type IntoIter = core::iter::Flatten<core::array::IntoIter<Option<SketchEntityId>, 3>>;
+
+    fn into_iter(self) -> Self::IntoIter {
+        self.0.into_iter().flatten()
+    }
+}
diff --git a/crates/bone-document/src/sketch/mod.rs b/crates/bone-document/src/sketch/mod.rs
new file mode 100644
index 0000000..fd25516
--- /dev/null
+++ b/crates/bone-document/src/sketch/mod.rs
@@ -0,0 +1,1611 @@
+use std::collections::{BTreeSet, HashMap};
+use std::sync::Arc;
+
+use bone_types::{
+    Length, Point2, SketchDimensionId, SketchEntityId, SketchParameterId, SketchPlaneBasis,
+    SketchRelationId,
+};
+use serde::{Deserialize, Serialize};
+use slotmap::SlotMap;
+
+pub mod dimension;
+pub mod edit;
+pub mod entity;
+pub mod parameter;
+pub mod relation;
+pub mod solve;
+
+pub use dimension::{
+    DimensionKind, DimensionRefs, DimensionValue, DimensionValueMismatch, SketchDimension,
+};
+pub use edit::{EditOutcome, SketchEdit};
+pub use entity::{
+    ArcData, CircleData, EntityRefs, LineData, PointData, SketchEntity, SketchEntityKind,
+};
+pub use parameter::SketchParameter;
+pub use relation::{RelationRefs, SketchRelation};
+pub use solve::{DragError, Mapping as SketchSolveMapping, SketchDofReport};
+
+type EntityMap = SlotMap<SketchEntityId, SketchEntity>;
+type RelationMap = SlotMap<SketchRelationId, SketchRelation>;
+type DimensionMap = SlotMap<SketchDimensionId, SketchDimension>;
+type ParameterMap = SlotMap<SketchParameterId, SketchParameter>;
+
+#[derive(Clone, Debug, Serialize, Deserialize)]
+#[serde(deny_unknown_fields)]
+pub struct Sketch {
+    plane: SketchPlaneBasis,
+    entities: Arc<EntityMap>,
+    entity_order: Arc<Vec<SketchEntityId>>,
+    relations: Arc<RelationMap>,
+    relation_order: Arc<Vec<SketchRelationId>>,
+    dimensions: Arc<DimensionMap>,
+    dimension_order: Arc<Vec<SketchDimensionId>>,
+    parameters: Arc<ParameterMap>,
+    parameter_order: Arc<Vec<SketchParameterId>>,
+}
+
+impl Sketch {
+    #[must_use]
+    pub fn new(plane: SketchPlaneBasis) -> Self {
+        Self {
+            plane,
+            entities: Arc::new(SlotMap::with_key()),
+            entity_order: Arc::new(Vec::new()),
+            relations: Arc::new(SlotMap::with_key()),
+            relation_order: Arc::new(Vec::new()),
+            dimensions: Arc::new(SlotMap::with_key()),
+            dimension_order: Arc::new(Vec::new()),
+            parameters: Arc::new(SlotMap::with_key()),
+            parameter_order: Arc::new(Vec::new()),
+        }
+    }
+
+    #[must_use]
+    pub fn plane(&self) -> SketchPlaneBasis {
+        self.plane
+    }
+
+    #[must_use]
+    pub fn entities(&self) -> &EntityMap {
+        &self.entities
+    }
+
+    #[must_use]
+    pub fn relations(&self) -> &RelationMap {
+        &self.relations
+    }
+
+    #[must_use]
+    pub fn dimensions(&self) -> &DimensionMap {
+        &self.dimensions
+    }
+
+    #[must_use]
+    pub fn parameters(&self) -> &ParameterMap {
+        &self.parameters
+    }
+
+    #[must_use]
+    pub fn entity_order(&self) -> &[SketchEntityId] {
+        &self.entity_order
+    }
+
+    #[must_use]
+    pub fn relation_order(&self) -> &[SketchRelationId] {
+        &self.relation_order
+    }
+
+    #[must_use]
+    pub fn dimension_order(&self) -> &[SketchDimensionId] {
+        &self.dimension_order
+    }
+
+    #[must_use]
+    pub fn parameter_order(&self) -> &[SketchParameterId] {
+        &self.parameter_order
+    }
+
+    fn entries_equal<K, V>(order: &[K], lhs: &SlotMap<K, V>, rhs: &SlotMap<K, V>) -> bool
+    where
+        K: slotmap::Key,
+        V: PartialEq,
+    {
+        order.iter().all(|id| lhs.get(*id) == rhs.get(*id))
+    }
+
+    pub fn validate(&self) -> Result<(), SketchEditError> {
+        ensure_order_matches("entities", &self.entity_order, &self.entities)?;
+        ensure_order_matches("relations", &self.relation_order, &self.relations)?;
+        ensure_order_matches("dimensions", &self.dimension_order, &self.dimensions)?;
+        ensure_order_matches("parameters", &self.parameter_order, &self.parameters)?;
+
+        self.entity_order.iter().copied().try_for_each(|id| {
+            let entity = *self.require_entity(id)?;
+            validate_entity_shape(entity)?;
+            entity
+                .references()
+                .into_iter()
+                .try_for_each(|reference| self.require_point(reference))
+        })?;
+
+        self.relation_order.iter().copied().try_for_each(|id| {
+            let rel = *self
+                .relations
+                .get(id)
+                .ok_or(SketchEditError::RelationNotFound(id))?;
+            self.validate_relation(rel)
+        })?;
+
+        self.dimension_order.iter().copied().try_for_each(|id| {
+            let dim = *self
+                .dimensions
+                .get(id)
+                .ok_or(SketchEditError::DimensionNotFound(id))?;
+            self.validate_dimension(dim)
+        })
+    }
+
+    pub fn apply(self, edit: SketchEdit) -> Result<(Self, EditOutcome), SketchEditError> {
+        let result = match edit {
+            SketchEdit::AddEntity(entity) => {
+                entity
+                    .references()
+                    .into_iter()
+                    .try_for_each(|ref_id| self.require_point(ref_id))?;
+                validate_entity_shape(entity)?;
+                Ok(self.add_entity(entity))
+            }
+            SketchEdit::AddRelation(rel) => self.add_relation(rel),
+            SketchEdit::AddDimension(dim) => self.add_dimension(dim),
+            SketchEdit::AddParameter(p) => Ok(self.add_parameter(p)),
+            SketchEdit::DeleteEntity(id) => self.delete_entity(id),
+            SketchEdit::DeleteRelation(id) => self.delete_relation(id),
+            SketchEdit::DeleteDimension(id) => self.delete_dimension(id),
+            SketchEdit::DeleteParameter(id) => self.delete_parameter(id),
+            SketchEdit::SetConstruction {
+                id,
+                for_construction,
+            } => self.set_construction(id, for_construction),
+            SketchEdit::UpdateDimensionValue { id, value } => self.update_dimension(id, value),
+        };
+        if let Ok((ref sketch, _)) = result {
+            sketch.assert_invariants();
+        }
+        result
+    }
+
+    fn assert_invariants(&self) {
+        debug_assert_eq!(self.entity_order.len(), self.entities.len());
+        debug_assert_eq!(self.relation_order.len(), self.relations.len());
+        debug_assert_eq!(self.dimension_order.len(), self.dimensions.len());
+        debug_assert_eq!(self.parameter_order.len(), self.parameters.len());
+        debug_assert!(
+            self.entity_order
+                .iter()
+                .all(|id| self.entities.contains_key(*id))
+        );
+        debug_assert!(
+            self.relation_order
+                .iter()
+                .all(|id| self.relations.contains_key(*id))
+        );
+        debug_assert!(
+            self.dimension_order
+                .iter()
+                .all(|id| self.dimensions.contains_key(*id))
+        );
+        debug_assert!(
+            self.parameter_order
+                .iter()
+                .all(|id| self.parameters.contains_key(*id))
+        );
+    }
+
+    pub fn apply_all<I>(&self, edits: I) -> Result<(Self, Vec<EditOutcome>), SketchEditError>
+    where
+        I: IntoIterator<Item = SketchEdit>,
+    {
+        edits.into_iter().try_fold(
+            (self.clone(), Vec::new()),
+            |(sketch, mut outcomes), edit| {
+                let (next, outcome) = sketch.apply(edit)?;
+                outcomes.push(outcome);
+                Ok((next, outcomes))
+            },
+        )
+    }
+
+    fn require_entity(&self, id: SketchEntityId) -> Result<&SketchEntity, SketchEditError> {
+        self.entities
+            .get(id)
+            .ok_or(SketchEditError::EntityNotFound(id))
+    }
+
+    fn require_point(&self, id: SketchEntityId) -> Result<(), SketchEditError> {
+        if self.require_entity(id)?.is_point() {
+            Ok(())
+        } else {
+            Err(SketchEditError::ExpectedPoint(id))
+        }
+    }
+
+    fn kind_of(&self, id: SketchEntityId) -> Result<SketchEntityKind, SketchEditError> {
+        self.require_entity(id).map(SketchEntity::kind)
+    }
+
+    fn validate_relation(&self, rel: SketchRelation) -> Result<(), SketchEditError> {
+        use SketchEntityKind as K;
+        if let Some((a, b)) = rel.pair()
+            && a == b
+        {
+            return Err(SketchEditError::SelfReferencingRelation(rel));
+        }
+        let ok = match rel {
+            SketchRelation::Coincident(a, b) => {
+                matches!(
+                    (self.kind_of(a)?, self.kind_of(b)?),
+                    (K::Point, _) | (_, K::Point)
+                )
+            }
+            SketchRelation::Horizontal(a) | SketchRelation::Vertical(a) => {
+                self.kind_of(a)? == K::Line
+            }
+            SketchRelation::Parallel(a, b) | SketchRelation::Perpendicular(a, b) => {
+                self.kind_of(a)? == K::Line && self.kind_of(b)? == K::Line
+            }
+            SketchRelation::Tangent(a, b) => {
+                let (ka, kb) = (self.kind_of(a)?, self.kind_of(b)?);
+                let curve = |k| matches!(k, K::Line | K::Arc | K::Circle);
+                let round = |k| matches!(k, K::Arc | K::Circle);
+                curve(ka) && curve(kb) && (round(ka) || round(kb))
+            }
+            SketchRelation::Equal(a, b) => matches!(
+                (self.kind_of(a)?, self.kind_of(b)?),
+                (K::Line, K::Line) | (K::Arc | K::Circle, K::Arc | K::Circle)
+            ),
+            SketchRelation::Concentric(a, b) => {
+                matches!(self.kind_of(a)?, K::Arc | K::Circle)
+                    && matches!(self.kind_of(b)?, K::Arc | K::Circle)
+            }
+            SketchRelation::Fix(a) => {
+                self.kind_of(a)?;
+                true
+            }
+        };
+        if ok {
+            Ok(())
+        } else {
+            Err(SketchEditError::InvalidRelationOperands(rel))
+        }
+    }
+
+    fn validate_dimension(&self, dim: SketchDimension) -> Result<(), SketchEditError> {
+        use SketchEntityKind as K;
+        let ok = match dim {
+            SketchDimension::Linear { a, b, .. } => {
+                self.kind_of(a)? == K::Point && self.kind_of(b)? == K::Point
+            }
+            SketchDimension::Radius { target, .. } | SketchDimension::Diameter { target, .. } => {
+                matches!(self.kind_of(target)?, K::Arc | K::Circle)
+            }
+            SketchDimension::Angular { a, b, .. } => {
+                self.kind_of(a)? == K::Line && self.kind_of(b)? == K::Line
+            }
+        };
+        if ok {
+            Ok(())
+        } else {
+            Err(SketchEditError::InvalidDimensionOperands(dim))
+        }
+    }
+
+    fn add_entity(self, entity: SketchEntity) -> (Self, EditOutcome) {
+        let (id, entities) = insert(self.entities, entity);
+        let entity_order = push(self.entity_order, id);
+        let next = Self {
+            entities,
+            entity_order,
+            ..self
+        };
+        (next, EditOutcome::Entity(id))
+    }
+
+    fn add_relation(self, rel: SketchRelation) -> Result<(Self, EditOutcome), SketchEditError> {
+        self.validate_relation(rel)?;
+        let (id, relations) = insert(self.relations, rel);
+        let relation_order = push(self.relation_order, id);
+        let next = Self {
+            relations,
+            relation_order,
+            ..self
+        };
+        Ok((next, EditOutcome::Relation(id)))
+    }
+
+    fn add_dimension(self, dim: SketchDimension) -> Result<(Self, EditOutcome), SketchEditError> {
+        self.validate_dimension(dim)?;
+        let (id, dimensions) = insert(self.dimensions, dim);
+        let dimension_order = push(self.dimension_order, id);
+        let next = Self {
+            dimensions,
+            dimension_order,
+            ..self
+        };
+        Ok((next, EditOutcome::Dimension(id)))
+    }
+
+    fn add_parameter(self, parameter: SketchParameter) -> (Self, EditOutcome) {
+        let (id, parameters) = insert(self.parameters, parameter);
+        let parameter_order = push(self.parameter_order, id);
+        let next = Self {
+            parameters,
+            parameter_order,
+            ..self
+        };
+        (next, EditOutcome::Parameter(id))
+    }
+
+    fn delete_entity(self, id: SketchEntityId) -> Result<(Self, EditOutcome), SketchEditError> {
+        if !self.entities.contains_key(id) {
+            return Err(SketchEditError::EntityNotFound(id));
+        }
+        let closure = entity_closure(&self.entities, id);
+        let entities = remove_many(self.entities, closure.iter().copied());
+        let entity_order = retain(&self.entity_order, |eid| !closure.contains(eid));
+        let touches_closure_rel =
+            |rel: &SketchRelation| rel.references().into_iter().any(|e| closure.contains(&e));
+        let touches_closure_dim =
+            |dim: &SketchDimension| dim.references().into_iter().any(|e| closure.contains(&e));
+        let (relations, dropped_relations) = remove_where(self.relations, touches_closure_rel);
+        let relation_order = retain(&self.relation_order, |rid| !dropped_relations.contains(rid));
+        let (dimensions, dropped_dimensions) = remove_where(self.dimensions, touches_closure_dim);
+        let dimension_order = retain(&self.dimension_order, |did| {
+            !dropped_dimensions.contains(did)
+        });
+        let next = Self {
+            entities,
+            entity_order,
+            relations,
+            relation_order,
+            dimensions,
+            dimension_order,
+            ..self
+        };
+        Ok((next, EditOutcome::None))
+    }
+
+    fn delete_relation(self, id: SketchRelationId) -> Result<(Self, EditOutcome), SketchEditError> {
+        if !self.relations.contains_key(id) {
+            return Err(SketchEditError::RelationNotFound(id));
+        }
+        let relations = remove_one(self.relations, id);
+        let relation_order = retain(&self.relation_order, |rid| *rid != id);
+        let next = Self {
+            relations,
+            relation_order,
+            ..self
+        };
+        Ok((next, EditOutcome::None))
+    }
+
+    fn delete_dimension(
+        self,
+        id: SketchDimensionId,
+    ) -> Result<(Self, EditOutcome), SketchEditError> {
+        if !self.dimensions.contains_key(id) {
+            return Err(SketchEditError::DimensionNotFound(id));
+        }
+        let dimensions = remove_one(self.dimensions, id);
+        let dimension_order = retain(&self.dimension_order, |did| *did != id);
+        let next = Self {
+            dimensions,
+            dimension_order,
+            ..self
+        };
+        Ok((next, EditOutcome::None))
+    }
+
+    fn delete_parameter(
+        self,
+        id: SketchParameterId,
+    ) -> Result<(Self, EditOutcome), SketchEditError> {
+        if !self.parameters.contains_key(id) {
+            return Err(SketchEditError::ParameterNotFound(id));
+        }
+        let parameters = remove_one(self.parameters, id);
+        let parameter_order = retain(&self.parameter_order, |pid| *pid != id);
+        let next = Self {
+            parameters,
+            parameter_order,
+            ..self
+        };
+        Ok((next, EditOutcome::None))
+    }
+
+    fn set_construction(
+        self,
+        id: SketchEntityId,
+        for_construction: bool,
+    ) -> Result<(Self, EditOutcome), SketchEditError> {
+        let entity = *self.require_entity(id)?;
+        let updated = match entity {
+            SketchEntity::Point(_) => return Err(SketchEditError::PointAlwaysConstruction),
+            SketchEntity::Line(l) => SketchEntity::Line(l.with_construction(for_construction)),
+            SketchEntity::Arc(a) => SketchEntity::Arc(a.with_construction(for_construction)),
+            SketchEntity::Circle(c) => SketchEntity::Circle(c.with_construction(for_construction)),
+        };
+        if updated == entity {
+            return Ok((self, EditOutcome::None));
+        }
+        let entities = replace(self.entities, id, updated);
+        let next = Self { entities, ..self };
+        Ok((next, EditOutcome::None))
+    }
+
+    fn update_dimension(
+        self,
+        id: SketchDimensionId,
+        value: DimensionValue,
+    ) -> Result<(Self, EditOutcome), SketchEditError> {
+        let existing = *self
+            .dimensions
+            .get(id)
+            .ok_or(SketchEditError::DimensionNotFound(id))?;
+        if existing.kind() == DimensionKind::Driven {
+            return Err(SketchEditError::DimensionIsDriven(id));
+        }
+        let updated = existing
+            .with_value(value)
+            .map_err(|_| SketchEditError::DimensionValueMismatch { id })?;
+        if updated == existing {
+            return Ok((self, EditOutcome::None));
+        }
+        let dimensions = replace(self.dimensions, id, updated);
+        let next = Self { dimensions, ..self };
+        Ok((next, EditOutcome::None))
+    }
+
+    #[must_use]
+    pub(crate) fn with_point_positions(self, updates: &HashMap<SketchEntityId, Point2>) -> Self {
+        if updates.is_empty() {
+            return self;
+        }
+        let entities = updates
+            .iter()
+            .fold(unwrap_arc(self.entities), |mut acc, (id, pt)| {
+                if let Some(slot @ SketchEntity::Point(_)) = acc.get_mut(*id) {
+                    *slot = SketchEntity::Point(PointData::new(*pt));
+                }
+                acc
+            });
+        Self {
+            entities: Arc::new(entities),
+            ..self
+        }
+    }
+
+    #[must_use]
+    pub(crate) fn with_circle_radii(self, updates: &HashMap<SketchEntityId, Length>) -> Self {
+        if updates.is_empty() {
+            return self;
+        }
+        let entities = updates
+            .iter()
+            .fold(unwrap_arc(self.entities), |mut acc, (id, r)| {
+                if let Some(SketchEntity::Circle(existing)) = acc.get(*id).copied()
+                    && let Some(slot) = acc.get_mut(*id)
+                {
+                    *slot = SketchEntity::Circle(CircleData::new(
+                        existing.center(),
+                        *r,
+                        existing.for_construction(),
+                    ));
+                }
+                acc
+            });
+        Self {
+            entities: Arc::new(entities),
+            ..self
+        }
+    }
+
+    #[must_use]
+    pub(crate) fn with_driven_dimension_values(
+        self,
+        updates: &[(SketchDimensionId, DimensionValue)],
+    ) -> Self {
+        if updates.is_empty() {
+            return self;
+        }
+        let dimensions =
+            updates
+                .iter()
+                .copied()
+                .fold(unwrap_arc(self.dimensions), |mut acc, (id, v)| {
+                    let existing = acc.get(id).copied();
+                    if let Some(dim) = existing
+                        && dim.kind() == DimensionKind::Driven
+                        && let Ok(next) = dim.with_value(v)
+                        && let Some(slot) = acc.get_mut(id)
+                    {
+                        *slot = next;
+                    }
+                    acc
+                });
+        Self {
+            dimensions: Arc::new(dimensions),
+            ..self
+        }
+    }
+}
+
+impl PartialEq for Sketch {
+    fn eq(&self, other: &Self) -> bool {
+        self.plane == other.plane
+            && *self.entity_order == *other.entity_order
+            && Self::entries_equal(&self.entity_order, &self.entities, &other.entities)
+            && *self.relation_order == *other.relation_order
+            && Self::entries_equal(&self.relation_order, &self.relations, &other.relations)
+            && *self.dimension_order == *other.dimension_order
+            && Self::entries_equal(&self.dimension_order, &self.dimensions, &other.dimensions)
+            && *self.parameter_order == *other.parameter_order
+            && Self::entries_equal(&self.parameter_order, &self.parameters, &other.parameters)
+    }
+}
+
+fn ensure_order_matches<K, V>(
+    container: &'static str,
+    order: &[K],
+    map: &SlotMap<K, V>,
+) -> Result<(), SketchEditError>
+where
+    K: slotmap::Key + Ord,
+{
+    let order_set: BTreeSet<K> = order.iter().copied().collect();
+    let has_duplicates = order_set.len() != order.len();
+    let same_size = order.len() == map.len();
+    let covers_map = order_set.iter().all(|id| map.contains_key(*id));
+    if has_duplicates || !same_size || !covers_map {
+        Err(SketchEditError::OrderMismatch { container })
+    } else {
+        Ok(())
+    }
+}
+
+fn validate_entity_shape(entity: SketchEntity) -> Result<(), SketchEditError> {
+    let ok = match entity {
+        SketchEntity::Point(_) => true,
+        SketchEntity::Line(l) => l.a() != l.b(),
+        SketchEntity::Arc(a) => {
+            a.center() != a.start() && a.start() != a.end() && a.center() != a.end()
+        }
+        SketchEntity::Circle(c) => c.radius() > Length::default(),
+    };
+    if ok {
+        Ok(())
+    } else {
+        Err(SketchEditError::DegenerateEntity(entity))
+    }
+}
+
+fn insert<K, V>(map: Arc<SlotMap<K, V>>, value: V) -> (K, Arc<SlotMap<K, V>>)
+where
+    K: slotmap::Key,
+    V: Clone,
+{
+    let mut owned = unwrap_arc(map);
+    let id = owned.insert(value);
+    (id, Arc::new(owned))
+}
+
+fn push<T: Clone>(list: Arc<Vec<T>>, value: T) -> Arc<Vec<T>> {
+    let mut owned = unwrap_arc(list);
+    owned.push(value);
+    Arc::new(owned)
+}
+
+fn retain<T, F>(list: &Arc<Vec<T>>, keep: F) -> Arc<Vec<T>>
+where
+    T: Clone,
+    F: Fn(&T) -> bool,
+{
+    Arc::new(list.iter().filter(|t| keep(t)).cloned().collect())
+}
+
+fn remove_one<K, V>(map: Arc<SlotMap<K, V>>, id: K) -> Arc<SlotMap<K, V>>
+where
+    K: slotmap::Key,
+    V: Clone,
+{
+    let mut owned = unwrap_arc(map);
+    owned.remove(id);
+    Arc::new(owned)
+}
+
+fn remove_many<K, V, I>(map: Arc<SlotMap<K, V>>, ids: I) -> Arc<SlotMap<K, V>>
+where
+    K: slotmap::Key,
+    V: Clone,
+    I: IntoIterator<Item = K>,
+{
+    let owned = ids.into_iter().fold(unwrap_arc(map), |mut acc, id| {
+        acc.remove(id);
+        acc
+    });
+    Arc::new(owned)
+}
+
+fn remove_where<K, V, F>(map: Arc<SlotMap<K, V>>, predicate: F) -> (Arc<SlotMap<K, V>>, BTreeSet<K>)
+where
+    K: slotmap::Key + Ord,
+    V: Clone,
+    F: Fn(&V) -> bool,
+{
+    let dropped: BTreeSet<K> = map
+        .iter()
+        .filter_map(|(k, v)| predicate(v).then_some(k))
+        .collect();
+    let next = remove_many(map, dropped.iter().copied());
+    (next, dropped)
+}
+
+fn replace<K, V>(map: Arc<SlotMap<K, V>>, id: K, value: V) -> Arc<SlotMap<K, V>>
+where
+    K: slotmap::Key,
+    V: Clone,
+{
+    let mut owned = unwrap_arc(map);
+    if let Some(slot) = owned.get_mut(id) {
+        *slot = value;
+    }
+    Arc::new(owned)
+}
+
+fn unwrap_arc<T: Clone>(arc: Arc<T>) -> T {
+    Arc::try_unwrap(arc).unwrap_or_else(|shared| (*shared).clone())
+}
+
+fn entity_closure(entities: &EntityMap, seed: SketchEntityId) -> BTreeSet<SketchEntityId> {
+    grow_closure(entities, BTreeSet::from([seed]))
+}
+
+fn grow_closure(
+    entities: &EntityMap,
+    closure: BTreeSet<SketchEntityId>,
+) -> BTreeSet<SketchEntityId> {
+    let grown: BTreeSet<SketchEntityId> = entities
+        .iter()
+        .filter_map(|(id, entity)| {
+            entity
+                .references()
+                .into_iter()
+                .any(|r| closure.contains(&r))
+                .then_some(id)
+        })
+        .chain(closure.iter().copied())
+        .collect();
+    if grown == closure {
+        closure
+    } else {
+        grow_closure(entities, grown)
+    }
+}
+
+#[derive(Debug, Clone, PartialEq, thiserror::Error)]
+pub enum SketchEditError {
+    #[error("entity not found: {0:?}")]
+    EntityNotFound(SketchEntityId),
+    #[error("relation not found: {0:?}")]
+    RelationNotFound(SketchRelationId),
+    #[error("dimension not found: {0:?}")]
+    DimensionNotFound(SketchDimensionId),
+    #[error("parameter not found: {0:?}")]
+    ParameterNotFound(SketchParameterId),
+    #[error("expected point entity: {0:?}")]
+    ExpectedPoint(SketchEntityId),
+    #[error("cannot toggle construction on a point")]
+    PointAlwaysConstruction,
+    #[error("dimension {id:?} value kind does not match variant")]
+    DimensionValueMismatch { id: SketchDimensionId },
+    #[error("cannot update driven dimension: {0:?}")]
+    DimensionIsDriven(SketchDimensionId),
+    #[error("invalid relation operands: {0:?}")]
+    InvalidRelationOperands(SketchRelation),
+    #[error("invalid dimension operands: {0:?}")]
+    InvalidDimensionOperands(SketchDimension),
+    #[error("entity is degenerate: {0:?}")]
+    DegenerateEntity(SketchEntity),
+    #[error("self-referencing relation: {0:?}")]
+    SelfReferencingRelation(SketchRelation),
+    #[error("{container} order list is out of step with its slotmap")]
+    OrderMismatch { container: &'static str },
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use bone_types::{
+        Angle, Length, Parameter, Point2, Point3, Tolerance, UnitVec3, degree, millimeter,
+    };
+
+    fn plane() -> SketchPlaneBasis {
+        let Ok(basis) = SketchPlaneBasis::new(
+            Point3::origin(),
+            UnitVec3::x_axis(),
+            UnitVec3::y_axis(),
+            Tolerance::new(1e-9),
+        ) else {
+            panic!("xy plane basis is orthogonal");
+        };
+        basis
+    }
+
+    fn len_mm(v: f64) -> Length {
+        Length::new::<millimeter>(v)
+    }
+
+    fn angle_deg(v: f64) -> Angle {
+        Angle::new::<degree>(v)
+    }
+
+    fn rectangle_script() -> Vec<SketchEdit> {
+        vec![
+            SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(0.0, 0.0))),
+            SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(10.0, 0.0))),
+            SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(10.0, 5.0))),
+            SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(0.0, 5.0))),
+        ]
+    }
+
+    fn apply_script(sketch: &Sketch, edits: Vec<SketchEdit>) -> (Sketch, Vec<EditOutcome>) {
+        let Ok(pair) = sketch.apply_all(edits) else {
+            panic!("fixture edits apply cleanly");
+        };
+        pair
+    }
+
+    #[test]
+    fn point_is_always_construction() {
+        let p = SketchEntity::point(Point2::from_mm(0.0, 0.0));
+        assert!(p.for_construction());
+    }
+
+    #[test]
+    fn new_sketch_is_empty() {
+        let s = Sketch::new(plane());
+        assert!(s.entities().is_empty());
+        assert!(s.relations().is_empty());
+        assert!(s.dimensions().is_empty());
+        assert!(s.parameters().is_empty());
+        assert!(s.entity_order().is_empty());
+    }
+
+    #[test]
+    fn add_point_records_insertion_order() {
+        let (sketch, outcomes) = apply_script(&Sketch::new(plane()), rectangle_script());
+        let ids: Vec<_> = outcomes
+            .iter()
+            .map(|o| match *o {
+                EditOutcome::Entity(id) => id,
+                _ => panic!("expected entity outcomes"),
+            })
+            .collect();
+        assert_eq!(sketch.entity_order(), ids.as_slice());
+        assert_eq!(sketch.entities().len(), 4);
+    }
+
+    #[test]
+    fn add_line_requires_point_endpoints() {
+        let mut sketch = Sketch::new(plane());
+        let Ok((s1, EditOutcome::Entity(a))) = sketch.apply(SketchEdit::AddEntity(
+            SketchEntity::point(Point2::from_mm(0.0, 0.0)),
+        )) else {
+            panic!("add point");
+        };
+        let Ok((s2, EditOutcome::Entity(b))) = s1.apply(SketchEdit::AddEntity(
+            SketchEntity::point(Point2::from_mm(1.0, 0.0)),
+        )) else {
+            panic!("add point");
+        };
+        let Ok((s3, EditOutcome::Entity(_))) =
+            s2.apply(SketchEdit::AddEntity(SketchEntity::line(a, b, false)))
+        else {
+            panic!("add line");
+        };
+        sketch = s3;
+        let bad = sketch.apply(SketchEdit::AddEntity(SketchEntity::line(
+            a,
+            SketchEntityId::default(),
+            false,
+        )));
+        assert!(matches!(bad, Err(SketchEditError::EntityNotFound(_))));
+    }
+
+    #[test]
+    fn add_line_rejects_non_point_endpoint() {
+        let Ok((s1, EditOutcome::Entity(a))) = Sketch::new(plane()).apply(SketchEdit::AddEntity(
+            SketchEntity::point(Point2::from_mm(0.0, 0.0)),
+        )) else {
+            panic!("add point");
+        };
+        let Ok((s2, EditOutcome::Entity(center))) = s1.apply(SketchEdit::AddEntity(
+            SketchEntity::point(Point2::from_mm(2.0, 0.0)),
+        )) else {
+            panic!("add center");
+        };
+        let Ok((s3, EditOutcome::Entity(circle))) = s2.apply(SketchEdit::AddEntity(
+            SketchEntity::circle(center, len_mm(1.0), false),
+        )) else {
+            panic!("add circle");
+        };
+        let bad = s3.apply(SketchEdit::AddEntity(SketchEntity::line(a, circle, false)));
+        assert!(matches!(bad, Err(SketchEditError::ExpectedPoint(_))));
+    }
+
+    #[test]
+    fn delete_entity_cascades_dependents() {
+        let s = Sketch::new(plane());
+        let Ok((s, EditOutcome::Entity(a))) = s.apply(SketchEdit::AddEntity(SketchEntity::point(
+            Point2::from_mm(0.0, 0.0),
+        ))) else {
+            panic!("a");
+        };
+        let Ok((s, EditOutcome::Entity(b))) = s.apply(SketchEdit::AddEntity(SketchEntity::point(
+            Point2::from_mm(1.0, 0.0),
+        ))) else {
+            panic!("b");
+        };
+        let Ok((s, EditOutcome::Entity(line))) =
+            s.apply(SketchEdit::AddEntity(SketchEntity::line(a, b, false)))
+        else {
+            panic!("line");
+        };
+        let Ok((s, EditOutcome::Relation(_))) =
+            s.apply(SketchEdit::AddRelation(SketchRelation::Horizontal(line)))
+        else {
+            panic!("rel");
+        };
+        let Ok((s, EditOutcome::Dimension(_))) =
+            s.apply(SketchEdit::AddDimension(SketchDimension::Linear {
+                a,
+                b,
+                value: len_mm(10.0),
+                kind: DimensionKind::Driving,
+            }))
+        else {
+            panic!("dim");
+        };
+        let Ok((after, EditOutcome::None)) = s.apply(SketchEdit::DeleteEntity(a)) else {
+            panic!("delete");
+        };
+        assert_eq!(after.entities().len(), 1);
+        assert!(after.entities().contains_key(b));
+        assert!(!after.entities().contains_key(line));
+        assert!(after.relations().is_empty());
+        assert!(after.dimensions().is_empty());
+        assert_eq!(after.entity_order(), [b]);
+        assert!(after.relation_order().is_empty());
+        assert!(after.dimension_order().is_empty());
+    }
+
+    #[test]
+    fn delete_entity_errors_on_unknown() {
+        let s = Sketch::new(plane());
+        let bad = s.apply(SketchEdit::DeleteEntity(SketchEntityId::default()));
+        assert!(matches!(bad, Err(SketchEditError::EntityNotFound(_))));
+    }
+
+    #[test]
+    fn set_construction_refuses_point() {
+        let Ok((s, EditOutcome::Entity(p))) = Sketch::new(plane()).apply(SketchEdit::AddEntity(
+            SketchEntity::point(Point2::from_mm(0.0, 0.0)),
+        )) else {
+            panic!("p");
+        };
+        let bad = s.apply(SketchEdit::SetConstruction {
+            id: p,
+            for_construction: false,
+        });
+        assert!(matches!(bad, Err(SketchEditError::PointAlwaysConstruction)));
+    }
+
+    #[test]
+    fn set_construction_toggles_line() {
+        let s = Sketch::new(plane());
+        let Ok((s, EditOutcome::Entity(a))) = s.apply(SketchEdit::AddEntity(SketchEntity::point(
+            Point2::from_mm(0.0, 0.0),
+        ))) else {
+            panic!("a");
+        };
+        let Ok((s, EditOutcome::Entity(b))) = s.apply(SketchEdit::AddEntity(SketchEntity::point(
+            Point2::from_mm(1.0, 0.0),
+        ))) else {
+            panic!("b");
+        };
+        let Ok((s, EditOutcome::Entity(line))) =
+            s.apply(SketchEdit::AddEntity(SketchEntity::line(a, b, false)))
+        else {
+            panic!("line");
+        };
+        assert!(!s.entities()[line].for_construction());
+        let Ok((s, _)) = s.apply(SketchEdit::SetConstruction {
+            id: line,
+            for_construction: true,
+        }) else {
+            panic!("toggle");
+        };
+        assert!(s.entities()[line].for_construction());
+    }
+
+    #[test]
+    fn update_dimension_value_rejects_unit_mismatch() {
+        let Ok((s, EditOutcome::Entity(a))) = Sketch::new(plane()).apply(SketchEdit::AddEntity(
+            SketchEntity::point(Point2::from_mm(0.0, 0.0)),
+        )) else {
+            panic!("a");
+        };
+        let Ok((s, EditOutcome::Entity(b))) = s.apply(SketchEdit::AddEntity(SketchEntity::point(
+            Point2::from_mm(1.0, 0.0),
+        ))) else {
+            panic!("b");
+        };
+        let Ok((s, EditOutcome::Dimension(id))) =
+            s.apply(SketchEdit::AddDimension(SketchDimension::Linear {
+                a,
+                b,
+                value: len_mm(1.0),
+                kind: DimensionKind::Driving,
+            }))
+        else {
+            panic!("dim");
+        };
+        let bad = s.apply(SketchEdit::UpdateDimensionValue {
+            id,
+            value: DimensionValue::Angle(angle_deg(90.0)),
+        });
+        assert!(matches!(
+            bad,
+            Err(SketchEditError::DimensionValueMismatch { .. })
+        ));
+    }
+
+    #[test]
+    fn update_dimension_value_replaces_length() {
+        let Ok((s, EditOutcome::Entity(a))) = Sketch::new(plane()).apply(SketchEdit::AddEntity(
+            SketchEntity::point(Point2::from_mm(0.0, 0.0)),
+        )) else {
+            panic!("a");
+        };
+        let Ok((s, EditOutcome::Entity(b))) = s.apply(SketchEdit::AddEntity(SketchEntity::point(
+            Point2::from_mm(1.0, 0.0),
+        ))) else {
+            panic!("b");
+        };
+        let Ok((s, EditOutcome::Dimension(id))) =
+            s.apply(SketchEdit::AddDimension(SketchDimension::Linear {
+                a,
+                b,
+                value: len_mm(1.0),
+                kind: DimensionKind::Driving,
+            }))
+        else {
+            panic!("dim");
+        };
+        let Ok((s, _)) = s.apply(SketchEdit::UpdateDimensionValue {
+            id,
+            value: DimensionValue::Length(len_mm(12.5)),
+        }) else {
+            panic!("update");
+        };
+        let SketchDimension::Linear { value, .. } = s.dimensions()[id] else {
+            panic!("variant unchanged");
+        };
+        assert!((value.get::<millimeter>() - 12.5).abs() < 1e-12);
+    }
+
+    #[test]
+    fn identical_scripts_produce_equal_sketches() {
+        let script = rich_script();
+        let a = Sketch::new(plane())
+            .apply_all(script.clone())
+            .map(|(s, _)| s);
+        let b = Sketch::new(plane()).apply_all(script).map(|(s, _)| s);
+        let (Ok(a), Ok(b)) = (a, b) else {
+            panic!("scripts apply cleanly");
+        };
+        assert_eq!(a, b);
+    }
+
+    #[test]
+    fn relation_edit_shares_entity_storage() {
+        let (base, _) = apply_script(&Sketch::new(plane()), rectangle_script());
+        let before = base.clone();
+        let Ok((base_with_rel, _)) = base.apply(SketchEdit::AddRelation(SketchRelation::Fix(
+            before.entity_order()[0],
+        ))) else {
+            panic!("relation");
+        };
+        assert!(Arc::ptr_eq(&before.entities, &base_with_rel.entities));
+        assert!(Arc::ptr_eq(
+            &before.entity_order,
+            &base_with_rel.entity_order
+        ));
+        assert!(Arc::ptr_eq(&before.dimensions, &base_with_rel.dimensions));
+        assert!(!Arc::ptr_eq(&before.relations, &base_with_rel.relations));
+    }
+
+    #[test]
+    fn add_parameter_roundtrips() {
+        let s = Sketch::new(plane());
+        let Ok((s, EditOutcome::Parameter(id))) = s.apply(SketchEdit::AddParameter(
+            SketchParameter::new(Parameter::new(0.5)),
+        )) else {
+            panic!("param");
+        };
+        assert!((s.parameters()[id].value().value() - 0.5).abs() < f64::EPSILON);
+        let Ok((s, _)) = s.apply(SketchEdit::DeleteParameter(id)) else {
+            panic!("delete param");
+        };
+        assert!(s.parameters().is_empty());
+        assert!(s.parameter_order().is_empty());
+    }
+
+    fn rich_script() -> Vec<SketchEdit> {
+        let mut edits = rectangle_script();
+        edits.extend([
+            SketchEdit::AddParameter(SketchParameter::new(Parameter::new(0.0))),
+            SketchEdit::AddParameter(SketchParameter::new(Parameter::new(1.0))),
+        ]);
+        edits
+    }
+
+    fn two_points() -> (Sketch, SketchEntityId, SketchEntityId) {
+        let Ok((s, EditOutcome::Entity(a))) = Sketch::new(plane()).apply(SketchEdit::AddEntity(
+            SketchEntity::point(Point2::from_mm(0.0, 0.0)),
+        )) else {
+            panic!("a");
+        };
+        let Ok((s, EditOutcome::Entity(b))) = s.apply(SketchEdit::AddEntity(SketchEntity::point(
+            Point2::from_mm(1.0, 0.0),
+        ))) else {
+            panic!("b");
+        };
+        (s, a, b)
+    }
+
+    #[test]
+    fn horizontal_on_point_is_rejected() {
+        let (s, a, _) = two_points();
+        let bad = s.apply(SketchEdit::AddRelation(SketchRelation::Horizontal(a)));
+        assert!(matches!(
+            bad,
+            Err(SketchEditError::InvalidRelationOperands(_))
+        ));
+    }
+
+    #[test]
+    fn tangent_between_two_lines_is_rejected() {
+        let (s, a, b) = two_points();
+        let Ok((s, EditOutcome::Entity(c))) = s.apply(SketchEdit::AddEntity(SketchEntity::point(
+            Point2::from_mm(0.0, 1.0),
+        ))) else {
+            panic!("c");
+        };
+        let Ok((s, EditOutcome::Entity(l1))) =
+            s.apply(SketchEdit::AddEntity(SketchEntity::line(a, b, false)))
+        else {
+            panic!("l1");
+        };
+        let Ok((s, EditOutcome::Entity(l2))) =
+            s.apply(SketchEdit::AddEntity(SketchEntity::line(a, c, false)))
+        else {
+            panic!("l2");
+        };
+        let bad = s.apply(SketchEdit::AddRelation(SketchRelation::Tangent(l1, l2)));
+        assert!(matches!(
+            bad,
+            Err(SketchEditError::InvalidRelationOperands(_))
+        ));
+    }
+
+    #[test]
+    fn equal_cross_kind_is_rejected() {
+        let (s, a, b) = two_points();
+        let Ok((s, EditOutcome::Entity(line))) =
+            s.apply(SketchEdit::AddEntity(SketchEntity::line(a, b, false)))
+        else {
+            panic!("line");
+        };
+        let Ok((s, EditOutcome::Entity(circle))) = s.apply(SketchEdit::AddEntity(
+            SketchEntity::circle(a, len_mm(1.0), false),
+        )) else {
+            panic!("circle");
+        };
+        let bad = s.apply(SketchEdit::AddRelation(SketchRelation::Equal(line, circle)));
+        assert!(matches!(
+            bad,
+            Err(SketchEditError::InvalidRelationOperands(_))
+        ));
+    }
+
+    #[test]
+    fn coincident_point_on_line_is_accepted() {
+        let (s, a, b) = two_points();
+        let Ok((s, EditOutcome::Entity(line))) =
+            s.apply(SketchEdit::AddEntity(SketchEntity::line(a, b, false)))
+        else {
+            panic!("line");
+        };
+        let Ok((s2, _)) = s.apply(SketchEdit::AddRelation(SketchRelation::Coincident(a, line)))
+        else {
+            panic!("coincident");
+        };
+        assert_eq!(s2.relations().len(), 1);
+    }
+
+    #[test]
+    fn linear_dimension_on_non_point_is_rejected() {
+        let (s, a, b) = two_points();
+        let Ok((s, EditOutcome::Entity(line))) =
+            s.apply(SketchEdit::AddEntity(SketchEntity::line(a, b, false)))
+        else {
+            panic!("line");
+        };
+        let bad = s.apply(SketchEdit::AddDimension(SketchDimension::Linear {
+            a: line,
+            b: a,
+            value: len_mm(1.0),
+            kind: DimensionKind::Driving,
+        }));
+        assert!(matches!(
+            bad,
+            Err(SketchEditError::InvalidDimensionOperands(_))
+        ));
+    }
+
+    #[test]
+    fn driven_dimension_cannot_be_updated() {
+        let (s, a, b) = two_points();
+        let Ok((s, EditOutcome::Dimension(id))) =
+            s.apply(SketchEdit::AddDimension(SketchDimension::Linear {
+                a,
+                b,
+                value: len_mm(1.0),
+                kind: DimensionKind::Driven,
+            }))
+        else {
+            panic!("dim");
+        };
+        let bad = s.apply(SketchEdit::UpdateDimensionValue {
+            id,
+            value: DimensionValue::Length(len_mm(2.0)),
+        });
+        assert!(matches!(bad, Err(SketchEditError::DimensionIsDriven(_))));
+    }
+
+    #[test]
+    fn apply_all_leaves_caller_unchanged_on_error() {
+        let (base, _) = apply_script(&Sketch::new(plane()), rectangle_script());
+        let snapshot = base.clone();
+        let bad_edit = SketchEdit::AddRelation(SketchRelation::Horizontal(base.entity_order()[0]));
+        let result = base.apply_all([bad_edit]);
+        assert!(matches!(
+            result,
+            Err(SketchEditError::InvalidRelationOperands(_))
+        ));
+        assert_eq!(base, snapshot);
+    }
+
+    #[test]
+    fn line_with_same_endpoints_is_rejected() {
+        let Ok((s, EditOutcome::Entity(a))) = Sketch::new(plane()).apply(SketchEdit::AddEntity(
+            SketchEntity::point(Point2::from_mm(0.0, 0.0)),
+        )) else {
+            panic!("a");
+        };
+        let bad = s.apply(SketchEdit::AddEntity(SketchEntity::line(a, a, false)));
+        assert!(matches!(bad, Err(SketchEditError::DegenerateEntity(_))));
+    }
+
+    #[test]
+    fn arc_with_repeated_endpoints_is_rejected() {
+        let Ok((s, EditOutcome::Entity(a))) = Sketch::new(plane()).apply(SketchEdit::AddEntity(
+            SketchEntity::point(Point2::from_mm(0.0, 0.0)),
+        )) else {
+            panic!("a");
+        };
+        let Ok((s, EditOutcome::Entity(b))) = s.apply(SketchEdit::AddEntity(SketchEntity::point(
+            Point2::from_mm(1.0, 0.0),
+        ))) else {
+            panic!("b");
+        };
+        let bad = s.apply(SketchEdit::AddEntity(SketchEntity::arc(a, b, a, false)));
+        assert!(matches!(bad, Err(SketchEditError::DegenerateEntity(_))));
+    }
+
+    #[test]
+    fn self_referencing_relation_is_rejected() {
+        let (s, a, b) = two_points();
+        let Ok((s, EditOutcome::Entity(line))) =
+            s.apply(SketchEdit::AddEntity(SketchEntity::line(a, b, false)))
+        else {
+            panic!("line");
+        };
+        let bad = s.apply(SketchEdit::AddRelation(SketchRelation::Parallel(
+            line, line,
+        )));
+        assert!(matches!(
+            bad,
+            Err(SketchEditError::SelfReferencingRelation(_))
+        ));
+    }
+
+    #[test]
+    fn coincident_self_reference_is_rejected() {
+        let (s, a, _) = two_points();
+        let bad = s.apply(SketchEdit::AddRelation(SketchRelation::Coincident(a, a)));
+        assert!(matches!(
+            bad,
+            Err(SketchEditError::SelfReferencingRelation(_))
+        ));
+    }
+
+    #[test]
+    fn set_construction_noop_shares_storage() {
+        let (s, a, b) = two_points();
+        let Ok((s, EditOutcome::Entity(line))) =
+            s.apply(SketchEdit::AddEntity(SketchEntity::line(a, b, true)))
+        else {
+            panic!("line");
+        };
+        let before = s.clone();
+        let Ok((after, _)) = s.apply(SketchEdit::SetConstruction {
+            id: line,
+            for_construction: true,
+        }) else {
+            panic!("set");
+        };
+        assert!(Arc::ptr_eq(&before.entities, &after.entities));
+    }
+
+    #[test]
+    fn circle_with_non_positive_radius_is_rejected() {
+        let Ok((s, EditOutcome::Entity(center))) = Sketch::new(plane()).apply(
+            SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(0.0, 0.0))),
+        ) else {
+            panic!("center");
+        };
+        let zero = s.clone().apply(SketchEdit::AddEntity(SketchEntity::circle(
+            center,
+            len_mm(0.0),
+            false,
+        )));
+        assert!(matches!(zero, Err(SketchEditError::DegenerateEntity(_))));
+        let negative = s.apply(SketchEdit::AddEntity(SketchEntity::circle(
+            center,
+            len_mm(-1.0),
+            false,
+        )));
+        assert!(matches!(
+            negative,
+            Err(SketchEditError::DegenerateEntity(_))
+        ));
+    }
+
+    #[test]
+    fn delete_relation_errors_on_unknown() {
+        let s = Sketch::new(plane());
+        let bad = s.apply(SketchEdit::DeleteRelation(SketchRelationId::default()));
+        assert!(matches!(bad, Err(SketchEditError::RelationNotFound(_))));
+    }
+
+    #[test]
+    fn delete_dimension_errors_on_unknown() {
+        let s = Sketch::new(plane());
+        let bad = s.apply(SketchEdit::DeleteDimension(SketchDimensionId::default()));
+        assert!(matches!(bad, Err(SketchEditError::DimensionNotFound(_))));
+    }
+
+    #[test]
+    fn delete_parameter_errors_on_unknown() {
+        let s = Sketch::new(plane());
+        let bad = s.apply(SketchEdit::DeleteParameter(SketchParameterId::default()));
+        assert!(matches!(bad, Err(SketchEditError::ParameterNotFound(_))));
+    }
+
+    #[test]
+    fn update_dimension_errors_on_unknown() {
+        let s = Sketch::new(plane());
+        let bad = s.apply(SketchEdit::UpdateDimensionValue {
+            id: SketchDimensionId::default(),
+            value: DimensionValue::Length(len_mm(1.0)),
+        });
+        assert!(matches!(bad, Err(SketchEditError::DimensionNotFound(_))));
+    }
+
+    #[test]
+    fn driven_radius_cannot_be_updated() {
+        let Ok((s, EditOutcome::Entity(center))) = Sketch::new(plane()).apply(
+            SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(0.0, 0.0))),
+        ) else {
+            panic!("center");
+        };
+        let Ok((s, EditOutcome::Entity(circle))) = s.apply(SketchEdit::AddEntity(
+            SketchEntity::circle(center, len_mm(1.0), false),
+        )) else {
+            panic!("circle");
+        };
+        let Ok((s, EditOutcome::Dimension(id))) =
+            s.apply(SketchEdit::AddDimension(SketchDimension::Radius {
+                target: circle,
+                value: len_mm(1.0),
+                kind: DimensionKind::Driven,
+            }))
+        else {
+            panic!("dim");
+        };
+        let bad = s.apply(SketchEdit::UpdateDimensionValue {
+            id,
+            value: DimensionValue::Length(len_mm(2.0)),
+        });
+        assert!(matches!(bad, Err(SketchEditError::DimensionIsDriven(_))));
+    }
+
+    #[test]
+    fn driven_diameter_cannot_be_updated() {
+        let Ok((s, EditOutcome::Entity(center))) = Sketch::new(plane()).apply(
+            SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(0.0, 0.0))),
+        ) else {
+            panic!("center");
+        };
+        let Ok((s, EditOutcome::Entity(circle))) = s.apply(SketchEdit::AddEntity(
+            SketchEntity::circle(center, len_mm(1.0), false),
+        )) else {
+            panic!("circle");
+        };
+        let Ok((s, EditOutcome::Dimension(id))) =
+            s.apply(SketchEdit::AddDimension(SketchDimension::Diameter {
+                target: circle,
+                value: len_mm(2.0),
+                kind: DimensionKind::Driven,
+            }))
+        else {
+            panic!("dim");
+        };
+        let bad = s.apply(SketchEdit::UpdateDimensionValue {
+            id,
+            value: DimensionValue::Length(len_mm(4.0)),
+        });
+        assert!(matches!(bad, Err(SketchEditError::DimensionIsDriven(_))));
+    }
+
+    #[test]
+    fn driven_angular_cannot_be_updated() {
+        let (s, a, b) = two_points();
+        let Ok((s, EditOutcome::Entity(c))) = s.apply(SketchEdit::AddEntity(SketchEntity::point(
+            Point2::from_mm(0.0, 1.0),
+        ))) else {
+            panic!("c");
+        };
+        let Ok((s, EditOutcome::Entity(l1))) =
+            s.apply(SketchEdit::AddEntity(SketchEntity::line(a, b, false)))
+        else {
+            panic!("l1");
+        };
+        let Ok((s, EditOutcome::Entity(l2))) =
+            s.apply(SketchEdit::AddEntity(SketchEntity::line(a, c, false)))
+        else {
+            panic!("l2");
+        };
+        let Ok((s, EditOutcome::Dimension(id))) =
+            s.apply(SketchEdit::AddDimension(SketchDimension::Angular {
+                a: l1,
+                b: l2,
+                value: angle_deg(90.0),
+                kind: DimensionKind::Driven,
+            }))
+        else {
+            panic!("dim");
+        };
+        let bad = s.apply(SketchEdit::UpdateDimensionValue {
+            id,
+            value: DimensionValue::Angle(angle_deg(45.0)),
+        });
+        assert!(matches!(bad, Err(SketchEditError::DimensionIsDriven(_))));
+    }
+
+    mod properties {
+        use super::*;
+        use proptest::prelude::*;
+
+        #[derive(Copy, Clone, Debug)]
+        enum Step {
+            Point(i16, i16),
+            Parameter(i16),
+            Line {
+                ai: u8,
+                bi: u8,
+            },
+            Circle {
+                ci: u8,
+                r: u16,
+            },
+            Horizontal {
+                li: u8,
+            },
+            LinearDim {
+                ai: u8,
+                bi: u8,
+                v: u16,
+                driven: bool,
+            },
+            UpdateDim {
+                di: u8,
+                v: u16,
+            },
+            Toggle {
+                ei: u8,
+            },
+            DelEntity {
+                ei: u8,
+            },
+            DelParam {
+                pi: u8,
+            },
+        }
+
+        fn arb_step() -> impl Strategy<Value = Step> {
+            prop_oneof![
+                (any::<i16>(), any::<i16>()).prop_map(|(x, y)| Step::Point(x, y)),
+                any::<i16>().prop_map(Step::Parameter),
+                (any::<u8>(), any::<u8>()).prop_map(|(ai, bi)| Step::Line { ai, bi }),
+                (any::<u8>(), any::<u16>()).prop_map(|(ci, r)| Step::Circle { ci, r: r % 100 + 1 }),
+                any::<u8>().prop_map(|li| Step::Horizontal { li }),
+                (any::<u8>(), any::<u8>(), any::<u16>(), any::<bool>())
+                    .prop_map(|(ai, bi, v, driven)| Step::LinearDim { ai, bi, v, driven }),
+                (any::<u8>(), any::<u16>()).prop_map(|(di, v)| Step::UpdateDim { di, v }),
+                any::<u8>().prop_map(|ei| Step::Toggle { ei }),
+                any::<u8>().prop_map(|ei| Step::DelEntity { ei }),
+                any::<u8>().prop_map(|pi| Step::DelParam { pi }),
+            ]
+        }
+
+        fn entities_of_kind(s: &Sketch, kind: SketchEntityKind) -> Vec<SketchEntityId> {
+            s.entity_order()
+                .iter()
+                .copied()
+                .filter(|id| s.entities()[*id].kind() == kind)
+                .collect()
+        }
+
+        fn pick<T: Copy>(xs: &[T], i: u8) -> Option<T> {
+            if xs.is_empty() {
+                None
+            } else {
+                Some(xs[usize::from(i) % xs.len()])
+            }
+        }
+
+        fn pick_two_distinct<T: Copy + Eq>(xs: &[T], ai: u8, bi: u8) -> Option<(T, T)> {
+            if xs.len() < 2 {
+                return None;
+            }
+            let ai = usize::from(ai) % xs.len();
+            let offset = usize::from(bi) % (xs.len() - 1) + 1;
+            let bi = (ai + offset) % xs.len();
+            Some((xs[ai], xs[bi]))
+        }
+
+        fn resolve_step(s: &Sketch, step: Step) -> Option<SketchEdit> {
+            match step {
+                Step::Point(x, y) => Some(SketchEdit::AddEntity(SketchEntity::point(
+                    Point2::from_mm(f64::from(x), f64::from(y)),
+                ))),
+                Step::Parameter(v) => Some(SketchEdit::AddParameter(SketchParameter::new(
+                    Parameter::new(f64::from(v)),
+                ))),
+                Step::Line { ai, bi } => {
+                    let (a, b) =
+                        pick_two_distinct(&entities_of_kind(s, SketchEntityKind::Point), ai, bi)?;
+                    Some(SketchEdit::AddEntity(SketchEntity::line(a, b, false)))
+                }
+                Step::Circle { ci, r } => {
+                    let c = pick(&entities_of_kind(s, SketchEntityKind::Point), ci)?;
+                    Some(SketchEdit::AddEntity(SketchEntity::circle(
+                        c,
+                        Length::new::<millimeter>(f64::from(r)),
+                        false,
+                    )))
+                }
+                Step::Horizontal { li } => {
+                    let l = pick(&entities_of_kind(s, SketchEntityKind::Line), li)?;
+                    Some(SketchEdit::AddRelation(SketchRelation::Horizontal(l)))
+                }
+                Step::LinearDim { ai, bi, v, driven } => {
+                    let (a, b) =
+                        pick_two_distinct(&entities_of_kind(s, SketchEntityKind::Point), ai, bi)?;
+                    let kind = if driven {
+                        DimensionKind::Driven
+                    } else {
+                        DimensionKind::Driving
+                    };
+                    Some(SketchEdit::AddDimension(SketchDimension::Linear {
+                        a,
+                        b,
+                        value: Length::new::<millimeter>(f64::from(v)),
+                        kind,
+                    }))
+                }
+                Step::UpdateDim { di, v } => {
+                    let id = pick(s.dimension_order(), di)?;
+                    Some(SketchEdit::UpdateDimensionValue {
+                        id,
+                        value: DimensionValue::Length(Length::new::<millimeter>(f64::from(v))),
+                    })
+                }
+                Step::Toggle { ei } => {
+                    let id = pick(s.entity_order(), ei)?;
+                    Some(SketchEdit::SetConstruction {
+                        id,
+                        for_construction: true,
+                    })
+                }
+                Step::DelEntity { ei } => {
+                    let id = pick(s.entity_order(), ei)?;
+                    Some(SketchEdit::DeleteEntity(id))
+                }
+                Step::DelParam { pi } => {
+                    let id = pick(s.parameter_order(), pi)?;
+                    Some(SketchEdit::DeleteParameter(id))
+                }
+            }
+        }
+
+        fn build_script(steps: Vec<Step>) -> Vec<SketchEdit> {
+            steps
+                .into_iter()
+                .fold(
+                    (Sketch::new(plane()), Vec::new()),
+                    |(sk, mut acc), step| match resolve_step(&sk, step) {
+                        Some(edit) => match sk.clone().apply(edit) {
+                            Ok((next, _)) => {
+                                acc.push(edit);
+                                (next, acc)
+                            }
+                            Err(_) => (sk, acc),
+                        },
+                        None => (sk, acc),
+                    },
+                )
+                .1
+        }
+
+        proptest! {
+            #[test]
+            fn apply_all_is_deterministic(steps in prop::collection::vec(arb_step(), 0..40)) {
+                let script = build_script(steps);
+                let Ok((a, _)) = Sketch::new(plane()).apply_all(script.clone()) else {
+                    unreachable!("build_script only emits successfully-applied edits");
+                };
+                let Ok((b, _)) = Sketch::new(plane()).apply_all(script) else {
+                    unreachable!();
+                };
+                prop_assert_eq!(a, b);
+            }
+
+            #[test]
+            fn apply_all_matches_apply_fold(steps in prop::collection::vec(arb_step(), 0..40)) {
+                let script = build_script(steps);
+                let Ok((via_all, _)) = Sketch::new(plane()).apply_all(script.clone()) else {
+                    unreachable!();
+                };
+                let via_fold = script.into_iter().try_fold(Sketch::new(plane()), |acc, e| {
+                    acc.apply(e).map(|(s, _)| s)
+                });
+                let Ok(via_fold) = via_fold else { unreachable!(); };
+                prop_assert_eq!(via_all, via_fold);
+            }
+        }
+    }
+}
diff --git a/crates/bone-document/src/sketch/parameter.rs b/crates/bone-document/src/sketch/parameter.rs
new file mode 100644
index 0000000..1f7c80b
--- /dev/null
+++ b/crates/bone-document/src/sketch/parameter.rs
@@ -0,0 +1,25 @@
+use bone_types::Parameter;
+use serde::{Deserialize, Serialize};
+
+#[derive(Copy, Clone, Debug, PartialEq, Serialize, Deserialize)]
+#[serde(deny_unknown_fields)]
+pub struct SketchParameter {
+    value: Parameter,
+}
+
+impl SketchParameter {
+    #[must_use]
+    pub const fn new(value: Parameter) -> Self {
+        Self { value }
+    }
+
+    #[must_use]
+    pub const fn value(self) -> Parameter {
+        self.value
+    }
+
+    #[must_use]
+    pub const fn with_value(self, value: Parameter) -> Self {
+        Self { value }
+    }
+}
diff --git a/crates/bone-document/src/sketch/relation.rs b/crates/bone-document/src/sketch/relation.rs
new file mode 100644
index 0000000..389f1f6
--- /dev/null
+++ b/crates/bone-document/src/sketch/relation.rs
@@ -0,0 +1,55 @@
+use bone_types::SketchEntityId;
+use serde::{Deserialize, Serialize};
+
+#[derive(Copy, Clone, Debug, PartialEq, Serialize, Deserialize)]
+pub enum SketchRelation {
+    Coincident(SketchEntityId, SketchEntityId),
+    Horizontal(SketchEntityId),
+    Vertical(SketchEntityId),
+    Parallel(SketchEntityId, SketchEntityId),
+    Perpendicular(SketchEntityId, SketchEntityId),
+    Tangent(SketchEntityId, SketchEntityId),
+    Equal(SketchEntityId, SketchEntityId),
+    Concentric(SketchEntityId, SketchEntityId),
+    Fix(SketchEntityId),
+}
+
+impl SketchRelation {
+    #[must_use]
+    pub const fn references(&self) -> RelationRefs {
+        match *self {
+            Self::Coincident(a, b)
+            | Self::Parallel(a, b)
+            | Self::Perpendicular(a, b)
+            | Self::Tangent(a, b)
+            | Self::Equal(a, b)
+            | Self::Concentric(a, b) => RelationRefs([Some(a), Some(b)]),
+            Self::Horizontal(a) | Self::Vertical(a) | Self::Fix(a) => RelationRefs([Some(a), None]),
+        }
+    }
+
+    #[must_use]
+    pub const fn pair(&self) -> Option<(SketchEntityId, SketchEntityId)> {
+        match *self {
+            Self::Coincident(a, b)
+            | Self::Parallel(a, b)
+            | Self::Perpendicular(a, b)
+            | Self::Tangent(a, b)
+            | Self::Equal(a, b)
+            | Self::Concentric(a, b) => Some((a, b)),
+            Self::Horizontal(_) | Self::Vertical(_) | Self::Fix(_) => None,
+        }
+    }
+}
+
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub struct RelationRefs([Option<SketchEntityId>; 2]);
+
+impl IntoIterator for RelationRefs {
+    type Item = SketchEntityId;
+    type IntoIter = core::iter::Flatten<core::array::IntoIter<Option<SketchEntityId>, 2>>;
+
+    fn into_iter(self) -> Self::IntoIter {
+        self.0.into_iter().flatten()
+    }
+}
diff --git a/crates/bone-document/src/sketch/snapshots/bone_document__sketch__solve__tests__rectangle_sparsity.snap b/crates/bone-document/src/sketch/snapshots/bone_document__sketch__solve__tests__rectangle_sparsity.snap
new file mode 100644
index 0000000..771fa04
--- /dev/null
+++ b/crates/bone-document/src/sketch/snapshots/bone_document__sketch__solve__tests__rectangle_sparsity.snap
@@ -0,0 +1,13 @@
+---
+source: crates/bone-document/src/sketch/solve.rs
+assertion_line: 692
+expression: pattern.display()
+---
+.X.X....
+.....X.X
+..X.X...
+X.....X.
+X.......
+.X......
+XXXX....
+..XXXX..
diff --git a/crates/bone-document/src/sketch/solve.rs b/crates/bone-document/src/sketch/solve.rs
new file mode 100644
index 0000000..5fa794b
--- /dev/null
+++ b/crates/bone-document/src/sketch/solve.rs
@@ -0,0 +1,1913 @@
+use std::collections::{BTreeMap, HashMap};
+use std::time::Instant;
+
+use bone_solver::{
+    ConstraintSystem, CurveRadius, DofConfig, DofReport, LineHandle, NewtonConfig, PointHandle,
+    Residual, SolverError, analyze_dof, analyze_dof_at, decompose, dedup_preserving_order,
+    minimal_unsatisfiable_subset, solve_newton_decomposed,
+};
+use bone_types::{
+    Angle, BudgetCeiling, DegreesOfFreedom, Length, Parameter, ParameterIndex, Point2,
+    ResidualIndex, SketchDimensionId, SketchEntityId, SketchItemId, SketchRelationId, millimeter,
+    radian,
+};
+
+use super::{
+    DimensionKind, DimensionValue, Sketch, SketchDimension, SketchEntity, SketchEntityKind,
+    SketchRelation,
+};
+
+#[derive(Debug, Clone, PartialEq, thiserror::Error)]
+pub enum DragError {
+    #[error("dragged entity {0:?} is not a Point")]
+    NotAPoint(SketchEntityId),
+    #[error("dragged entity {0:?} not found in sketch")]
+    NotFound(SketchEntityId),
+    #[error(transparent)]
+    Solver(#[from] SolverError),
+}
+
+#[derive(Clone, Debug)]
+pub struct Mapping {
+    points: BTreeMap<SketchEntityId, PointHandle>,
+    radii: BTreeMap<SketchEntityId, ParameterIndex>,
+    residual_origin: Vec<Origin>,
+}
+
+#[derive(Copy, Clone, Debug, PartialEq)]
+enum Origin {
+    Relation(SketchRelationId),
+    Dimension(SketchDimensionId),
+    Intrinsic(SketchEntityId),
+}
+
+impl Mapping {
+    #[must_use]
+    pub fn point(&self, id: SketchEntityId) -> Option<PointHandle> {
+        self.points.get(&id).copied()
+    }
+
+    #[must_use]
+    pub fn radius(&self, id: SketchEntityId) -> Option<ParameterIndex> {
+        self.radii.get(&id).copied()
+    }
+
+    #[must_use]
+    pub fn item_of_residual(&self, idx: ResidualIndex) -> Option<SketchItemId> {
+        self.residual_origin.get(idx.as_usize()).map(|o| match *o {
+            Origin::Relation(id) => SketchItemId::Relation(id),
+            Origin::Dimension(id) => SketchItemId::Dimension(id),
+            Origin::Intrinsic(id) => SketchItemId::Entity(id),
+        })
+    }
+
+    #[must_use]
+    pub fn items(&self, indices: impl IntoIterator<Item = ResidualIndex>) -> Vec<SketchItemId> {
+        indices
+            .into_iter()
+            .filter_map(|i| self.item_of_residual(i))
+            .collect()
+    }
+
+    #[must_use]
+    pub fn entity_of_parameter(&self, idx: ParameterIndex) -> Option<SketchEntityId> {
+        self.points
+            .iter()
+            .find_map(|(id, h)| (h.x == idx || h.y == idx).then_some(*id))
+            .or_else(|| {
+                self.radii
+                    .iter()
+                    .find_map(|(id, p)| (*p == idx).then_some(*id))
+            })
+    }
+}
+
+#[derive(Clone, Debug, PartialEq)]
+pub struct SketchDofReport {
+    dof: DegreesOfFreedom,
+    under_constrained: Vec<SketchEntityId>,
+    over_constrained: Vec<SketchItemId>,
+    redundant_consistent: Vec<SketchItemId>,
+}
+
+impl SketchDofReport {
+    #[must_use]
+    pub fn dof(&self) -> DegreesOfFreedom {
+        self.dof
+    }
+
+    #[must_use]
+    pub fn under_constrained(&self) -> &[SketchEntityId] {
+        &self.under_constrained
+    }
+
+    #[must_use]
+    pub fn over_constrained(&self) -> &[SketchItemId] {
+        &self.over_constrained
+    }
+
+    #[must_use]
+    pub fn redundant_consistent(&self) -> &[SketchItemId] {
+        &self.redundant_consistent
+    }
+
+    fn from_solver_report(report: &DofReport, mapping: &Mapping) -> Self {
+        Self {
+            dof: report.dof(),
+            under_constrained: dedup_preserving_order(
+                report
+                    .under_constrained()
+                    .iter()
+                    .filter_map(|p| mapping.entity_of_parameter(*p)),
+            ),
+            over_constrained: dedup_preserving_order(
+                mapping.items(report.over_constrained().iter().copied()),
+            ),
+            redundant_consistent: dedup_preserving_order(
+                mapping.items(report.redundant_consistent().iter().copied()),
+            ),
+        }
+    }
+}
+
+impl Sketch {
+    #[must_use]
+    pub fn lower(&self) -> (ConstraintSystem, Mapping) {
+        let (parameters, points, radii) = self.entity_order().iter().fold(
+            (
+                Vec::<Parameter>::new(),
+                BTreeMap::<SketchEntityId, PointHandle>::new(),
+                BTreeMap::<SketchEntityId, ParameterIndex>::new(),
+            ),
+            |(mut parameters, mut points, mut radii), id| {
+                match self.entities()[*id] {
+                    SketchEntity::Point(p) => {
+                        let (x, y) = p.at().coords_mm();
+                        let handle = PointHandle {
+                            x: push_parameter(&mut parameters, x),
+                            y: push_parameter(&mut parameters, y),
+                        };
+                        points.insert(*id, handle);
+                    }
+                    SketchEntity::Circle(c) => {
+                        let idx = push_parameter(&mut parameters, c.radius().get::<millimeter>());
+                        radii.insert(*id, idx);
+                    }
+                    SketchEntity::Line(_) | SketchEntity::Arc(_) => {}
+                }
+                (parameters, points, radii)
+            },
+        );
+
+        let intrinsic_rows: Vec<(Residual, Origin)> = self
+            .entity_order()
+            .iter()
+            .flat_map(|eid| {
+                intrinsic_residuals(self, *eid, &points)
+                    .into_iter()
+                    .map(move |r| (r, Origin::Intrinsic(*eid)))
+            })
+            .collect();
+
+        let relation_rows: Vec<(Residual, Origin)> = self
+            .relation_order()
+            .iter()
+            .flat_map(|rid| {
+                lower_relation(self, self.relations()[*rid], &points, &radii, &parameters)
+                    .into_iter()
+                    .map(move |r| (r, Origin::Relation(*rid)))
+            })
+            .collect();
+
+        let dimension_rows: Vec<(Residual, Origin)> = self
+            .dimension_order()
+            .iter()
+            .filter(|did| self.dimensions()[**did].kind() != DimensionKind::Driven)
+            .flat_map(|did| {
+                lower_dimension(self, self.dimensions()[*did], &points, &radii)
+                    .into_iter()
+                    .map(move |r| (r, Origin::Dimension(*did)))
+            })
+            .collect();
+
+        let combined: Vec<(Residual, Origin)> = intrinsic_rows
+            .into_iter()
+            .chain(relation_rows)
+            .chain(dimension_rows)
+            .collect();
+        let residual_origin: Vec<Origin> = combined
+            .iter()
+            .flat_map(|(r, o)| std::iter::repeat_n(*o, r.rows()))
+            .collect();
+        let residuals: Vec<Residual> = combined.into_iter().map(|(r, _)| r).collect();
+        let system = ConstraintSystem::new(parameters, residuals);
+        let mapping = Mapping {
+            points,
+            radii,
+            residual_origin,
+        };
+        (system, mapping)
+    }
+
+    pub fn solve(&self) -> Result<Sketch, SolverError> {
+        let (system, mapping) = self.lower();
+        let decomposition = decompose(&system);
+        match solve_newton_decomposed(&system, &decomposition, NewtonConfig::DEFAULT) {
+            Ok(parameters) => {
+                let report = analyze_dof_at(&system, &parameters, DofConfig::DEFAULT);
+                if report.over_constrained().is_empty() {
+                    Ok(self.clone().apply_solution(&parameters, &mapping))
+                } else {
+                    Err(narrow_over_defined(
+                        &system,
+                        &mapping,
+                        report.over_constrained(),
+                    ))
+                }
+            }
+            Err(SolverError::NoSolutionFound { last }) => {
+                let report = analyze_dof(&system, DofConfig::DEFAULT);
+                if report.over_constrained().is_empty() {
+                    Err(SolverError::NoSolutionFound { last })
+                } else {
+                    Err(narrow_over_defined(
+                        &system,
+                        &mapping,
+                        report.over_constrained(),
+                    ))
+                }
+            }
+            Err(other) => Err(other),
+        }
+    }
+
+    pub fn solve_with_drag(
+        &self,
+        dragged: SketchEntityId,
+        target: Point2,
+        budget: BudgetCeiling,
+    ) -> Result<Sketch, DragError> {
+        let started = Instant::now();
+        let entity = self
+            .entities()
+            .get(dragged)
+            .copied()
+            .ok_or(DragError::NotFound(dragged))?;
+        if !matches!(entity, SketchEntity::Point(_)) {
+            return Err(DragError::NotAPoint(dragged));
+        }
+        let warm = self
+            .clone()
+            .with_point_positions(&HashMap::from([(dragged, target)]));
+        let (system, mapping) = warm.lower();
+        let Some(handle) = mapping.point(dragged) else {
+            unreachable!("Point entity must lower to a PointHandle in the mapping")
+        };
+        let (tx, ty) = target.coords_mm();
+        let system = system.with_extra_residuals(vec![
+            Residual::Pin {
+                param: handle.x,
+                target: tx,
+            },
+            Residual::Pin {
+                param: handle.y,
+                target: ty,
+            },
+        ]);
+        let decomposition = decompose(&system);
+        let remaining = BudgetCeiling::new(budget.duration().saturating_sub(started.elapsed()));
+        let cfg = NewtonConfig {
+            budget: Some(remaining),
+            ..NewtonConfig::DEFAULT
+        };
+        let parameters = solve_newton_decomposed(&system, &decomposition, cfg)?;
+        Ok(warm.apply_solution(&parameters, &mapping))
+    }
+
+    #[must_use]
+    pub fn analyze_dof(&self) -> SketchDofReport {
+        let (system, mapping) = self.lower();
+        let decomposition = decompose(&system);
+        let report = match solve_newton_decomposed(&system, &decomposition, NewtonConfig::DEFAULT) {
+            Ok(parameters) => analyze_dof_at(&system, &parameters, DofConfig::DEFAULT),
+            Err(_) => analyze_dof(&system, DofConfig::DEFAULT),
+        };
+        SketchDofReport::from_solver_report(&report, &mapping)
+    }
+
+    #[must_use]
+    pub fn apply_solution(self, parameters: &[Parameter], mapping: &Mapping) -> Self {
+        let point_updates: HashMap<SketchEntityId, bone_types::Point2> = mapping
+            .points
+            .iter()
+            .map(|(id, h)| {
+                let p = bone_types::Point2::from_mm(
+                    parameters[h.x.as_usize()].value(),
+                    parameters[h.y.as_usize()].value(),
+                );
+                (*id, p)
+            })
+            .collect();
+        let radius_updates: HashMap<SketchEntityId, Length> = mapping
+            .radii
+            .iter()
+            .map(|(id, idx)| {
+                (
+                    *id,
+                    Length::new::<millimeter>(parameters[idx.as_usize()].value()),
+                )
+            })
+            .collect();
+        let next = self
+            .with_point_positions(&point_updates)
+            .with_circle_radii(&radius_updates);
+        recompute_driven(next)
+    }
+}
+
+fn narrow_over_defined(
+    system: &ConstraintSystem,
+    mapping: &Mapping,
+    over_flagged: &[ResidualIndex],
+) -> SolverError {
+    let mus = minimal_unsatisfiable_subset(system, over_flagged, DofConfig::DEFAULT);
+    let rows = if mus.is_empty() { over_flagged } else { &mus };
+    SolverError::OverDefined {
+        conflicts: dedup_preserving_order(mapping.items(rows.iter().copied())),
+    }
+}
+
+fn push_parameter(out: &mut Vec<Parameter>, value: f64) -> ParameterIndex {
+    let idx = ParameterIndex::new(u32::try_from(out.len()).unwrap_or(u32::MAX));
+    out.push(Parameter::new(value));
+    idx
+}
+
+fn intrinsic_residuals(
+    sketch: &Sketch,
+    id: SketchEntityId,
+    points: &BTreeMap<SketchEntityId, PointHandle>,
+) -> Vec<Residual> {
+    match sketch.entities().get(id) {
+        Some(SketchEntity::Arc(a)) => match (
+            points.get(&a.center()).copied(),
+            points.get(&a.start()).copied(),
+            points.get(&a.end()).copied(),
+        ) {
+            (Some(center), Some(spoke), Some(end)) => vec![Residual::CoincidentPointCurve {
+                point: end,
+                curve: CurveRadius::FromSpoke { center, spoke },
+            }],
+            _ => Vec::new(),
+        },
+        _ => Vec::new(),
+    }
+}
+
+fn lower_relation(
+    sketch: &Sketch,
+    relation: SketchRelation,
+    points: &BTreeMap<SketchEntityId, PointHandle>,
+    radii: &BTreeMap<SketchEntityId, ParameterIndex>,
+    parameters: &[Parameter],
+) -> Vec<Residual> {
+    match relation {
+        SketchRelation::Coincident(a, b) => lower_coincident(sketch, a, b, points, radii),
+        SketchRelation::Horizontal(id) => line_handle(sketch, id, points)
+            .map(Residual::Horizontal)
+            .into_iter()
+            .collect(),
+        SketchRelation::Vertical(id) => line_handle(sketch, id, points)
+            .map(Residual::Vertical)
+            .into_iter()
+            .collect(),
+        SketchRelation::Parallel(a, b) => combine_lines(sketch, a, b, points, Residual::Parallel),
+        SketchRelation::Perpendicular(a, b) => {
+            combine_lines(sketch, a, b, points, Residual::Perpendicular)
+        }
+        SketchRelation::Tangent(a, b) => lower_tangent(sketch, a, b, points, radii),
+        SketchRelation::Equal(a, b) => lower_equal(sketch, a, b, points, radii),
+        SketchRelation::Concentric(a, b) => lower_concentric(sketch, a, b, points),
+        SketchRelation::Fix(id) => lower_fix(sketch, id, points, radii, parameters),
+    }
+}
+
+fn lower_coincident(
+    sketch: &Sketch,
+    a: SketchEntityId,
+    b: SketchEntityId,
+    points: &BTreeMap<SketchEntityId, PointHandle>,
+    radii: &BTreeMap<SketchEntityId, ParameterIndex>,
+) -> Vec<Residual> {
+    let ka = kind(sketch, a);
+    let kb = kind(sketch, b);
+    let (point_id, other_id, other_kind) = match (ka, kb) {
+        (SketchEntityKind::Point, _) => (a, b, kb),
+        (_, SketchEntityKind::Point) => (b, a, ka),
+        _ => unreachable!("Coincident validation guarantees at least one point operand"),
+    };
+    let Some(point) = points.get(&point_id).copied() else {
+        return Vec::new();
+    };
+    match other_kind {
+        SketchEntityKind::Point => points
+            .get(&other_id)
+            .copied()
+            .map(|q| vec![Residual::CoincidentPointPoint(point, q)])
+            .unwrap_or_default(),
+        SketchEntityKind::Line => line_handle(sketch, other_id, points)
+            .map(|line| vec![Residual::CoincidentPointLine { point, line }])
+            .unwrap_or_default(),
+        SketchEntityKind::Circle | SketchEntityKind::Arc => {
+            curve_radius(sketch, other_id, points, radii)
+                .map(|curve| vec![Residual::CoincidentPointCurve { point, curve }])
+                .unwrap_or_default()
+        }
+    }
+}
+
+fn combine_lines(
+    sketch: &Sketch,
+    a: SketchEntityId,
+    b: SketchEntityId,
+    points: &BTreeMap<SketchEntityId, PointHandle>,
+    build: impl Fn(LineHandle, LineHandle) -> Residual,
+) -> Vec<Residual> {
+    match (
+        line_handle(sketch, a, points),
+        line_handle(sketch, b, points),
+    ) {
+        (Some(la), Some(lb)) => vec![build(la, lb)],
+        _ => Vec::new(),
+    }
+}
+
+fn lower_tangent(
+    sketch: &Sketch,
+    a: SketchEntityId,
+    b: SketchEntityId,
+    points: &BTreeMap<SketchEntityId, PointHandle>,
+    radii: &BTreeMap<SketchEntityId, ParameterIndex>,
+) -> Vec<Residual> {
+    let ka = kind(sketch, a);
+    let kb = kind(sketch, b);
+    let line_id;
+    let curve_id;
+    match (ka, kb) {
+        (SketchEntityKind::Line, _) => {
+            line_id = Some(a);
+            curve_id = b;
+        }
+        (_, SketchEntityKind::Line) => {
+            line_id = Some(b);
+            curve_id = a;
+        }
+        _ => {
+            line_id = None;
+            curve_id = a;
+        }
+    }
+    if let Some(lid) = line_id {
+        return match (
+            line_handle(sketch, lid, points),
+            curve_radius(sketch, curve_id, points, radii),
+        ) {
+            (Some(line), Some(curve)) => vec![Residual::TangentLineCurve { line, curve }],
+            _ => Vec::new(),
+        };
+    }
+    match (
+        curve_radius(sketch, a, points, radii),
+        curve_radius(sketch, b, points, radii),
+    ) {
+        (Some(ca), Some(cb)) => vec![Residual::TangentCurveCurve { a: ca, b: cb }],
+        _ => Vec::new(),
+    }
+}
+
+fn lower_equal(
+    sketch: &Sketch,
+    a: SketchEntityId,
+    b: SketchEntityId,
+    points: &BTreeMap<SketchEntityId, PointHandle>,
+    radii: &BTreeMap<SketchEntityId, ParameterIndex>,
+) -> Vec<Residual> {
+    match (kind(sketch, a), kind(sketch, b)) {
+        (SketchEntityKind::Line, SketchEntityKind::Line) => {
+            combine_lines(sketch, a, b, points, Residual::EqualLength)
+        }
+        (ka, kb) if is_curve(ka) && is_curve(kb) => match (
+            curve_radius(sketch, a, points, radii),
+            curve_radius(sketch, b, points, radii),
+        ) {
+            (Some(ca), Some(cb)) => vec![Residual::EqualRadius(ca, cb)],
+            _ => Vec::new(),
+        },
+        _ => unreachable!("Equal validation guarantees matching-kind operands"),
+    }
+}
+
+fn lower_concentric(
+    sketch: &Sketch,
+    a: SketchEntityId,
+    b: SketchEntityId,
+    points: &BTreeMap<SketchEntityId, PointHandle>,
+) -> Vec<Residual> {
+    let ca = curve_center(sketch, a).and_then(|c| points.get(&c).copied());
+    let cb = curve_center(sketch, b).and_then(|c| points.get(&c).copied());
+    match (ca, cb) {
+        (Some(p), Some(q)) => vec![Residual::CoincidentPointPoint(p, q)],
+        _ => Vec::new(),
+    }
+}
+
+fn lower_fix(
+    sketch: &Sketch,
+    id: SketchEntityId,
+    points: &BTreeMap<SketchEntityId, PointHandle>,
+    radii: &BTreeMap<SketchEntityId, ParameterIndex>,
+    parameters: &[Parameter],
+) -> Vec<Residual> {
+    let Some(entity) = sketch.entities().get(id).copied() else {
+        return Vec::new();
+    };
+    let point_ids: Vec<SketchEntityId> = match entity {
+        SketchEntity::Point(_) => vec![id],
+        _ => entity.references().into_iter().collect(),
+    };
+    let pins_from_points = point_ids
+        .into_iter()
+        .filter_map(|pid| points.get(&pid).copied())
+        .flat_map(|h| {
+            [
+                Residual::Pin {
+                    param: h.x,
+                    target: parameters[h.x.as_usize()].value(),
+                },
+                Residual::Pin {
+                    param: h.y,
+                    target: parameters[h.y.as_usize()].value(),
+                },
+            ]
+        });
+    let radius_pin = matches!(entity, SketchEntity::Circle(_))
+        .then(|| radii.get(&id).copied())
+        .flatten()
+        .map(|idx| Residual::Pin {
+            param: idx,
+            target: parameters[idx.as_usize()].value(),
+        });
+    pins_from_points.chain(radius_pin).collect()
+}
+
+fn lower_dimension(
+    sketch: &Sketch,
+    dim: SketchDimension,
+    points: &BTreeMap<SketchEntityId, PointHandle>,
+    radii: &BTreeMap<SketchEntityId, ParameterIndex>,
+) -> Vec<Residual> {
+    match dim {
+        SketchDimension::Linear { a, b, value, .. } => match (points.get(&a), points.get(&b)) {
+            (Some(pa), Some(pb)) => vec![Residual::LinearDistance {
+                a: *pa,
+                b: *pb,
+                value_mm: value.get::<millimeter>(),
+            }],
+            _ => Vec::new(),
+        },
+        SketchDimension::Radius { target, value, .. } => {
+            curve_radius(sketch, target, points, radii)
+                .map(|curve| {
+                    vec![Residual::RadiusCurve {
+                        curve,
+                        value_mm: value.get::<millimeter>(),
+                    }]
+                })
+                .unwrap_or_default()
+        }
+        SketchDimension::Diameter { target, value, .. } => {
+            curve_radius(sketch, target, points, radii)
+                .map(|curve| {
+                    vec![Residual::RadiusCurve {
+                        curve,
+                        value_mm: value.get::<millimeter>() * 0.5,
+                    }]
+                })
+                .unwrap_or_default()
+        }
+        SketchDimension::Angular { a, b, value, .. } => {
+            combine_lines(sketch, a, b, points, move |la, lb| {
+                Residual::AngularBetweenLines {
+                    a: la,
+                    b: lb,
+                    angle_rad: value.get::<radian>(),
+                }
+            })
+        }
+    }
+}
+
+fn kind(sketch: &Sketch, id: SketchEntityId) -> SketchEntityKind {
+    sketch
+        .entities()
+        .get(id)
+        .map_or(SketchEntityKind::Point, SketchEntity::kind)
+}
+
+fn is_curve(k: SketchEntityKind) -> bool {
+    matches!(k, SketchEntityKind::Circle | SketchEntityKind::Arc)
+}
+
+fn line_handle(
+    sketch: &Sketch,
+    id: SketchEntityId,
+    points: &BTreeMap<SketchEntityId, PointHandle>,
+) -> Option<LineHandle> {
+    let entity = sketch.entities().get(id)?;
+    let SketchEntity::Line(l) = entity else {
+        return None;
+    };
+    Some(LineHandle {
+        a: *points.get(&l.a())?,
+        b: *points.get(&l.b())?,
+    })
+}
+
+fn curve_radius(
+    sketch: &Sketch,
+    id: SketchEntityId,
+    points: &BTreeMap<SketchEntityId, PointHandle>,
+    radii: &BTreeMap<SketchEntityId, ParameterIndex>,
+) -> Option<CurveRadius> {
+    match sketch.entities().get(id)? {
+        SketchEntity::Circle(c) => {
+            let center = *points.get(&c.center())?;
+            let radius = *radii.get(&id)?;
+            Some(CurveRadius::Explicit { center, radius })
+        }
+        SketchEntity::Arc(a) => {
+            let center = *points.get(&a.center())?;
+            let spoke = *points.get(&a.start())?;
+            Some(CurveRadius::FromSpoke { center, spoke })
+        }
+        _ => None,
+    }
+}
+
+fn curve_center(sketch: &Sketch, id: SketchEntityId) -> Option<SketchEntityId> {
+    match sketch.entities().get(id)? {
+        SketchEntity::Circle(c) => Some(c.center()),
+        SketchEntity::Arc(a) => Some(a.center()),
+        _ => None,
+    }
+}
+
+fn recompute_driven(sketch: Sketch) -> Sketch {
+    let driven_updates: Vec<(SketchDimensionId, DimensionValue)> = sketch
+        .dimension_order()
+        .iter()
+        .filter_map(|did| {
+            let dim = *sketch.dimensions().get(*did)?;
+            if dim.kind() != DimensionKind::Driven {
+                return None;
+            }
+            let value = measure_dimension(&sketch, dim)?;
+            Some((*did, value))
+        })
+        .collect();
+    sketch.with_driven_dimension_values(&driven_updates)
+}
+
+fn measure_dimension(sketch: &Sketch, dim: SketchDimension) -> Option<DimensionValue> {
+    match dim {
+        SketchDimension::Linear { a, b, .. } => {
+            let pa = point_position(sketch, a)?;
+            let pb = point_position(sketch, b)?;
+            let (ax, ay) = pa.coords_mm();
+            let (bx, by) = pb.coords_mm();
+            let dx = bx - ax;
+            let dy = by - ay;
+            Some(DimensionValue::Length(Length::new::<millimeter>(
+                (dx * dx + dy * dy).sqrt(),
+            )))
+        }
+        SketchDimension::Radius { target, .. } => measure_radius(sketch, target)
+            .map(|r| DimensionValue::Length(Length::new::<millimeter>(r))),
+        SketchDimension::Diameter { target, .. } => measure_radius(sketch, target)
+            .map(|r| DimensionValue::Length(Length::new::<millimeter>(2.0 * r))),
+        SketchDimension::Angular { a, b, .. } => {
+            measure_angle(sketch, a, b).map(|r| DimensionValue::Angle(Angle::new::<radian>(r)))
+        }
+    }
+}
+
+fn point_position(sketch: &Sketch, id: SketchEntityId) -> Option<bone_types::Point2> {
+    match sketch.entities().get(id)? {
+        SketchEntity::Point(p) => Some(p.at()),
+        _ => None,
+    }
+}
+
+fn measure_radius(sketch: &Sketch, id: SketchEntityId) -> Option<f64> {
+    match sketch.entities().get(id)? {
+        SketchEntity::Circle(c) => Some(c.radius().get::<millimeter>()),
+        SketchEntity::Arc(a) => {
+            let center = point_position(sketch, a.center())?;
+            let start = point_position(sketch, a.start())?;
+            let (cx, cy) = center.coords_mm();
+            let (sx, sy) = start.coords_mm();
+            let dx = sx - cx;
+            let dy = sy - cy;
+            Some((dx * dx + dy * dy).sqrt())
+        }
+        _ => None,
+    }
+}
+
+fn measure_angle(sketch: &Sketch, a: SketchEntityId, b: SketchEntityId) -> Option<f64> {
+    let la = line_endpoints(sketch, a)?;
+    let lb = line_endpoints(sketch, b)?;
+    let (d1x, d1y) = (la.1.0 - la.0.0, la.1.1 - la.0.1);
+    let (d2x, d2y) = (lb.1.0 - lb.0.0, lb.1.1 - lb.0.1);
+    let cross = d1x * d2y - d1y * d2x;
+    let dot = d1x * d2x + d1y * d2y;
+    Some(cross.atan2(dot))
+}
+
+fn line_endpoints(sketch: &Sketch, id: SketchEntityId) -> Option<((f64, f64), (f64, f64))> {
+    match sketch.entities().get(id)? {
+        SketchEntity::Line(l) => {
+            let a = point_position(sketch, l.a())?.coords_mm();
+            let b = point_position(sketch, l.b())?.coords_mm();
+            Some((a, b))
+        }
+        _ => None,
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use std::collections::BTreeSet;
+
+    use super::*;
+    use crate::sketch::{EditOutcome, SketchEdit};
+    use bone_solver::{assemble_jacobian, evaluate_residuals, sparsity_pattern};
+    use bone_types::{Point2, Point3, SketchPlaneBasis, Tolerance, UnitVec3};
+    use nalgebra::DMatrix;
+
+    fn xy_plane() -> SketchPlaneBasis {
+        let Ok(basis) = SketchPlaneBasis::new(
+            Point3::origin(),
+            UnitVec3::x_axis(),
+            UnitVec3::y_axis(),
+            Tolerance::new(1e-9),
+        ) else {
+            panic!("xy axes are orthogonal");
+        };
+        basis
+    }
+
+    fn apply(sketch: &Sketch, edits: Vec<SketchEdit>) -> (Sketch, Vec<EditOutcome>) {
+        let Ok(pair) = sketch.apply_all(edits) else {
+            panic!("fixture edits apply cleanly");
+        };
+        pair
+    }
+
+    fn mm_val(v: f64) -> Length {
+        Length::new::<millimeter>(v)
+    }
+
+    fn entity_id(outcome: &EditOutcome) -> SketchEntityId {
+        let EditOutcome::Entity(id) = outcome else {
+            panic!("expected entity outcome");
+        };
+        *id
+    }
+
+    fn generous_budget() -> BudgetCeiling {
+        BudgetCeiling::new(core::time::Duration::from_secs(5))
+    }
+
+    fn rectangle_sketch() -> (Sketch, [SketchEntityId; 4]) {
+        let base = Sketch::new(xy_plane());
+        let (after, out) = apply(
+            &base,
+            vec![
+                SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(0.0, 0.0))),
+                SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(7.0, 0.5))),
+                SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(7.5, 3.2))),
+                SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(0.3, 2.9))),
+            ],
+        );
+        let ids = [0_usize, 1, 2, 3].map(|i| entity_id(&out[i]));
+        (after, ids)
+    }
+
+    fn constrained_rectangle() -> Sketch {
+        let (base, corners) = rectangle_sketch();
+        let [corner_0, corner_1, corner_2, corner_3] = corners;
+        let (with_edges, out) = apply(
+            &base,
+            vec![
+                SketchEdit::AddEntity(SketchEntity::line(corner_0, corner_1, false)),
+                SketchEdit::AddEntity(SketchEntity::line(corner_1, corner_2, false)),
+                SketchEdit::AddEntity(SketchEntity::line(corner_2, corner_3, false)),
+                SketchEdit::AddEntity(SketchEntity::line(corner_3, corner_0, false)),
+            ],
+        );
+        let [edge_bottom, edge_right, edge_top, edge_left] =
+            [0_usize, 1, 2, 3].map(|i| entity_id(&out[i]));
+        apply(
+            &with_edges,
+            vec![
+                SketchEdit::AddRelation(SketchRelation::Horizontal(edge_bottom)),
+                SketchEdit::AddRelation(SketchRelation::Horizontal(edge_top)),
+                SketchEdit::AddRelation(SketchRelation::Vertical(edge_right)),
+                SketchEdit::AddRelation(SketchRelation::Vertical(edge_left)),
+                SketchEdit::AddRelation(SketchRelation::Fix(corner_0)),
+                SketchEdit::AddDimension(SketchDimension::Linear {
+                    a: corner_0,
+                    b: corner_1,
+                    value: mm_val(10.0),
+                    kind: DimensionKind::Driving,
+                }),
+                SketchEdit::AddDimension(SketchDimension::Linear {
+                    a: corner_1,
+                    b: corner_2,
+                    value: mm_val(5.0),
+                    kind: DimensionKind::Driving,
+                }),
+            ],
+        )
+        .0
+    }
+
+    fn finite_difference_jacobian(
+        system: &ConstraintSystem,
+        params: &[f64],
+        eps: f64,
+    ) -> DMatrix<f64> {
+        let rows = system.row_count();
+        let cols = params.len();
+        (0..cols).fold(DMatrix::<f64>::zeros(rows, cols), |mut m, col| {
+            let mut plus = params.to_vec();
+            plus[col] += eps;
+            let mut minus = params.to_vec();
+            minus[col] -= eps;
+            let rp = evaluate_residuals(system, &plus);
+            let rn = evaluate_residuals(system, &minus);
+            (0..rows).for_each(|row| {
+                m[(row, col)] = (rp[row] - rn[row]) / (2.0 * eps);
+            });
+            m
+        })
+    }
+
+    fn point_coords(sketch: &Sketch) -> Vec<(f64, f64)> {
+        sketch
+            .entity_order()
+            .iter()
+            .filter_map(|id| match sketch.entities().get(*id)? {
+                SketchEntity::Point(p) => Some(p.at().coords_mm()),
+                _ => None,
+            })
+            .collect()
+    }
+
+    #[test]
+    fn rectangle_solves_to_exact_dimensions() {
+        let Ok(solved) = constrained_rectangle().solve() else {
+            panic!("rectangle solves");
+        };
+        let coords = point_coords(&solved);
+        let expected = [(0.0, 0.0), (10.0, 0.0), (10.0, 5.0), (0.0, 5.0)];
+        expected.iter().zip(coords.iter()).for_each(|(want, got)| {
+            assert!((want.0 - got.0).abs() < 1e-6, "x: {want:?} vs {got:?}");
+            assert!((want.1 - got.1).abs() < 1e-6, "y: {want:?} vs {got:?}");
+        });
+    }
+
+    #[test]
+    fn rectangle_analytic_jacobian_matches_finite_difference() {
+        let sketch = constrained_rectangle();
+        let (system, _) = sketch.lower();
+        let params: Vec<f64> = system.parameters().iter().map(|p| p.value()).collect();
+        let analytic = assemble_jacobian(&system, &params);
+        let fd = finite_difference_jacobian(&system, &params, 1e-6);
+        let diff = (&analytic - &fd).abs().max();
+        assert!(diff < 1e-4, "jacobian mismatch (max diff {diff})");
+    }
+
+    #[test]
+    fn rectangle_sparsity_pattern_is_stable() {
+        let (system, _) = constrained_rectangle().lower();
+        let pattern = sparsity_pattern(&system);
+        insta::assert_snapshot!("rectangle_sparsity", pattern.display());
+    }
+
+    #[test]
+    fn tangent_line_to_circle_solves() {
+        let base = Sketch::new(xy_plane());
+        let (with_points, out) = apply(
+            &base,
+            vec![
+                SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(0.0, 0.0))),
+                SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(10.0, 0.0))),
+                SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(3.0, 3.5))),
+            ],
+        );
+        let [line_start, line_end, circle_center] = [0_usize, 1, 2].map(|i| entity_id(&out[i]));
+        let (with_shapes, out2) = apply(
+            &with_points,
+            vec![
+                SketchEdit::AddEntity(SketchEntity::line(line_start, line_end, false)),
+                SketchEdit::AddEntity(SketchEntity::circle(circle_center, mm_val(2.0), false)),
+            ],
+        );
+        let [line_id, circle_id] = [0_usize, 1].map(|i| entity_id(&out2[i]));
+        let sketch = apply(
+            &with_shapes,
+            vec![
+                SketchEdit::AddRelation(SketchRelation::Fix(line_start)),
+                SketchEdit::AddRelation(SketchRelation::Fix(line_end)),
+                SketchEdit::AddDimension(SketchDimension::Linear {
+                    a: line_start,
+                    b: circle_center,
+                    value: mm_val(5.0),
+                    kind: DimensionKind::Driving,
+                }),
+                SketchEdit::AddDimension(SketchDimension::Radius {
+                    target: circle_id,
+                    value: mm_val(2.0),
+                    kind: DimensionKind::Driving,
+                }),
+                SketchEdit::AddRelation(SketchRelation::Tangent(line_id, circle_id)),
+            ],
+        )
+        .0;
+        let Ok(solved) = sketch.solve() else {
+            panic!("tangent line-circle solves");
+        };
+        let coords = point_coords(&solved);
+        let (start_x, start_y) = coords[0];
+        let (end_x, end_y) = coords[1];
+        let (center_x, center_y) = coords[2];
+        assert!(start_x.abs() < 1e-6 && start_y.abs() < 1e-6);
+        assert!((end_x - 10.0).abs() < 1e-6 && end_y.abs() < 1e-6);
+        assert!(
+            (center_y.abs() - 2.0).abs() < 1e-6,
+            "center-to-line {center_y}"
+        );
+        let radial = (center_x * center_x + center_y * center_y).sqrt();
+        assert!((radial - 5.0).abs() < 1e-6, "|c| {radial}");
+    }
+
+    #[test]
+    fn item_of_residual_tracks_rows_across_multi_row_residuals() {
+        let base = Sketch::new(xy_plane());
+        let (s, outs) = apply(
+            &base,
+            vec![
+                SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(0.0, 0.0))),
+                SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(0.5, 0.5))),
+                SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(5.0, 0.0))),
+                SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(5.5, 0.5))),
+            ],
+        );
+        let [p0, p1, p2, p3] = [0_usize, 1, 2, 3].map(|i| entity_id(&outs[i]));
+        let (sketch, rel_outs) = apply(
+            &s,
+            vec![
+                SketchEdit::AddRelation(SketchRelation::Coincident(p0, p1)),
+                SketchEdit::AddRelation(SketchRelation::Fix(p2)),
+                SketchEdit::AddRelation(SketchRelation::Coincident(p2, p3)),
+            ],
+        );
+        let relation_ids: Vec<SketchRelationId> = rel_outs
+            .iter()
+            .map(|o| match *o {
+                EditOutcome::Relation(id) => id,
+                _ => panic!("expected relation outcome"),
+            })
+            .collect();
+        let (system, mapping) = sketch.lower();
+        assert_eq!(system.row_count(), 6);
+        let expected: [SketchItemId; 6] = [
+            SketchItemId::Relation(relation_ids[0]),
+            SketchItemId::Relation(relation_ids[0]),
+            SketchItemId::Relation(relation_ids[1]),
+            SketchItemId::Relation(relation_ids[1]),
+            SketchItemId::Relation(relation_ids[2]),
+            SketchItemId::Relation(relation_ids[2]),
+        ];
+        expected.iter().enumerate().for_each(|(i, want)| {
+            let Ok(row) = u32::try_from(i) else {
+                unreachable!("fixture row count fits in u32")
+            };
+            let got = mapping.item_of_residual(ResidualIndex::new(row));
+            assert_eq!(got, Some(*want), "row {i}");
+        });
+    }
+
+    #[test]
+    fn under_constrained_horizontal_line_solves() {
+        let base = Sketch::new(xy_plane());
+        let (with_points, outs) = apply(
+            &base,
+            vec![
+                SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(0.0, 0.0))),
+                SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(1.0, 3.0))),
+            ],
+        );
+        let [a, b] = [0_usize, 1].map(|i| entity_id(&outs[i]));
+        let (with_line, out_line) = apply(
+            &with_points,
+            vec![SketchEdit::AddEntity(SketchEntity::line(a, b, false))],
+        );
+        let line = entity_id(&out_line[0]);
+        let sketch = apply(
+            &with_line,
+            vec![SketchEdit::AddRelation(SketchRelation::Horizontal(line))],
+        )
+        .0;
+        let Ok(solved) = sketch.solve() else {
+            panic!("under-constrained Horizontal should solve, not fail")
+        };
+        let coords = point_coords(&solved);
+        assert!(
+            (coords[0].1 - coords[1].1).abs() < 1e-6,
+            "y-values should match after Horizontal: {coords:?}"
+        );
+    }
+
+    #[test]
+    fn driven_dimension_is_recomputed_post_solve() {
+        let (base, corners) = rectangle_sketch();
+        let [corner_0, corner_1, _corner_2, _corner_3] = corners;
+        let sketch = apply(
+            &base,
+            vec![SketchEdit::AddDimension(SketchDimension::Linear {
+                a: corner_0,
+                b: corner_1,
+                value: mm_val(0.0),
+                kind: DimensionKind::Driven,
+            })],
+        )
+        .0;
+        let Ok(solved) = sketch.solve() else {
+            panic!("solve with driven dim");
+        };
+        let driven_id = solved.dimension_order()[0];
+        let SketchDimension::Linear { value, .. } = solved.dimensions()[driven_id] else {
+            panic!("driven should remain Linear");
+        };
+        let expected = (7.0_f64.powi(2) + 0.5_f64.powi(2)).sqrt();
+        assert!((value.get::<millimeter>() - expected).abs() < 1e-9);
+    }
+
+    #[test]
+    fn arc_end_stays_on_implicit_circle_after_solve() {
+        let base = Sketch::new(xy_plane());
+        let (with_points, outs) = apply(
+            &base,
+            vec![
+                SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(0.0, 0.0))),
+                SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(2.0, 0.0))),
+                SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(0.3, 2.5))),
+            ],
+        );
+        let [center, start, end] = [0_usize, 1, 2].map(|i| entity_id(&outs[i]));
+        let (with_arc, out_arc) = apply(
+            &with_points,
+            vec![SketchEdit::AddEntity(SketchEntity::arc(
+                center, start, end, false,
+            ))],
+        );
+        let _arc = entity_id(&out_arc[0]);
+        let sketch = apply(
+            &with_arc,
+            vec![
+                SketchEdit::AddRelation(SketchRelation::Fix(center)),
+                SketchEdit::AddRelation(SketchRelation::Fix(start)),
+            ],
+        )
+        .0;
+        let Ok(solved) = sketch.solve() else {
+            panic!("arc coherence solve");
+        };
+        let coords = point_coords(&solved);
+        let (cx, cy) = coords[0];
+        let (sx, sy) = coords[1];
+        let (ex, ey) = coords[2];
+        let r_start = ((sx - cx).powi(2) + (sy - cy).powi(2)).sqrt();
+        let r_end = ((ex - cx).powi(2) + (ey - cy).powi(2)).sqrt();
+        assert!(
+            (r_start - r_end).abs() < 1e-6,
+            "arc end must lie on implicit circle: r_start={r_start} r_end={r_end}"
+        );
+    }
+
+    fn horizontal_line_fixture(p0: (f64, f64), p1: (f64, f64), length_mm: f64) -> Sketch {
+        let base = Sketch::new(xy_plane());
+        let (with_points, outs) = apply(
+            &base,
+            vec![
+                SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(p0.0, p0.1))),
+                SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(p1.0, p1.1))),
+            ],
+        );
+        let [a, b] = [0_usize, 1].map(|i| entity_id(&outs[i]));
+        let (with_line, line_outs) = apply(
+            &with_points,
+            vec![SketchEdit::AddEntity(SketchEntity::line(a, b, false))],
+        );
+        let line = entity_id(&line_outs[0]);
+        apply(
+            &with_line,
+            vec![
+                SketchEdit::AddRelation(SketchRelation::Fix(a)),
+                SketchEdit::AddRelation(SketchRelation::Horizontal(line)),
+                SketchEdit::AddDimension(SketchDimension::Linear {
+                    a,
+                    b,
+                    value: mm_val(length_mm),
+                    kind: DimensionKind::Driving,
+                }),
+            ],
+        )
+        .0
+    }
+
+    fn disjoint_pair_fixture() -> Sketch {
+        let base = Sketch::new(xy_plane());
+        let (with_points, outs) = apply(
+            &base,
+            vec![
+                SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(0.0, 0.0))),
+                SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(7.0, 0.3))),
+                SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(100.0, 0.0))),
+                SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(107.0, 0.3))),
+            ],
+        );
+        let [a0, a1, b0, b1] = [0_usize, 1, 2, 3].map(|i| entity_id(&outs[i]));
+        let (with_lines, line_outs) = apply(
+            &with_points,
+            vec![
+                SketchEdit::AddEntity(SketchEntity::line(a0, a1, false)),
+                SketchEdit::AddEntity(SketchEntity::line(b0, b1, false)),
+            ],
+        );
+        let [line_a, line_b] = [0_usize, 1].map(|i| entity_id(&line_outs[i]));
+        apply(
+            &with_lines,
+            vec![
+                SketchEdit::AddRelation(SketchRelation::Fix(a0)),
+                SketchEdit::AddRelation(SketchRelation::Horizontal(line_a)),
+                SketchEdit::AddRelation(SketchRelation::Fix(b0)),
+                SketchEdit::AddRelation(SketchRelation::Horizontal(line_b)),
+                SketchEdit::AddDimension(SketchDimension::Linear {
+                    a: a0,
+                    b: a1,
+                    value: mm_val(5.0),
+                    kind: DimensionKind::Driving,
+                }),
+                SketchEdit::AddDimension(SketchDimension::Linear {
+                    a: b0,
+                    b: b1,
+                    value: mm_val(9.0),
+                    kind: DimensionKind::Driving,
+                }),
+            ],
+        )
+        .0
+    }
+
+    #[test]
+    fn disjoint_sketches_solve_per_component() {
+        let Ok(solved_a) = horizontal_line_fixture((0.0, 0.0), (7.0, 0.3), 5.0).solve() else {
+            panic!("component A solves alone");
+        };
+        let Ok(solved_b) = horizontal_line_fixture((100.0, 0.0), (107.0, 0.3), 9.0).solve() else {
+            panic!("component B solves alone");
+        };
+        let Ok(joint_solved) = disjoint_pair_fixture().solve() else {
+            panic!("joint disjoint-component solve");
+        };
+        let joint_coords = point_coords(&joint_solved);
+        let expected_joint: Vec<(f64, f64)> = point_coords(&solved_a)
+            .into_iter()
+            .chain(point_coords(&solved_b))
+            .collect();
+        expected_joint
+            .iter()
+            .zip(joint_coords.iter())
+            .for_each(|(want, got)| {
+                assert!(
+                    (want.0 - got.0).abs() < 1e-9,
+                    "x alone-vs-joint {want:?} vs {got:?}"
+                );
+                assert!(
+                    (want.1 - got.1).abs() < 1e-9,
+                    "y alone-vs-joint {want:?} vs {got:?}"
+                );
+            });
+        let expected_absolute = [(0.0, 0.0), (5.0, 0.0), (100.0, 0.0), (109.0, 0.0)];
+        expected_absolute
+            .iter()
+            .zip(joint_coords.iter())
+            .for_each(|(want, got)| {
+                assert!((want.0 - got.0).abs() < 1e-6, "x {want:?} vs {got:?}");
+                assert!((want.1 - got.1).abs() < 1e-6, "y {want:?} vs {got:?}");
+            });
+    }
+
+    #[test]
+    fn conflicting_dimensions_report_over_defined() {
+        let base = Sketch::new(xy_plane());
+        let (with_points, outs) = apply(
+            &base,
+            vec![
+                SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(0.0, 0.0))),
+                SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(5.0, 0.0))),
+            ],
+        );
+        let [a, b] = [0_usize, 1].map(|i| entity_id(&outs[i]));
+        let (sketch, dim_outs) = apply(
+            &with_points,
+            vec![
+                SketchEdit::AddRelation(SketchRelation::Fix(a)),
+                SketchEdit::AddDimension(SketchDimension::Linear {
+                    a,
+                    b,
+                    value: mm_val(10.0),
+                    kind: DimensionKind::Driving,
+                }),
+                SketchEdit::AddDimension(SketchDimension::Linear {
+                    a,
+                    b,
+                    value: mm_val(3.0),
+                    kind: DimensionKind::Driving,
+                }),
+            ],
+        );
+        let dim_ids: Vec<SketchDimensionId> = dim_outs
+            .iter()
+            .filter_map(|o| match *o {
+                EditOutcome::Dimension(id) => Some(id),
+                _ => None,
+            })
+            .collect();
+        let Err(err) = sketch.solve() else {
+            panic!("conflicting dimensions must surface OverDefined");
+        };
+        let SolverError::OverDefined { conflicts } = err else {
+            panic!("expected OverDefined, got {err:?}");
+        };
+        assert!(!conflicts.is_empty(), "conflicts populated");
+        let has_dim = conflicts
+            .iter()
+            .any(|c| matches!(*c, SketchItemId::Dimension(id) if dim_ids.contains(&id)));
+        assert!(has_dim, "at least one of the conflicting dimensions names");
+    }
+
+    #[test]
+    fn redundant_dimension_does_not_block_solve() {
+        let base = Sketch::new(xy_plane());
+        let (with_points, outs) = apply(
+            &base,
+            vec![
+                SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(0.0, 0.0))),
+                SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(10.0, 0.0))),
+            ],
+        );
+        let [a, b] = [0_usize, 1].map(|i| entity_id(&outs[i]));
+        let sketch = apply(
+            &with_points,
+            vec![
+                SketchEdit::AddRelation(SketchRelation::Fix(a)),
+                SketchEdit::AddDimension(SketchDimension::Linear {
+                    a,
+                    b,
+                    value: mm_val(10.0),
+                    kind: DimensionKind::Driving,
+                }),
+                SketchEdit::AddDimension(SketchDimension::Linear {
+                    a,
+                    b,
+                    value: mm_val(10.0),
+                    kind: DimensionKind::Driving,
+                }),
+            ],
+        )
+        .0;
+        let report = sketch.analyze_dof();
+        assert_eq!(report.dof().value(), 1);
+        assert!(report.over_constrained().is_empty());
+        assert!(!report.redundant_consistent().is_empty());
+        let Ok(_) = sketch.solve() else {
+            panic!("redundant-consistent system must solve");
+        };
+    }
+
+    #[test]
+    fn redundant_dimension_at_unsatisfied_seed_still_solves() {
+        let base = Sketch::new(xy_plane());
+        let (with_points, outs) = apply(
+            &base,
+            vec![
+                SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(0.0, 0.0))),
+                SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(5.0, 0.0))),
+            ],
+        );
+        let [a, b] = [0_usize, 1].map(|i| entity_id(&outs[i]));
+        let sketch = apply(
+            &with_points,
+            vec![
+                SketchEdit::AddRelation(SketchRelation::Fix(a)),
+                SketchEdit::AddDimension(SketchDimension::Linear {
+                    a,
+                    b,
+                    value: mm_val(10.0),
+                    kind: DimensionKind::Driving,
+                }),
+                SketchEdit::AddDimension(SketchDimension::Linear {
+                    a,
+                    b,
+                    value: mm_val(10.0),
+                    kind: DimensionKind::Driving,
+                }),
+            ],
+        )
+        .0;
+        let report = sketch.analyze_dof();
+        assert_eq!(report.dof().value(), 1);
+        assert!(
+            report.over_constrained().is_empty(),
+            "redundant-consistent dims at unsatisfied seed must not be flagged as conflicts"
+        );
+        assert!(!report.redundant_consistent().is_empty());
+        let Ok(solved) = sketch.solve() else {
+            panic!("redundant-consistent dims at unsatisfied seed must solve");
+        };
+        let coords = point_coords(&solved);
+        let (bx, by) = coords[1];
+        let radial = (bx * bx + by * by).sqrt();
+        assert!(
+            (radial - 10.0).abs() < 1e-6,
+            "free endpoint should move to |b|=10mm, got {radial}"
+        );
+    }
+
+    #[test]
+    fn analyze_dof_reports_fully_constrained_rectangle() {
+        let sketch = constrained_rectangle();
+        let report = sketch.analyze_dof();
+        assert_eq!(report.dof().value(), 0);
+        assert!(report.over_constrained().is_empty());
+        assert!(report.redundant_consistent().is_empty());
+        assert!(report.under_constrained().is_empty());
+    }
+
+    #[test]
+    fn analyze_dof_reports_free_line() {
+        let base = Sketch::new(xy_plane());
+        let (with_points, outs) = apply(
+            &base,
+            vec![
+                SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(0.0, 0.0))),
+                SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(1.0, 3.0))),
+            ],
+        );
+        let [a, b] = [0_usize, 1].map(|i| entity_id(&outs[i]));
+        let (with_line, line_outs) = apply(
+            &with_points,
+            vec![SketchEdit::AddEntity(SketchEntity::line(a, b, false))],
+        );
+        let line = entity_id(&line_outs[0]);
+        let sketch = apply(
+            &with_line,
+            vec![SketchEdit::AddRelation(SketchRelation::Horizontal(line))],
+        )
+        .0;
+        let report = sketch.analyze_dof();
+        assert_eq!(report.dof().value(), 3);
+        assert!(report.over_constrained().is_empty());
+        assert!(!report.under_constrained().is_empty());
+    }
+
+    fn three_conflicting_linear_dims_fixture() -> (Sketch, Vec<SketchDimensionId>) {
+        let base = Sketch::new(xy_plane());
+        let (with_points, outs) = apply(
+            &base,
+            vec![
+                SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(0.0, 0.0))),
+                SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(5.0, 0.0))),
+            ],
+        );
+        let [a, b] = [0_usize, 1].map(|i| entity_id(&outs[i]));
+        let (sketch, dim_outs) = apply(
+            &with_points,
+            vec![
+                SketchEdit::AddRelation(SketchRelation::Fix(a)),
+                SketchEdit::AddDimension(SketchDimension::Linear {
+                    a,
+                    b,
+                    value: mm_val(10.0),
+                    kind: DimensionKind::Driving,
+                }),
+                SketchEdit::AddDimension(SketchDimension::Linear {
+                    a,
+                    b,
+                    value: mm_val(7.0),
+                    kind: DimensionKind::Driving,
+                }),
+                SketchEdit::AddDimension(SketchDimension::Linear {
+                    a,
+                    b,
+                    value: mm_val(10.0),
+                    kind: DimensionKind::Driving,
+                }),
+            ],
+        );
+        let dim_ids: Vec<SketchDimensionId> = dim_outs
+            .iter()
+            .filter_map(|o| match *o {
+                EditOutcome::Dimension(id) => Some(id),
+                _ => None,
+            })
+            .collect();
+        (sketch, dim_ids)
+    }
+
+    #[test]
+    fn mus_narrows_three_conflicting_dims_to_odd_one_out() {
+        let (sketch, dim_ids) = three_conflicting_linear_dims_fixture();
+        let Err(err) = sketch.solve() else {
+            panic!("three conflicting dims must surface OverDefined");
+        };
+        let SolverError::OverDefined { conflicts } = err else {
+            panic!("expected OverDefined, got {err:?}");
+        };
+        let added: BTreeSet<SketchItemId> = dim_ids
+            .iter()
+            .copied()
+            .map(SketchItemId::Dimension)
+            .collect();
+        assert!(
+            !conflicts.is_empty() && conflicts.len() < dim_ids.len(),
+            "MUS must narrow {{10, 7, 10}} strictly below the full dim set: \
+             conflicts={conflicts:?} dim_ids={dim_ids:?}",
+        );
+        if let Some(stray) = conflicts.iter().find(|c| !added.contains(c)) {
+            panic!("unexpected conflict: {stray:?} not in {dim_ids:?}");
+        }
+        assert!(
+            conflicts.contains(&SketchItemId::Dimension(dim_ids[1])),
+            "MUS must flag the 7mm outlier (dim_ids[1]): \
+             conflicts={conflicts:?} dim_ids={dim_ids:?}",
+        );
+    }
+
+    #[test]
+    fn mus_on_three_distinct_conflicting_dims_returns_minimal_conflict() {
+        let base = Sketch::new(xy_plane());
+        let (with_points, outs) = apply(
+            &base,
+            vec![
+                SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(0.0, 0.0))),
+                SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(5.0, 0.0))),
+            ],
+        );
+        let [a, b] = [0_usize, 1].map(|i| entity_id(&outs[i]));
+        let (sketch, dim_outs) = apply(
+            &with_points,
+            vec![
+                SketchEdit::AddRelation(SketchRelation::Fix(a)),
+                SketchEdit::AddDimension(SketchDimension::Linear {
+                    a,
+                    b,
+                    value: mm_val(10.0),
+                    kind: DimensionKind::Driving,
+                }),
+                SketchEdit::AddDimension(SketchDimension::Linear {
+                    a,
+                    b,
+                    value: mm_val(7.0),
+                    kind: DimensionKind::Driving,
+                }),
+                SketchEdit::AddDimension(SketchDimension::Linear {
+                    a,
+                    b,
+                    value: mm_val(4.0),
+                    kind: DimensionKind::Driving,
+                }),
+            ],
+        );
+        let dim_ids: Vec<SketchDimensionId> = dim_outs
+            .iter()
+            .filter_map(|o| match *o {
+                EditOutcome::Dimension(id) => Some(id),
+                _ => None,
+            })
+            .collect();
+        let Err(SolverError::OverDefined { conflicts }) = sketch.solve() else {
+            panic!("three distinct-value conflicting dims must surface OverDefined");
+        };
+        assert!(!conflicts.is_empty(), "MUS must be non-empty");
+        assert!(
+            conflicts.len() <= dim_ids.len(),
+            "MUS must not exceed the added-dim count: conflicts={conflicts:?}",
+        );
+        if let Some(bad) = conflicts
+            .iter()
+            .find(|c| !matches!(c, SketchItemId::Dimension(id) if dim_ids.contains(id)))
+        {
+            panic!("unexpected conflict: {bad:?} not in {dim_ids:?}");
+        }
+        assert_eq!(
+            conflicts.iter().collect::<BTreeSet<_>>().len(),
+            conflicts.len(),
+            "conflicts must be deduped: {conflicts:?}",
+        );
+    }
+
+    #[test]
+    fn narrow_over_defined_dedups_multi_row_parent() {
+        let base = Sketch::new(xy_plane());
+        let (with_points, outs) = apply(
+            &base,
+            vec![
+                SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(0.0, 0.0))),
+                SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(1.0, 1.0))),
+            ],
+        );
+        let [a, b] = [0_usize, 1].map(|i| entity_id(&outs[i]));
+        let (sketch, rel_outs) = apply(
+            &with_points,
+            vec![SketchEdit::AddRelation(SketchRelation::Coincident(a, b))],
+        );
+        let EditOutcome::Relation(coincident_id) = rel_outs[0] else {
+            panic!("expected relation outcome");
+        };
+        let (system, mapping) = sketch.lower();
+        let rows: Vec<ResidualIndex> = (0..system.row_count())
+            .map(|i| {
+                let Ok(iv) = u32::try_from(i) else {
+                    unreachable!("fixture row count fits in u32")
+                };
+                ResidualIndex::new(iv)
+            })
+            .collect();
+        assert_eq!(rows.len(), 2, "Coincident is a two-row residual");
+        let err = narrow_over_defined(&system, &mapping, &rows);
+        let SolverError::OverDefined { conflicts } = err else {
+            panic!("narrow_over_defined must surface OverDefined");
+        };
+        assert_eq!(
+            conflicts,
+            vec![SketchItemId::Relation(coincident_id)],
+            "two rows of one Coincident must collapse to a single conflict: {conflicts:?}",
+        );
+    }
+
+    fn draggable_horizontal_line() -> (Sketch, SketchEntityId, SketchEntityId) {
+        let base = Sketch::new(xy_plane());
+        let (with_points, outs) = apply(
+            &base,
+            vec![
+                SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(0.0, 0.0))),
+                SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(5.0, 0.0))),
+            ],
+        );
+        let [a, b] = [0_usize, 1].map(|i| entity_id(&outs[i]));
+        let (with_line, line_outs) = apply(
+            &with_points,
+            vec![SketchEdit::AddEntity(SketchEntity::line(a, b, false))],
+        );
+        let line = entity_id(&line_outs[0]);
+        let sketch = apply(
+            &with_line,
+            vec![
+                SketchEdit::AddRelation(SketchRelation::Fix(a)),
+                SketchEdit::AddRelation(SketchRelation::Horizontal(line)),
+            ],
+        )
+        .0;
+        (sketch, a, b)
+    }
+
+    #[test]
+    fn solve_with_drag_along_horizontal_lands_on_target() {
+        let (sketch, a, b) = draggable_horizontal_line();
+        let target = Point2::from_mm(8.0, 0.0);
+        let Ok(dragged) = sketch.solve_with_drag(b, target, generous_budget()) else {
+            panic!("feasible drag along Horizontal must converge");
+        };
+        let coords = point_coords(&dragged);
+        let (bx, by) = coords[1];
+        assert!(
+            (bx - 8.0).abs() < 1e-6,
+            "dragged x must land at target.x: {bx}",
+        );
+        assert!(
+            by.abs() < 1e-6,
+            "Horizontal holds: a.y={} b.y={by}",
+            coords[0].1,
+        );
+        let (ax, ay) = coords[0];
+        assert!(
+            ax.abs() < 1e-9 && ay.abs() < 1e-9,
+            "Fix on a holds: ({ax}, {ay})",
+        );
+        let _ = a;
+    }
+
+    #[test]
+    fn solve_with_drag_rejects_non_point() {
+        let base = Sketch::new(xy_plane());
+        let (with_points, outs) = apply(
+            &base,
+            vec![
+                SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(0.0, 0.0))),
+                SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(1.0, 0.0))),
+            ],
+        );
+        let [a, b] = [0_usize, 1].map(|i| entity_id(&outs[i]));
+        let (sketch, line_outs) = apply(
+            &with_points,
+            vec![SketchEdit::AddEntity(SketchEntity::line(a, b, false))],
+        );
+        let line = entity_id(&line_outs[0]);
+        let result = sketch.solve_with_drag(line, Point2::from_mm(0.0, 0.0), generous_budget());
+        let Err(err) = result else {
+            panic!("dragging a Line must reject, got {result:?}");
+        };
+        assert!(matches!(err, DragError::NotAPoint(id) if id == line));
+    }
+
+    #[test]
+    fn solve_with_drag_rejects_unknown_entity_id() {
+        let sketch = constrained_rectangle();
+        let stranger = SketchEntityId::default();
+        let result = sketch.solve_with_drag(stranger, Point2::from_mm(0.0, 0.0), generous_budget());
+        let Err(err) = result else {
+            panic!("unknown entity id must reject, got {result:?}");
+        };
+        assert!(matches!(err, DragError::NotFound(id) if id == stranger));
+    }
+
+    #[test]
+    fn solve_with_drag_returns_budget_when_ceiling_is_zero() {
+        let sketch = constrained_rectangle();
+        let target = Point2::from_mm(20.0, 20.0);
+        let drag_id = sketch.entity_order()[1];
+        let result = sketch.solve_with_drag(
+            drag_id,
+            target,
+            BudgetCeiling::new(core::time::Duration::ZERO),
+        );
+        match result {
+            Err(DragError::Solver(SolverError::Budget { .. })) => {}
+            other => panic!("expected Budget, got {other:?}"),
+        }
+    }
+
+    #[test]
+    fn solve_with_drag_on_fixed_point_reanchors_to_target() {
+        let base = Sketch::new(xy_plane());
+        let (with_point, outs) = apply(
+            &base,
+            vec![SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(
+                0.0, 0.0,
+            )))],
+        );
+        let p = entity_id(&outs[0]);
+        let sketch = apply(
+            &with_point,
+            vec![SketchEdit::AddRelation(SketchRelation::Fix(p))],
+        )
+        .0;
+        let target = Point2::from_mm(8.0, 5.0);
+        let Ok(dragged) = sketch.solve_with_drag(p, target, generous_budget()) else {
+            panic!("Fix'd drag re-anchors via warm-start and must converge");
+        };
+        let (x, y) = point_coords(&dragged)[0];
+        assert!(
+            (x - 8.0).abs() < 1e-9,
+            "warm-start re-anchors Fix's x pin to target: {x}",
+        );
+        assert!(
+            (y - 5.0).abs() < 1e-9,
+            "warm-start re-anchors Fix's y pin to target: {y}",
+        );
+    }
+
+    #[test]
+    fn solve_is_bit_identical_across_repeated_invocations() {
+        let sketch = constrained_rectangle();
+        let Ok(first) = sketch.solve() else {
+            panic!("rectangle solves once");
+        };
+        let Ok(second) = sketch.solve() else {
+            panic!("rectangle solves twice");
+        };
+        let lhs = point_coords(&first);
+        let rhs = point_coords(&second);
+        lhs.iter().zip(rhs.iter()).for_each(|(a, b)| {
+            assert_eq!(
+                a.0.to_bits(),
+                b.0.to_bits(),
+                "x must match bit-for-bit: {a:?} vs {b:?}",
+            );
+            assert_eq!(
+                a.1.to_bits(),
+                b.1.to_bits(),
+                "y must match bit-for-bit: {a:?} vs {b:?}",
+            );
+        });
+    }
+
+    #[test]
+    fn over_defined_conflicts_are_identical_across_repeated_invocations() {
+        let (sketch, _) = three_conflicting_linear_dims_fixture();
+        let Err(SolverError::OverDefined { conflicts: first }) = sketch.solve() else {
+            panic!("conflicting fixture must surface OverDefined on first solve");
+        };
+        let Err(SolverError::OverDefined { conflicts: second }) = sketch.solve() else {
+            panic!("conflicting fixture must surface OverDefined on second solve");
+        };
+        assert_eq!(
+            first, second,
+            "MUS conflict ordering must be identical across runs",
+        );
+    }
+
+    #[test]
+    fn drag_is_bit_identical_across_repeated_invocations() {
+        let (sketch, _a, b) = draggable_horizontal_line();
+        let target = Point2::from_mm(13.5, 0.0);
+        let Ok(first) = sketch.solve_with_drag(b, target, generous_budget()) else {
+            panic!("first drag solves");
+        };
+        let Ok(second) = sketch.solve_with_drag(b, target, generous_budget()) else {
+            panic!("second drag solves");
+        };
+        let lhs = point_coords(&first);
+        let rhs = point_coords(&second);
+        lhs.iter().zip(rhs.iter()).for_each(|(a, b)| {
+            assert_eq!(a.0.to_bits(), b.0.to_bits());
+            assert_eq!(a.1.to_bits(), b.1.to_bits());
+        });
+    }
+
+    #[cfg(not(debug_assertions))]
+    #[test]
+    fn drag_resolve_completes_inside_frame_budget() {
+        use std::time::Instant;
+        let (sketch, _a, b) = draggable_horizontal_line();
+        (0..16).for_each(|i| {
+            let f = f64::from(i);
+            let target = Point2::from_mm(8.0 + 0.1 * f, 0.0);
+            let started = Instant::now();
+            let Ok(_) = sketch.solve_with_drag(b, target, BudgetCeiling::FRAME_16MS) else {
+                panic!("drag solve must converge inside budget at step {i}");
+            };
+            let elapsed = started.elapsed();
+            assert!(
+                elapsed <= core::time::Duration::from_millis(16),
+                "drag step {i} took {elapsed:?}, exceeding 16 ms budget",
+            );
+        });
+    }
+
+    mod determinism {
+        use super::*;
+        use proptest::prelude::*;
+
+        #[derive(Copy, Clone, Debug)]
+        struct DragSeed {
+            base_x: i32,
+            base_y: i32,
+            target_dx: i32,
+            target_dy: i32,
+            length: u32,
+        }
+
+        fn arb_drag_seed() -> impl Strategy<Value = DragSeed> {
+            (
+                -10_000_i32..10_000,
+                -10_000_i32..10_000,
+                -20_000_i32..20_000,
+                -20_000_i32..20_000,
+                500_u32..50_000,
+            )
+                .prop_map(|(base_x, base_y, target_dx, target_dy, length)| DragSeed {
+                    base_x,
+                    base_y,
+                    target_dx,
+                    target_dy,
+                    length,
+                })
+        }
+
+        fn build_horizontal_line(seed: DragSeed) -> (Sketch, SketchEntityId) {
+            let bx = f64::from(seed.base_x) * 1e-3;
+            let by = f64::from(seed.base_y) * 1e-3;
+            let length_mm_value = f64::from(seed.length) * 1e-3;
+            let base = Sketch::new(xy_plane());
+            let (with_points, outs) = apply(
+                &base,
+                vec![
+                    SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(bx, by))),
+                    SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(bx + 1.0, by + 1.0))),
+                ],
+            );
+            let [a, b] = [0_usize, 1].map(|i| entity_id(&outs[i]));
+            let (with_line, line_outs) = apply(
+                &with_points,
+                vec![SketchEdit::AddEntity(SketchEntity::line(a, b, false))],
+            );
+            let line = entity_id(&line_outs[0]);
+            let sketch = apply(
+                &with_line,
+                vec![
+                    SketchEdit::AddRelation(SketchRelation::Fix(a)),
+                    SketchEdit::AddRelation(SketchRelation::Horizontal(line)),
+                    SketchEdit::AddDimension(SketchDimension::Linear {
+                        a,
+                        b,
+                        value: mm_val(length_mm_value),
+                        kind: DimensionKind::Driving,
+                    }),
+                ],
+            )
+            .0;
+            (sketch, b)
+        }
+
+        fn assert_coord_pairs_bit_identical(
+            lhs: &[(f64, f64)],
+            rhs: &[(f64, f64)],
+        ) -> Result<(), TestCaseError> {
+            lhs.iter().zip(rhs.iter()).try_for_each(|(a, b)| {
+                prop_assert_eq!(a.0.to_bits(), b.0.to_bits());
+                prop_assert_eq!(a.1.to_bits(), b.1.to_bits());
+                Ok(())
+            })
+        }
+
+        proptest! {
+            #[test]
+            fn solve_is_bit_identical_for_random_horizontal_lines(seed in arb_drag_seed()) {
+                let (sketch, _b) = build_horizontal_line(seed);
+                let Ok(first) = sketch.solve() else {
+                    return Ok(());
+                };
+                let Ok(second) = sketch.solve() else {
+                    unreachable!("first solve succeeded; second cannot diverge under determinism");
+                };
+                assert_coord_pairs_bit_identical(&point_coords(&first), &point_coords(&second))?;
+            }
+
+            #[test]
+            fn drag_is_bit_identical_for_random_horizontal_lines(seed in arb_drag_seed()) {
+                let (sketch, drag_id) = build_horizontal_line(seed);
+                let tx = f64::from(seed.base_x + seed.target_dx) * 1e-3;
+                let ty = f64::from(seed.base_y + seed.target_dy) * 1e-3;
+                let target = Point2::from_mm(tx, ty);
+                let Ok(first) = sketch.solve_with_drag(drag_id, target, generous_budget()) else {
+                    return Ok(());
+                };
+                let Ok(second) = sketch.solve_with_drag(drag_id, target, generous_budget()) else {
+                    unreachable!("first drag succeeded; second must too under determinism");
+                };
+                assert_coord_pairs_bit_identical(&point_coords(&first), &point_coords(&second))?;
+            }
+        }
+    }
+
+    #[test]
+    fn arc_intrinsic_residual_tags_entity_origin() {
+        let base = Sketch::new(xy_plane());
+        let (with_points, outs) = apply(
+            &base,
+            vec![
+                SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(0.0, 0.0))),
+                SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(2.0, 0.0))),
+                SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(0.0, 2.0))),
+            ],
+        );
+        let [c, s, e] = [0_usize, 1, 2].map(|i| entity_id(&outs[i]));
+        let sketch = apply(
+            &with_points,
+            vec![SketchEdit::AddEntity(SketchEntity::arc(c, s, e, false))],
+        )
+        .0;
+        let arc_id = sketch.entity_order()[3];
+        let (system, mapping) = sketch.lower();
+        assert_eq!(system.row_count(), 1, "one intrinsic row per arc");
+        let got = mapping.item_of_residual(ResidualIndex::new(0));
+        assert_eq!(got, Some(SketchItemId::Entity(arc_id)));
+    }
+}
diff --git a/crates/bone-document/src/undo.rs b/crates/bone-document/src/undo.rs
new file mode 100644
index 0000000..6193fae
--- /dev/null
+++ b/crates/bone-document/src/undo.rs
@@ -0,0 +1,74 @@
+use std::collections::VecDeque;
+use std::mem;
+use std::num::NonZeroUsize;
+
+use crate::Document;
+
+#[derive(Clone, Debug)]
+pub struct UndoStack {
+    past: VecDeque<Document>,
+    future: VecDeque<Document>,
+    capacity: NonZeroUsize,
+}
+
+impl UndoStack {
+    #[must_use]
+    pub fn with_capacity(capacity: NonZeroUsize) -> Self {
+        Self {
+            past: VecDeque::new(),
+            future: VecDeque::new(),
+            capacity,
+        }
+    }
+
+    pub fn record(&mut self, previous: Document) {
+        self.future.clear();
+        if self.past.len() == self.capacity.get() {
+            self.past.pop_front();
+        }
+        self.past.push_back(previous);
+    }
+
+    pub fn undo(&mut self, current: &mut Document) -> bool {
+        let Some(previous) = self.past.pop_back() else {
+            return false;
+        };
+        let old = mem::replace(current, previous);
+        self.future.push_back(old);
+        true
+    }
+
+    pub fn redo(&mut self, current: &mut Document) -> bool {
+        let Some(next) = self.future.pop_back() else {
+            return false;
+        };
+        let old = mem::replace(current, next);
+        self.past.push_back(old);
+        true
+    }
+
+    #[must_use]
+    pub fn can_undo(&self) -> bool {
+        !self.past.is_empty()
+    }
+
+    #[must_use]
+    pub fn can_redo(&self) -> bool {
+        !self.future.is_empty()
+    }
+
+    #[must_use]
+    pub fn capacity(&self) -> NonZeroUsize {
+        self.capacity
+    }
+
+    #[must_use]
+    pub fn past_len(&self) -> usize {
+        self.past.len()
+    }
+
+    #[must_use]
+    pub fn future_len(&self) -> usize {
+        self.future.len()
+    }
+}
diff --git a/crates/bone-document/tests/evaluator.rs b/crates/bone-document/tests/evaluator.rs
new file mode 100644
index 0000000..d61c724
--- /dev/null
+++ b/crates/bone-document/tests/evaluator.rs
@@ -0,0 +1,254 @@
+use bone_document::{
+    DimensionKind, Document, EditOutcome, EvaluatedSketch, FeatureCache, FeatureNode, Sketch,
+    SketchDimension, SketchEdit, SketchEntity, SketchRelation, evaluate_sketch,
+};
+use bone_types::{
+    DocumentId, FeatureId, Length, Point2, Point3, SketchId, SketchPlaneBasis, Tolerance, UnitVec3,
+    millimeter,
+};
+use slotmap::KeyData;
+
+fn plane() -> SketchPlaneBasis {
+    let Ok(basis) = SketchPlaneBasis::new(
+        Point3::origin(),
+        UnitVec3::x_axis(),
+        UnitVec3::y_axis(),
+        Tolerance::new(1e-9),
+    ) else {
+        panic!("xy plane");
+    };
+    basis
+}
+
+fn mm(v: f64) -> Length {
+    Length::new::<millimeter>(v)
+}
+
+fn sketch_id(idx: u32) -> SketchId {
+    SketchId::from(KeyData::from_ffi((1u64 << 32) | u64::from(idx)))
+}
+
+fn document_id(idx: u32) -> DocumentId {
+    DocumentId::from(KeyData::from_ffi((1u64 << 32) | u64::from(idx)))
+}
+
+fn feature_id(idx: u32) -> FeatureId {
+    FeatureId::from(KeyData::from_ffi((1u64 << 32) | u64::from(idx)))
+}
+
+fn horizontal_line_sketch(dim_mm: f64) -> Sketch {
+    let Ok((s, EditOutcome::Entity(a))) = Sketch::new(plane()).apply(SketchEdit::AddEntity(
+        SketchEntity::point(Point2::from_mm(0.0, 0.0)),
+    )) else {
+        panic!("a");
+    };
+    let Ok((s, EditOutcome::Entity(b))) = s.apply(SketchEdit::AddEntity(SketchEntity::point(
+        Point2::from_mm(1.0, 0.0),
+    ))) else {
+        panic!("b");
+    };
+    let Ok((s, EditOutcome::Entity(line))) =
+        s.apply(SketchEdit::AddEntity(SketchEntity::line(a, b, false)))
+    else {
+        panic!("line");
+    };
+    let Ok((s, _)) = s.apply(SketchEdit::AddRelation(SketchRelation::Fix(a))) else {
+        panic!("fix");
+    };
+    let Ok((s, _)) = s.apply(SketchEdit::AddRelation(SketchRelation::Horizontal(line))) else {
+        panic!("horizontal");
+    };
+    let Ok((s, _)) = s.apply(SketchEdit::AddDimension(SketchDimension::Linear {
+        a,
+        b,
+        value: mm(dim_mm),
+        kind: DimensionKind::Driving,
+    })) else {
+        panic!("dim");
+    };
+    s
+}
+
+fn conflicting_sketch() -> Sketch {
+    let Ok((s, EditOutcome::Entity(a))) = Sketch::new(plane()).apply(SketchEdit::AddEntity(
+        SketchEntity::point(Point2::from_mm(0.0, 0.0)),
+    )) else {
+        panic!("a");
+    };
+    let Ok((s, EditOutcome::Entity(b))) = s.apply(SketchEdit::AddEntity(SketchEntity::point(
+        Point2::from_mm(1.0, 0.0),
+    ))) else {
+        panic!("b");
+    };
+    let Ok((s, _)) = s.apply(SketchEdit::AddRelation(SketchRelation::Fix(a))) else {
+        panic!("fix");
+    };
+    let Ok((s, _)) = s.apply(SketchEdit::AddDimension(SketchDimension::Linear {
+        a,
+        b,
+        value: mm(5.0),
+        kind: DimensionKind::Driving,
+    })) else {
+        panic!("dim1");
+    };
+    let Ok((s, _)) = s.apply(SketchEdit::AddDimension(SketchDimension::Linear {
+        a,
+        b,
+        value: mm(10.0),
+        kind: DimensionKind::Driving,
+    })) else {
+        panic!("dim2");
+    };
+    s
+}
+
+#[test]
+fn evaluate_sketch_returns_solved_variant_on_success() {
+    let sketch = horizontal_line_sketch(10.0);
+    let output = evaluate_sketch(&sketch);
+    let EvaluatedSketch::Solved(solved) = output else {
+        panic!("expected Solved, got {output:?}");
+    };
+    let order = solved.entity_order();
+    let b = order[1];
+    let SketchEntity::Point(p) = solved.entities()[b] else {
+        panic!("b is point");
+    };
+    let (x, y) = p.at().coords_mm();
+    assert!((x - 10.0).abs() < 1e-9, "x = {x}");
+    assert!(y.abs() < 1e-9, "y = {y}");
+}
+
+#[test]
+fn evaluate_sketch_returns_failed_variant_on_conflict() {
+    let sketch = conflicting_sketch();
+    let output = evaluate_sketch(&sketch);
+    assert!(
+        matches!(output, EvaluatedSketch::Failed(_)),
+        "expected Failed, got {output:?}"
+    );
+}
+
+#[test]
+fn feature_cache_reuses_solution_for_identical_input() {
+    let sketch = horizontal_line_sketch(10.0);
+    let mut cache = FeatureCache::new();
+    let fid = feature_id(7);
+    let first = cache.evaluate(fid, &sketch).clone();
+    let second = cache.evaluate(fid, &sketch).clone();
+    assert_eq!(first, second);
+    assert_eq!(cache.len(), 1);
+    assert!(cache.lookup(fid).is_some());
+}
+
+#[test]
+fn feature_cache_refreshes_on_input_change() {
+    let mut cache = FeatureCache::new();
+    let fid = feature_id(7);
+    let first = cache.evaluate(fid, &horizontal_line_sketch(10.0)).clone();
+    let second = cache.evaluate(fid, &horizontal_line_sketch(20.0)).clone();
+    let EvaluatedSketch::Solved(first_solved) = first else {
+        panic!("first Solved");
+    };
+    let EvaluatedSketch::Solved(second_solved) = second else {
+        panic!("second Solved");
+    };
+    assert_ne!(first_solved, second_solved);
+    assert_eq!(cache.len(), 1);
+}
+
+#[test]
+fn feature_cache_keeps_entries_per_feature() {
+    let mut cache = FeatureCache::new();
+    let a = feature_id(1);
+    let b = feature_id(2);
+    cache.evaluate(a, &horizontal_line_sketch(10.0));
+    cache.evaluate(b, &horizontal_line_sketch(20.0));
+    assert_eq!(cache.len(), 2);
+    assert!(cache.lookup(a).is_some());
+    assert!(cache.lookup(b).is_some());
+}
+
+#[test]
+fn feature_cache_invalidate_drops_entry() {
+    let mut cache = FeatureCache::new();
+    let fid = feature_id(7);
+    cache.evaluate(fid, &horizontal_line_sketch(10.0));
+    assert!(cache.invalidate(fid));
+    assert!(cache.lookup(fid).is_none());
+    assert!(!cache.invalidate(fid));
+}
+
+#[test]
+fn feature_cache_retain_drops_absent_features() {
+    let mut cache = FeatureCache::new();
+    let keep = feature_id(1);
+    let drop = feature_id(2);
+    cache.evaluate(keep, &horizontal_line_sketch(10.0));
+    cache.evaluate(drop, &horizontal_line_sketch(20.0));
+    cache.retain([keep]);
+    assert!(cache.lookup(keep).is_some());
+    assert!(cache.lookup(drop).is_none());
+    assert_eq!(cache.len(), 1);
+}
+
+#[test]
+fn document_resolves_feature_id_back_to_sketch() {
+    let mut doc = Document::new(document_id(1), "d".to_owned());
+    let sid = sketch_id(7);
+    doc.insert_sketch(sid, "S".to_owned(), horizontal_line_sketch(10.0));
+    let feature = doc.feature_tree().iter().find_map(|(fid, node)| {
+        matches!(node, FeatureNode::Sketch(id) if id == sid).then_some(fid)
+    });
+    let Some(fid) = feature else {
+        panic!("sketch feature present");
+    };
+    let Some(sketch) = doc.sketch_of_feature(fid) else {
+        panic!("sketch addressable through feature id");
+    };
+    let Some(direct) = doc.sketch(sid) else {
+        panic!("sketch in map");
+    };
+    assert_eq!(sketch, direct);
+}
+
+#[test]
+fn feature_cache_refreshes_when_feature_id_is_reused() {
+    let mut doc = Document::new(document_id(1), "d".to_owned());
+    let sid_a = sketch_id(10);
+    let sid_b = sketch_id(11);
+    doc.insert_sketch(sid_a, "A".to_owned(), horizontal_line_sketch(10.0));
+    let Some(fid_a) = doc.feature_tree().iter().find_map(|(fid, node)| {
+        matches!(node, FeatureNode::Sketch(id) if id == sid_a).then_some(fid)
+    }) else {
+        panic!("A feature present");
+    };
+
+    let mut cache = FeatureCache::new();
+    let Some(sketch_a) = doc.sketch(sid_a) else {
+        panic!("A in map");
+    };
+    let EvaluatedSketch::Solved(solved_a) = cache.evaluate(fid_a, sketch_a).clone() else {
+        panic!("A solves");
+    };
+
+    doc.remove_sketch(sid_a);
+    doc.insert_sketch(sid_b, "B".to_owned(), horizontal_line_sketch(20.0));
+    let Some(fid_b) = doc.feature_tree().iter().find_map(|(fid, node)| {
+        matches!(node, FeatureNode::Sketch(id) if id == sid_b).then_some(fid)
+    }) else {
+        panic!("B feature present");
+    };
+    assert_eq!(
+        fid_a, fid_b,
+        "precondition: FeatureTree reuses the id of a removed feature"
+    );
+
+    let Some(sketch_b) = doc.sketch(sid_b) else {
+        panic!("B in map");
+    };
+    let EvaluatedSketch::Solved(solved_b) = cache.evaluate(fid_b, sketch_b).clone() else {
+        panic!("B solves");
+    };
+    assert_ne!(solved_a, solved_b, "cache must refresh on reused FeatureId");
+}
diff --git a/crates/bone-document/tests/folder_jj.rs b/crates/bone-document/tests/folder_jj.rs
new file mode 100644
index 0000000..cd4db41
--- /dev/null
+++ b/crates/bone-document/tests/folder_jj.rs
@@ -0,0 +1,178 @@
+use std::process::Command;
+
+use bone_document::{
+    DimensionKind, Document, DocumentFolder, EditOutcome, Sketch, SketchDimension, SketchEdit,
+    SketchEntity, load, save,
+};
+use bone_types::{
+    DocumentId, Length, Point2, Point3, SketchId, SketchPlaneBasis, Tolerance, UnitVec3, millimeter,
+};
+use slotmap::KeyData;
+use tempfile::{TempDir, tempdir};
+
+fn plane() -> SketchPlaneBasis {
+    let Ok(basis) = SketchPlaneBasis::new(
+        Point3::origin(),
+        UnitVec3::x_axis(),
+        UnitVec3::y_axis(),
+        Tolerance::new(1e-9),
+    ) else {
+        panic!("xy plane");
+    };
+    basis
+}
+
+fn mm(v: f64) -> Length {
+    Length::new::<millimeter>(v)
+}
+
+fn ok_dir() -> TempDir {
+    let Ok(dir) = tempdir() else {
+        panic!("tempdir");
+    };
+    dir
+}
+
+fn sketch_id(idx: u32) -> SketchId {
+    SketchId::from(KeyData::from_ffi((1u64 << 32) | u64::from(idx)))
+}
+
+fn document_id(idx: u32) -> DocumentId {
+    DocumentId::from(KeyData::from_ffi((1u64 << 32) | u64::from(idx)))
+}
+
+fn jj_available() -> bool {
+    let Ok(out) = Command::new("jj").arg("--version").output() else {
+        return false;
+    };
+    out.status.success()
+}
+
+fn run(cmd: &mut Command) -> String {
+    let Ok(out) = cmd.output() else {
+        panic!("{cmd:?}");
+    };
+    assert!(
+        out.status.success(),
+        "{cmd:?} failed: {}\n{}",
+        String::from_utf8_lossy(&out.stdout),
+        String::from_utf8_lossy(&out.stderr),
+    );
+    String::from_utf8_lossy(&out.stdout).into_owned()
+}
+
+fn rectangle() -> Sketch {
+    let script = [
+        SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(0.0, 0.0))),
+        SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(10.0, 0.0))),
+        SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(10.0, 5.0))),
+        SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(0.0, 5.0))),
+    ];
+    let Ok((s, _)) = Sketch::new(plane()).apply_all(script) else {
+        panic!("rectangle");
+    };
+    s
+}
+
+fn assert_save(doc: &Document, folder: &DocumentFolder) {
+    let Ok(()) = save(doc, folder) else {
+        panic!("save");
+    };
+}
+
+fn assert_load(folder: &DocumentFolder) -> Document {
+    let Ok(doc) = load(folder) else {
+        panic!("load");
+    };
+    doc
+}
+
+#[test]
+fn jj_diff_on_dimension_edit_is_text_only() {
+    if !jj_available() {
+        eprintln!("skip: jj not on PATH");
+        return;
+    }
+
+    let dir = ok_dir();
+    let folder = DocumentFolder::new(dir.path().join("demo.bone"));
+
+    let mut doc = Document::new(document_id(1), "demo".to_owned());
+    let sid = sketch_id(1);
+    let rect = rectangle();
+    let Some(&a) = rect.entity_order().first() else {
+        panic!("rect has points");
+    };
+    let Some(&b) = rect.entity_order().get(1) else {
+        panic!("rect has points");
+    };
+    let Ok((rect, EditOutcome::Dimension(dim_id))) =
+        rect.apply(SketchEdit::AddDimension(SketchDimension::Linear {
+            a,
+            b,
+            value: mm(10.0),
+            kind: DimensionKind::Driving,
+        }))
+    else {
+        panic!("dim");
+    };
+    doc.insert_sketch(sid, "Sketch1".to_owned(), rect.clone());
+    assert_save(&doc, &folder);
+
+    run(Command::new("jj")
+        .arg("git")
+        .arg("init")
+        .arg("--colocate")
+        .current_dir(folder.path()));
+    run(Command::new("jj")
+        .arg("config")
+        .arg("set")
+        .arg("--repo")
+        .arg("user.name")
+        .arg("test")
+        .current_dir(folder.path()));
+    run(Command::new("jj")
+        .arg("config")
+        .arg("set")
+        .arg("--repo")
+        .arg("user.email")
+        .arg("test@example.com")
+        .current_dir(folder.path()));
+    run(Command::new("jj")
+        .arg("describe")
+        .arg("--message")
+        .arg("initial")
+        .current_dir(folder.path()));
+    run(Command::new("jj").arg("new").current_dir(folder.path()));
+
+    let reopened = assert_load(&folder);
+    let Some(sketch) = reopened.sketch(sid) else {
+        panic!("sketch missing");
+    };
+    let Ok((updated_sketch, _)) = sketch.clone().apply(SketchEdit::UpdateDimensionValue {
+        id: dim_id,
+        value: bone_document::DimensionValue::Length(mm(12.5)),
+    }) else {
+        panic!("update dim");
+    };
+    let mut next_doc = reopened.clone();
+    next_doc.replace_sketch(sid, updated_sketch);
+    assert_save(&next_doc, &folder);
+
+    let diff = run(Command::new("jj")
+        .arg("diff")
+        .arg("--git")
+        .current_dir(folder.path()));
+    assert!(
+        diff.contains("sketches/"),
+        "expected sketch file in diff:\n{diff}"
+    );
+    assert!(
+        diff.contains('+') && diff.contains('-'),
+        "expected text diff markers:\n{diff}"
+    );
+    assert!(
+        !diff.contains("Binary files"),
+        "expected no binary diff:\n{diff}"
+    );
+}
diff --git a/crates/bone-document/tests/folder_property.proptest-regressions b/crates/bone-document/tests/folder_property.proptest-regressions
new file mode 100644
index 0000000..ea91ddc
--- /dev/null
+++ b/crates/bone-document/tests/folder_property.proptest-regressions
@@ -0,0 +1,7 @@
+# Seeds for failure cases proptest has generated in the past. It is
+# automatically read and these particular cases re-run before any
+# novel cases are generated.
+#
+# It is recommended to check this file in to source control so that
+# everyone who runs the test benefits from these saved cases.
+cc b546c8b5dd00350941227512211737f6516fcf6278d102fa85ee6aa30bb41584 # shrinks to steps = [Point(0, 0), Point(0, 0), LinearDim { ai: 0, bi: 0, v: 56114, driven: false }]
diff --git a/crates/bone-document/tests/folder_property.rs b/crates/bone-document/tests/folder_property.rs
new file mode 100644
index 0000000..454b810
--- /dev/null
+++ b/crates/bone-document/tests/folder_property.rs
@@ -0,0 +1,428 @@
+use bone_document::{
+    DimensionKind, Document, DocumentFolder, Sketch, SketchDimension, SketchEdit, SketchEntity,
+    SketchEntityKind, SketchParameter, SketchRelation, load, save,
+};
+use bone_types::{
+    Angle, DocumentId, Length, Parameter, Point2, Point3, SketchId, SketchPlaneBasis, Tolerance,
+    UnitVec3, degree, millimeter,
+};
+use proptest::prelude::*;
+use slotmap::KeyData;
+use tempfile::tempdir;
+
+fn plane() -> SketchPlaneBasis {
+    let Ok(basis) = SketchPlaneBasis::new(
+        Point3::origin(),
+        UnitVec3::x_axis(),
+        UnitVec3::y_axis(),
+        Tolerance::new(1e-9),
+    ) else {
+        panic!("xy plane");
+    };
+    basis
+}
+
+fn mm(v: f64) -> Length {
+    Length::new::<millimeter>(v)
+}
+
+fn deg(v: f64) -> Angle {
+    Angle::new::<degree>(v)
+}
+
+fn sketch_id(idx: u32) -> SketchId {
+    SketchId::from(KeyData::from_ffi((1u64 << 32) | u64::from(idx)))
+}
+
+fn document_id(idx: u32) -> DocumentId {
+    DocumentId::from(KeyData::from_ffi((1u64 << 32) | u64::from(idx)))
+}
+
+#[derive(Copy, Clone, Debug)]
+enum Step {
+    Point(i16, i16),
+    Parameter(i16),
+    Line {
+        ai: u8,
+        bi: u8,
+        cons: bool,
+    },
+    Circle {
+        ci: u8,
+        r: u16,
+        cons: bool,
+    },
+    Arc {
+        ci: u8,
+        si: u8,
+        ei: u8,
+    },
+    Coincident {
+        pi: u8,
+        xi: u8,
+    },
+    Horizontal {
+        li: u8,
+    },
+    Vertical {
+        li: u8,
+    },
+    Parallel {
+        ai: u8,
+        bi: u8,
+    },
+    Perpendicular {
+        ai: u8,
+        bi: u8,
+    },
+    Tangent {
+        ai: u8,
+        bi: u8,
+    },
+    EqualLines {
+        ai: u8,
+        bi: u8,
+    },
+    EqualRounds {
+        ai: u8,
+        bi: u8,
+    },
+    Concentric {
+        ai: u8,
+        bi: u8,
+    },
+    Fix {
+        ei: u8,
+    },
+    LinearDim {
+        ai: u8,
+        bi: u8,
+        v: u16,
+        driven: bool,
+    },
+    RadiusDim {
+        ti: u8,
+        v: u16,
+        driven: bool,
+    },
+    DiameterDim {
+        ti: u8,
+        v: u16,
+        driven: bool,
+    },
+    AngularDim {
+        ai: u8,
+        bi: u8,
+        deg: u16,
+    },
+    ToggleConstruction {
+        ei: u8,
+    },
+}
+
+fn arb_step() -> impl Strategy<Value = Step> {
+    prop_oneof![
+        (any::<i16>(), any::<i16>()).prop_map(|(x, y)| Step::Point(x, y)),
+        any::<i16>().prop_map(Step::Parameter),
+        (any::<u8>(), any::<u8>(), any::<bool>()).prop_map(|(ai, bi, cons)| Step::Line {
+            ai,
+            bi,
+            cons
+        }),
+        (any::<u8>(), any::<u16>(), any::<bool>()).prop_map(|(ci, r, cons)| Step::Circle {
+            ci,
+            r: r % 200 + 1,
+            cons
+        }),
+        (any::<u8>(), any::<u8>(), any::<u8>()).prop_map(|(ci, si, ei)| Step::Arc { ci, si, ei }),
+        (any::<u8>(), any::<u8>()).prop_map(|(pi, xi)| Step::Coincident { pi, xi }),
+        any::<u8>().prop_map(|li| Step::Horizontal { li }),
+        any::<u8>().prop_map(|li| Step::Vertical { li }),
+        (any::<u8>(), any::<u8>()).prop_map(|(ai, bi)| Step::Parallel { ai, bi }),
+        (any::<u8>(), any::<u8>()).prop_map(|(ai, bi)| Step::Perpendicular { ai, bi }),
+        (any::<u8>(), any::<u8>()).prop_map(|(ai, bi)| Step::Tangent { ai, bi }),
+        (any::<u8>(), any::<u8>()).prop_map(|(ai, bi)| Step::EqualLines { ai, bi }),
+        (any::<u8>(), any::<u8>()).prop_map(|(ai, bi)| Step::EqualRounds { ai, bi }),
+        (any::<u8>(), any::<u8>()).prop_map(|(ai, bi)| Step::Concentric { ai, bi }),
+        any::<u8>().prop_map(|ei| Step::Fix { ei }),
+        (any::<u8>(), any::<u8>(), any::<u16>(), any::<bool>())
+            .prop_map(|(ai, bi, v, driven)| Step::LinearDim { ai, bi, v, driven }),
+        (any::<u8>(), any::<u16>(), any::<bool>()).prop_map(|(ti, v, driven)| Step::RadiusDim {
+            ti,
+            v: v % 200 + 1,
+            driven
+        }),
+        (any::<u8>(), any::<u16>(), any::<bool>()).prop_map(|(ti, v, driven)| Step::DiameterDim {
+            ti,
+            v: v % 200 + 1,
+            driven
+        }),
+        (any::<u8>(), any::<u8>(), any::<u16>()).prop_map(|(ai, bi, deg)| Step::AngularDim {
+            ai,
+            bi,
+            deg: deg % 359 + 1
+        }),
+        any::<u8>().prop_map(|ei| Step::ToggleConstruction { ei }),
+    ]
+}
+
+fn entities_of_kind(s: &Sketch, kind: SketchEntityKind) -> Vec<bone_types::SketchEntityId> {
+    s.entity_order()
+        .iter()
+        .copied()
+        .filter(|id| s.entities()[*id].kind() == kind)
+        .collect()
+}
+
+fn pick<T: Copy>(xs: &[T], i: u8) -> Option<T> {
+    if xs.is_empty() {
+        None
+    } else {
+        Some(xs[usize::from(i) % xs.len()])
+    }
+}
+
+fn pick_two_distinct<T: Copy + Eq>(xs: &[T], ai: u8, bi: u8) -> Option<(T, T)> {
+    if xs.len() < 2 {
+        return None;
+    }
+    let a = usize::from(ai) % xs.len();
+    let offset = usize::from(bi) % (xs.len() - 1) + 1;
+    let b = (a + offset) % xs.len();
+    Some((xs[a], xs[b]))
+}
+
+fn pick_three_distinct<T: Copy + Eq>(xs: &[T], ai: u8, bi: u8, ci: u8) -> Option<(T, T, T)> {
+    if xs.len() < 3 {
+        return None;
+    }
+    let a = usize::from(ai) % xs.len();
+    let b_off = usize::from(bi) % (xs.len() - 1) + 1;
+    let b = (a + b_off) % xs.len();
+    let remaining: Vec<_> = (0..xs.len()).filter(|&i| i != a && i != b).collect();
+    let c = remaining[usize::from(ci) % remaining.len()];
+    Some((xs[a], xs[b], xs[c]))
+}
+
+fn dim_kind(driven: bool) -> DimensionKind {
+    if driven {
+        DimensionKind::Driven
+    } else {
+        DimensionKind::Driving
+    }
+}
+
+fn rounds(s: &Sketch) -> Vec<bone_types::SketchEntityId> {
+    entities_of_kind(s, SketchEntityKind::Arc)
+        .into_iter()
+        .chain(entities_of_kind(s, SketchEntityKind::Circle))
+        .collect()
+}
+
+fn resolve(s: &Sketch, step: Step) -> Option<SketchEdit> {
+    match step {
+        Step::Point(..) | Step::Parameter(..) => Some(resolve_atom(step)),
+        Step::Line { .. } | Step::Circle { .. } | Step::Arc { .. } => resolve_entity(s, step),
+        Step::Coincident { .. }
+        | Step::Horizontal { .. }
+        | Step::Vertical { .. }
+        | Step::Parallel { .. }
+        | Step::Perpendicular { .. }
+        | Step::Tangent { .. }
+        | Step::EqualLines { .. }
+        | Step::EqualRounds { .. }
+        | Step::Concentric { .. }
+        | Step::Fix { .. } => resolve_relation(s, step),
+        Step::LinearDim { .. }
+        | Step::RadiusDim { .. }
+        | Step::DiameterDim { .. }
+        | Step::AngularDim { .. } => resolve_dimension(s, step),
+        Step::ToggleConstruction { ei } => {
+            let id = pick(s.entity_order(), ei)?;
+            (!s.entities()[id].is_point()).then_some(SketchEdit::SetConstruction {
+                id,
+                for_construction: true,
+            })
+        }
+    }
+}
+
+fn resolve_atom(step: Step) -> SketchEdit {
+    match step {
+        Step::Point(x, y) => SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(
+            f64::from(x) / 100.0,
+            f64::from(y) / 100.0,
+        ))),
+        Step::Parameter(v) => {
+            SketchEdit::AddParameter(SketchParameter::new(Parameter::new(f64::from(v) / 100.0)))
+        }
+        _ => unreachable!("caller routes only Point and Parameter here"),
+    }
+}
+
+fn resolve_entity(s: &Sketch, step: Step) -> Option<SketchEdit> {
+    match step {
+        Step::Line { ai, bi, cons } => {
+            let (a, b) = pick_two_distinct(&entities_of_kind(s, SketchEntityKind::Point), ai, bi)?;
+            Some(SketchEdit::AddEntity(SketchEntity::line(a, b, cons)))
+        }
+        Step::Circle { ci, r, cons } => {
+            let c = pick(&entities_of_kind(s, SketchEntityKind::Point), ci)?;
+            Some(SketchEdit::AddEntity(SketchEntity::circle(
+                c,
+                mm(f64::from(r)),
+                cons,
+            )))
+        }
+        Step::Arc { ci, si, ei } => {
+            let (c, start, end) =
+                pick_three_distinct(&entities_of_kind(s, SketchEntityKind::Point), ci, si, ei)?;
+            Some(SketchEdit::AddEntity(SketchEntity::arc(
+                c, start, end, false,
+            )))
+        }
+        _ => unreachable!("caller routes only Line, Circle, Arc here"),
+    }
+}
+
+fn resolve_relation(s: &Sketch, step: Step) -> Option<SketchEdit> {
+    let rel = match step {
+        Step::Coincident { pi, xi } => {
+            let p = pick(&entities_of_kind(s, SketchEntityKind::Point), pi)?;
+            let others: Vec<_> = s
+                .entity_order()
+                .iter()
+                .copied()
+                .filter(|id| *id != p)
+                .collect();
+            SketchRelation::Coincident(p, pick(&others, xi)?)
+        }
+        Step::Horizontal { li } => {
+            SketchRelation::Horizontal(pick(&entities_of_kind(s, SketchEntityKind::Line), li)?)
+        }
+        Step::Vertical { li } => {
+            SketchRelation::Vertical(pick(&entities_of_kind(s, SketchEntityKind::Line), li)?)
+        }
+        Step::Parallel { ai, bi } => {
+            let (a, b) = pick_two_distinct(&entities_of_kind(s, SketchEntityKind::Line), ai, bi)?;
+            SketchRelation::Parallel(a, b)
+        }
+        Step::Perpendicular { ai, bi } => {
+            let (a, b) = pick_two_distinct(&entities_of_kind(s, SketchEntityKind::Line), ai, bi)?;
+            SketchRelation::Perpendicular(a, b)
+        }
+        Step::Tangent { ai, bi } => {
+            let round = rounds(s);
+            let lines = entities_of_kind(s, SketchEntityKind::Line);
+            let a = pick(&round, ai)?;
+            let b = if lines.is_empty() {
+                let remaining: Vec<_> = round.iter().copied().filter(|id| *id != a).collect();
+                pick(&remaining, bi)?
+            } else {
+                pick(&lines, bi)?
+            };
+            SketchRelation::Tangent(a, b)
+        }
+        Step::EqualLines { ai, bi } => {
+            let (a, b) = pick_two_distinct(&entities_of_kind(s, SketchEntityKind::Line), ai, bi)?;
+            SketchRelation::Equal(a, b)
+        }
+        Step::EqualRounds { ai, bi } => {
+            let (a, b) = pick_two_distinct(&rounds(s), ai, bi)?;
+            SketchRelation::Equal(a, b)
+        }
+        Step::Concentric { ai, bi } => {
+            let (a, b) = pick_two_distinct(&rounds(s), ai, bi)?;
+            SketchRelation::Concentric(a, b)
+        }
+        Step::Fix { ei } => SketchRelation::Fix(pick(s.entity_order(), ei)?),
+        _ => unreachable!("caller routes only relations here"),
+    };
+    Some(SketchEdit::AddRelation(rel))
+}
+
+fn resolve_dimension(s: &Sketch, step: Step) -> Option<SketchEdit> {
+    let dim = match step {
+        Step::LinearDim { ai, bi, v, driven } => {
+            let (a, b) = pick_two_distinct(&entities_of_kind(s, SketchEntityKind::Point), ai, bi)?;
+            SketchDimension::Linear {
+                a,
+                b,
+                value: mm(f64::from(v)),
+                kind: dim_kind(driven),
+            }
+        }
+        Step::RadiusDim { ti, v, driven } => SketchDimension::Radius {
+            target: pick(&rounds(s), ti)?,
+            value: mm(f64::from(v)),
+            kind: dim_kind(driven),
+        },
+        Step::DiameterDim { ti, v, driven } => SketchDimension::Diameter {
+            target: pick(&rounds(s), ti)?,
+            value: mm(f64::from(v)),
+            kind: dim_kind(driven),
+        },
+        Step::AngularDim { ai, bi, deg: d } => {
+            let (a, b) = pick_two_distinct(&entities_of_kind(s, SketchEntityKind::Line), ai, bi)?;
+            SketchDimension::Angular {
+                a,
+                b,
+                value: deg(f64::from(d)),
+                kind: DimensionKind::Driving,
+            }
+        }
+        _ => unreachable!("caller routes only dimensions here"),
+    };
+    Some(SketchEdit::AddDimension(dim))
+}
+
+fn build(steps: Vec<Step>) -> Sketch {
+    steps.into_iter().fold(Sketch::new(plane()), |sk, step| {
+        resolve(&sk, step)
+            .and_then(|edit| sk.clone().apply(edit).ok().map(|(s, _)| s))
+            .unwrap_or(sk)
+    })
+}
+
+proptest! {
+    #![proptest_config(ProptestConfig {
+        cases: 32,
+        .. ProptestConfig::default()
+    })]
+
+    #[test]
+    fn load_save_roundtrip_preserves_sketch(steps in prop::collection::vec(arb_step(), 0..30)) {
+        let sketch = build(steps);
+        let dir = tempdir().map_err(|e| TestCaseError::Fail(format!("tempdir: {e}").into()))?;
+        let folder = DocumentFolder::new(dir.path().join("fuzz.bone"));
+        let mut doc = Document::new(document_id(1), "fuzz".to_owned());
+        doc.insert_sketch(sketch_id(1), "S".to_owned(), sketch.clone());
+        save(&doc, &folder).map_err(|e| TestCaseError::Fail(format!("save: {e}").into()))?;
+        let loaded = load(&folder).map_err(|e| TestCaseError::Fail(format!("load: {e}").into()))?;
+        let Some(round) = loaded.sketch(sketch_id(1)) else {
+            return Err(TestCaseError::Fail("sketch missing".into()));
+        };
+        prop_assert_eq!(round, &sketch);
+    }
+
+    #[test]
+    fn double_save_is_bit_identical(steps in prop::collection::vec(arb_step(), 0..30)) {
+        let sketch = build(steps);
+        let dir = tempdir().map_err(|e| TestCaseError::Fail(format!("tempdir: {e}").into()))?;
+        let folder = DocumentFolder::new(dir.path().join("det.bone"));
+        let mut doc = Document::new(document_id(1), "det".to_owned());
+        doc.insert_sketch(sketch_id(1), "S".to_owned(), sketch);
+        save(&doc, &folder).map_err(|e| TestCaseError::Fail(format!("save: {e}").into()))?;
+        let first_doc = std::fs::read(folder.document_file())
+            .map_err(|e| TestCaseError::Fail(format!("read: {e}").into()))?;
+        let first_sketch = std::fs::read(folder.sketch_path(sketch_id(1)))
+            .map_err(|e| TestCaseError::Fail(format!("read: {e}").into()))?;
+        save(&doc, &folder).map_err(|e| TestCaseError::Fail(format!("save2: {e}").into()))?;
+        let second_doc = std::fs::read(folder.document_file())
+            .map_err(|e| TestCaseError::Fail(format!("read: {e}").into()))?;
+        let second_sketch = std::fs::read(folder.sketch_path(sketch_id(1)))
+            .map_err(|e| TestCaseError::Fail(format!("read: {e}").into()))?;
+        prop_assert_eq!(first_doc, second_doc);
+        prop_assert_eq!(first_sketch, second_sketch);
+    }
+}
diff --git a/crates/bone-document/tests/folder_roundtrip.rs b/crates/bone-document/tests/folder_roundtrip.rs
new file mode 100644
index 0000000..bedff61
--- /dev/null
+++ b/crates/bone-document/tests/folder_roundtrip.rs
@@ -0,0 +1,610 @@
+use bone_document::{
+    BlobHash, BlobKind, Document, DocumentFolder, DocumentHeader, EditOutcome, Sketch,
+    SketchDimension, SketchEdit, SketchEntity, SketchRegistry, SketchRegistryEntry, SketchRelation,
+    from_str, load, save, to_string,
+};
+use bone_types::{
+    Angle, DocumentId, Length, Point2, Point3, SketchId, SketchPlaneBasis, Tolerance, UnitVec3,
+    degree, millimeter,
+};
+use slotmap::{Key, KeyData};
+use tempfile::{TempDir, tempdir};
+
+fn plane() -> SketchPlaneBasis {
+    let Ok(basis) = SketchPlaneBasis::new(
+        Point3::origin(),
+        UnitVec3::x_axis(),
+        UnitVec3::y_axis(),
+        Tolerance::new(1e-9),
+    ) else {
+        panic!("xy plane basis is orthogonal");
+    };
+    basis
+}
+
+fn mm(v: f64) -> Length {
+    Length::new::<millimeter>(v)
+}
+
+fn deg(v: f64) -> Angle {
+    Angle::new::<degree>(v)
+}
+
+fn ok_dir() -> TempDir {
+    let Ok(dir) = tempdir() else {
+        panic!("tempdir");
+    };
+    dir
+}
+
+fn sketch_id(idx: u32) -> SketchId {
+    SketchId::from(KeyData::from_ffi((1u64 << 32) | u64::from(idx)))
+}
+
+fn document_id(idx: u32) -> DocumentId {
+    DocumentId::from(KeyData::from_ffi((1u64 << 32) | u64::from(idx)))
+}
+
+fn rectangle() -> Sketch {
+    let script = [
+        SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(0.0, 0.0))),
+        SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(10.0, 0.0))),
+        SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(10.0, 5.0))),
+        SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(0.0, 5.0))),
+    ];
+    let Ok((s, _)) = Sketch::new(plane()).apply_all(script) else {
+        panic!("rectangle");
+    };
+    s
+}
+
+fn assert_save(doc: &Document, folder: &DocumentFolder) {
+    let Ok(()) = save(doc, folder) else {
+        panic!("save clean");
+    };
+}
+
+fn assert_load(folder: &DocumentFolder) -> Document {
+    let Ok(doc) = load(folder) else {
+        panic!("load clean");
+    };
+    doc
+}
+
+fn read_file(path: &std::path::Path) -> String {
+    let Ok(text) = std::fs::read_to_string(path) else {
+        panic!("read {}", path.display());
+    };
+    text
+}
+
+fn write_file(path: &std::path::Path, contents: &str) {
+    let Ok(()) = std::fs::write(path, contents) else {
+        panic!("write {}", path.display());
+    };
+}
+
+#[test]
+fn rectangle_roundtrips_through_folder() {
+    let dir = ok_dir();
+    let folder = DocumentFolder::new(dir.path().join("part.bone"));
+
+    let mut doc = Document::new(document_id(1), "part".to_owned());
+    doc.insert_sketch(sketch_id(1), "Sketch1".to_owned(), rectangle());
+    doc.set_parameter("width".to_owned(), 10.0);
+
+    assert_save(&doc, &folder);
+    let loaded = assert_load(&folder);
+
+    assert_eq!(loaded.name(), doc.name());
+    assert_eq!(loaded.registry().order(), doc.registry().order());
+    assert_eq!(loaded.feature_tree(), doc.feature_tree());
+    assert_eq!(loaded.sketches_map(), doc.sketches_map());
+    assert_eq!(loaded.parameters().get("width"), Some(10.0));
+}
+
+#[test]
+fn rich_sketch_with_every_variant_roundtrips() {
+    let dir = ok_dir();
+    let folder = DocumentFolder::new(dir.path().join("rich.bone"));
+
+    let mut s = Sketch::new(plane());
+    let Ok((s_next, EditOutcome::Entity(p0))) = s.apply(SketchEdit::AddEntity(
+        SketchEntity::point(Point2::from_mm(0.0, 0.0)),
+    )) else {
+        panic!("p0");
+    };
+    s = s_next;
+    let Ok((s_next, EditOutcome::Entity(p1))) = s.apply(SketchEdit::AddEntity(
+        SketchEntity::point(Point2::from_mm(10.0, 0.0)),
+    )) else {
+        panic!("p1");
+    };
+    s = s_next;
+    let Ok((s_next, EditOutcome::Entity(p2))) = s.apply(SketchEdit::AddEntity(
+        SketchEntity::point(Point2::from_mm(5.0, 6.0)),
+    )) else {
+        panic!("p2");
+    };
+    s = s_next;
+
+    let Ok((s_next, EditOutcome::Entity(line))) =
+        s.apply(SketchEdit::AddEntity(SketchEntity::line(p0, p1, false)))
+    else {
+        panic!("line");
+    };
+    s = s_next;
+    let Ok((s_next, EditOutcome::Entity(line2))) =
+        s.apply(SketchEdit::AddEntity(SketchEntity::line(p0, p2, true)))
+    else {
+        panic!("line2");
+    };
+    s = s_next;
+    let Ok((s_next, EditOutcome::Entity(arc))) =
+        s.apply(SketchEdit::AddEntity(SketchEntity::arc(p0, p1, p2, false)))
+    else {
+        panic!("arc");
+    };
+    s = s_next;
+    let Ok((s_next, EditOutcome::Entity(circle))) = s.apply(SketchEdit::AddEntity(
+        SketchEntity::circle(p0, mm(3.0), false),
+    )) else {
+        panic!("circle");
+    };
+    s = s_next;
+
+    for relation in [
+        SketchRelation::Coincident(p0, line),
+        SketchRelation::Horizontal(line),
+        SketchRelation::Vertical(line2),
+        SketchRelation::Parallel(line, line2),
+        SketchRelation::Perpendicular(line, line2),
+        SketchRelation::Tangent(line, circle),
+        SketchRelation::Equal(line, line2),
+        SketchRelation::Concentric(arc, circle),
+        SketchRelation::Fix(p0),
+    ] {
+        let Ok((next, _)) = s.apply(SketchEdit::AddRelation(relation)) else {
+            panic!("relation {relation:?}");
+        };
+        s = next;
+    }
+
+    for dim in [
+        SketchDimension::Linear {
+            a: p0,
+            b: p1,
+            value: mm(10.0),
+            kind: bone_document::DimensionKind::Driving,
+        },
+        SketchDimension::Radius {
+            target: circle,
+            value: mm(3.0),
+            kind: bone_document::DimensionKind::Driven,
+        },
+        SketchDimension::Diameter {
+            target: circle,
+            value: mm(6.0),
+            kind: bone_document::DimensionKind::Driving,
+        },
+        SketchDimension::Angular {
+            a: line,
+            b: line2,
+            value: deg(45.0),
+            kind: bone_document::DimensionKind::Driving,
+        },
+    ] {
+        let Ok((next, _)) = s.apply(SketchEdit::AddDimension(dim)) else {
+            panic!("dim {dim:?}");
+        };
+        s = next;
+    }
+
+    let mut doc = Document::new(document_id(1), "rich".to_owned());
+    doc.insert_sketch(sketch_id(1), "Rich".to_owned(), s.clone());
+
+    assert_save(&doc, &folder);
+    let loaded = assert_load(&folder);
+    let Some(loaded_sketch) = loaded.sketch(sketch_id(1)) else {
+        panic!("sketch missing after load");
+    };
+    assert_eq!(loaded_sketch, &s);
+}
+
+#[test]
+fn load_refuses_unknown_schema_major() {
+    use bone_types::{SchemaHeader, SchemaVersion};
+
+    let dir = ok_dir();
+    let folder = DocumentFolder::new(dir.path().join("future.bone"));
+    let mut doc = Document::new(document_id(1), "future".to_owned());
+    doc.insert_sketch(sketch_id(1), "Sketch1".to_owned(), rectangle());
+    assert_save(&doc, &folder);
+
+    let header_path = folder.document_file();
+    let text = read_file(&header_path);
+    write_file(&header_path, &text.replace("major: 1,", "major: 9999,"));
+
+    let load_result = load(&folder).map_err(bone_document::FolderError::into_kind);
+    let Err(bone_document::FolderErrorKind::UnsupportedMajor {
+        found, supported, ..
+    }) = load_result
+    else {
+        panic!("expected UnsupportedMajor");
+    };
+    assert_eq!(found, SchemaVersion::new(9999, 0));
+    assert_eq!(
+        supported,
+        SchemaVersion::new(
+            SchemaHeader::BONE_DOCUMENT_MAJOR,
+            SchemaHeader::BONE_DOCUMENT_MINOR
+        )
+    );
+}
+
+#[test]
+fn load_refuses_unknown_schema_name() {
+    let dir = ok_dir();
+    let folder = DocumentFolder::new(dir.path().join("foreign.bone"));
+    let mut doc = Document::new(document_id(1), "foreign".to_owned());
+    doc.insert_sketch(sketch_id(1), "Sketch1".to_owned(), rectangle());
+    assert_save(&doc, &folder);
+
+    let header_path = folder.document_file();
+    let text = read_file(&header_path);
+    write_file(
+        &header_path,
+        &text.replace("\"bone-document\"", "\"kicad-pcb\""),
+    );
+
+    let load_result = load(&folder).map_err(bone_document::FolderError::into_kind);
+    let Err(bone_document::FolderErrorKind::UnknownSchema { found, .. }) = load_result else {
+        panic!("expected UnknownSchema");
+    };
+    assert_eq!(found, "kicad-pcb");
+}
+
+#[test]
+fn save_ships_vcs_files() {
+    let dir = ok_dir();
+    let folder = DocumentFolder::new(dir.path().join("vcs.bone"));
+    let doc = Document::new(document_id(1), "vcs".to_owned());
+    assert_save(&doc, &folder);
+
+    let gitignore = read_file(&folder.path().join(".gitignore"));
+    assert!(gitignore.contains("caches/"));
+    let gitattributes = read_file(&folder.path().join(".gitattributes"));
+    assert!(gitattributes.contains("* text=auto eol=lf"));
+    assert!(gitattributes.contains("*.boneblob binary"));
+    let caches_tag = read_file(&folder.caches_dir().join("CACHEDIR.TAG"));
+    assert!(caches_tag.starts_with("Signature: 8a477f597d28d172789f06886806bc55"));
+    let caches_gitignore = read_file(&folder.caches_dir().join(".gitignore"));
+    assert_eq!(caches_gitignore, "*\n!.gitignore\n!CACHEDIR.TAG\n");
+}
+
+#[test]
+fn save_is_idempotent_byte_for_byte() {
+    let dir = ok_dir();
+    let folder = DocumentFolder::new(dir.path().join("idem.bone"));
+    let mut doc = Document::new(document_id(1), "idem".to_owned());
+    doc.insert_sketch(sketch_id(1), "S".to_owned(), rectangle());
+    assert_save(&doc, &folder);
+    let Ok(first) = std::fs::read(folder.document_file()) else {
+        panic!("read first");
+    };
+    assert_save(&doc, &folder);
+    let Ok(second) = std::fs::read(folder.document_file()) else {
+        panic!("read second");
+    };
+    assert_eq!(first, second, "document.ron drifted between saves");
+}
+
+#[test]
+fn loaded_document_preserves_slotmap_keys() {
+    let dir = ok_dir();
+    let folder = DocumentFolder::new(dir.path().join("keys.bone"));
+
+    let mut s = Sketch::new(plane());
+    let Ok((s_next, EditOutcome::Entity(a))) = s.apply(SketchEdit::AddEntity(SketchEntity::point(
+        Point2::from_mm(0.0, 0.0),
+    ))) else {
+        panic!("a");
+    };
+    s = s_next;
+    let Ok((s_next, EditOutcome::Entity(b))) = s.apply(SketchEdit::AddEntity(SketchEntity::point(
+        Point2::from_mm(1.0, 0.0),
+    ))) else {
+        panic!("b");
+    };
+    s = s_next;
+    let Ok((s_next, _)) = s.apply(SketchEdit::AddEntity(SketchEntity::line(a, b, false))) else {
+        panic!("line");
+    };
+    s = s_next;
+
+    let mut doc = Document::new(document_id(1), "keys".to_owned());
+    let id = sketch_id(1);
+    doc.insert_sketch(id, "S".to_owned(), s.clone());
+    assert_save(&doc, &folder);
+
+    let loaded = assert_load(&folder);
+    let Some(loaded_sketch) = loaded.sketch(id) else {
+        panic!("sketch");
+    };
+    let first_id = loaded_sketch.entity_order()[0];
+    assert_eq!(first_id.data(), a.data());
+    let entries: Vec<(SketchId, SketchRegistryEntry)> = loaded
+        .registry()
+        .iter()
+        .map(|(id, e)| (id, SketchRegistryEntry::clone(e)))
+        .collect();
+    assert_eq!(entries.len(), 1);
+    let (_, entry) = &entries[0];
+    assert_eq!(entry.label, "S");
+    assert!(
+        std::path::Path::new(&entry.filename)
+            .extension()
+            .is_some_and(|ext| ext.eq_ignore_ascii_case("ron"))
+    );
+}
+
+#[test]
+fn feature_tree_bytes_survive_insert_remove_insert_cycle() {
+    let dir = ok_dir();
+    let with_history = DocumentFolder::new(dir.path().join("hist.bone"));
+    let fresh = DocumentFolder::new(dir.path().join("fresh.bone"));
+
+    let rect = rectangle();
+    let mut doc_a = Document::new(document_id(1), "doc".to_owned());
+    doc_a.insert_sketch(sketch_id(1), "S".to_owned(), rect.clone());
+    doc_a.insert_sketch(sketch_id(2), "T".to_owned(), rect.clone());
+    doc_a.remove_sketch(sketch_id(2));
+    assert_save(&doc_a, &with_history);
+
+    let mut doc_b = Document::new(document_id(1), "doc".to_owned());
+    doc_b.insert_sketch(sketch_id(1), "S".to_owned(), rect);
+    assert_save(&doc_b, &fresh);
+
+    let bytes_a = read_file(&with_history.document_file());
+    let bytes_b = read_file(&fresh.document_file());
+    assert_eq!(
+        bytes_a, bytes_b,
+        "remove then re-insert leaks tombstones into document.ron"
+    );
+}
+
+#[test]
+fn push_sketch_is_idempotent_on_duplicate_id() {
+    let mut doc = Document::new(document_id(1), "dup".to_owned());
+    let id = sketch_id(1);
+    doc.insert_sketch(id, "S".to_owned(), rectangle());
+    doc.insert_sketch(id, "S-renamed".to_owned(), rectangle());
+    let sketch_nodes = doc
+        .feature_tree()
+        .iter()
+        .filter(|(_, n)| matches!(n, bone_document::FeatureNode::Sketch(_)))
+        .count();
+    assert_eq!(
+        sketch_nodes, 1,
+        "duplicate sketch id must not spawn a second feature node"
+    );
+}
+
+#[test]
+fn removing_sketch_drops_stale_file() {
+    let dir = ok_dir();
+    let folder = DocumentFolder::new(dir.path().join("drop.bone"));
+    let mut doc = Document::new(document_id(1), "drop".to_owned());
+    let id = sketch_id(1);
+    doc.insert_sketch(id, "S".to_owned(), rectangle());
+    assert_save(&doc, &folder);
+    assert!(folder.sketch_path(id).exists());
+    doc.remove_sketch(id);
+    assert_save(&doc, &folder);
+    assert!(!folder.sketch_path(id).exists());
+}
+
+#[test]
+fn registry_kept_sorted_by_order_across_roundtrip() {
+    let dir = ok_dir();
+    let folder = DocumentFolder::new(dir.path().join("order.bone"));
+    let mut doc = Document::new(document_id(1), "order".to_owned());
+    let rect = rectangle();
+    let a = sketch_id(1);
+    let b = sketch_id(2);
+    let c = sketch_id(3);
+    doc.insert_sketch(a, "A".to_owned(), rect.clone());
+    doc.insert_sketch(b, "B".to_owned(), rect.clone());
+    doc.insert_sketch(c, "C".to_owned(), rect);
+    assert_save(&doc, &folder);
+    let loaded = assert_load(&folder);
+    assert_eq!(loaded.registry().order(), &[a, b, c]);
+    let _: &SketchRegistry = loaded.registry();
+}
+
+#[test]
+fn f64_special_values_roundtrip_bit_identically() {
+    let dir = ok_dir();
+    let folder = DocumentFolder::new(dir.path().join("bits.bone"));
+    let specials: [(f64, f64); 7] = [
+        (-0.0, 0.0),
+        (0.0, 1.0),
+        (f64::INFINITY, 2.0),
+        (f64::NEG_INFINITY, 3.0),
+        (f64::MIN_POSITIVE, 4.0),
+        (f64::MIN_POSITIVE / 2.0, 5.0),
+        (1.0 + f64::EPSILON, 6.0),
+    ];
+    let sketch = specials
+        .into_iter()
+        .try_fold(Sketch::new(plane()), |s, (x, y)| {
+            s.apply(SketchEdit::AddEntity(SketchEntity::point(Point2::from_mm(
+                x, y,
+            ))))
+            .map(|(next, _)| next)
+        });
+    let Ok(sketch) = sketch else {
+        panic!("apply points");
+    };
+    let mut doc = Document::new(document_id(1), "bits".to_owned());
+    doc.insert_sketch(sketch_id(1), "S".to_owned(), sketch.clone());
+    assert_save(&doc, &folder);
+    let loaded = assert_load(&folder);
+    let Some(back) = loaded.sketch(sketch_id(1)) else {
+        panic!("sketch missing");
+    };
+    let coords = |s: &Sketch| -> Vec<(f64, f64)> {
+        s.entity_order()
+            .iter()
+            .map(|id| match s.entities()[*id] {
+                SketchEntity::Point(p) => p.at().coords_mm(),
+                other => panic!("expected Point, got {other:?}"),
+            })
+            .collect()
+    };
+    let originals = coords(&sketch);
+    let reloaded = coords(back);
+    assert_eq!(originals.len(), reloaded.len());
+    originals
+        .iter()
+        .zip(reloaded.iter())
+        .for_each(|((ax, ay), (bx, by))| {
+            assert_eq!(ax.to_bits(), bx.to_bits(), "x bits drift: {ax} -> {bx}");
+            assert_eq!(ay.to_bits(), by.to_bits(), "y bits drift: {ay} -> {by}");
+        });
+}
+
+#[test]
+fn load_refuses_sketch_file_unsupported_major() {
+    let dir = ok_dir();
+    let folder = DocumentFolder::new(dir.path().join("bad-sketch-schema.bone"));
+    let mut doc = Document::new(document_id(1), "bad".to_owned());
+    doc.insert_sketch(sketch_id(1), "S".to_owned(), rectangle());
+    assert_save(&doc, &folder);
+
+    let sketch_path = folder.sketch_path(sketch_id(1));
+    let text = read_file(&sketch_path);
+    let patched = text.replace("major: 1,", "major: 9999,");
+    assert_ne!(patched, text, "schema major pattern missing");
+    write_file(&sketch_path, &patched);
+
+    let result = load(&folder).map_err(bone_document::FolderError::into_kind);
+    let Err(bone_document::FolderErrorKind::UnsupportedMajor { name, found, .. }) = result else {
+        panic!("expected UnsupportedMajor on sketch file, got {result:?}");
+    };
+    assert_eq!(name, "bone-document");
+    assert_eq!(found.major, 9999);
+}
+
+#[test]
+fn load_refuses_tampered_sketch_integrity() {
+    let dir = ok_dir();
+    let folder = DocumentFolder::new(dir.path().join("neg.bone"));
+    let Ok((s, EditOutcome::Entity(center))) = Sketch::new(plane()).apply(SketchEdit::AddEntity(
+        SketchEntity::point(Point2::from_mm(0.0, 0.0)),
+    )) else {
+        panic!("center");
+    };
+    let Ok((s, _)) = s.apply(SketchEdit::AddEntity(SketchEntity::circle(
+        center,
+        mm(1.0),
+        false,
+    ))) else {
+        panic!("circle");
+    };
+    let mut doc = Document::new(document_id(1), "neg".to_owned());
+    doc.insert_sketch(sketch_id(1), "S".to_owned(), s);
+    assert_save(&doc, &folder);
+
+    let sketch_path = folder.sketch_path(sketch_id(1));
+    let text = read_file(&sketch_path);
+    let patched = text.replace("radius: 0.001,", "radius: -0.001,");
+    assert_ne!(patched, text, "radius pattern missing");
+    write_file(&sketch_path, &patched);
+
+    let result = load(&folder).map_err(bone_document::FolderError::into_kind);
+    let Err(bone_document::FolderErrorKind::SketchIntegrity { source, .. }) = result else {
+        panic!("expected SketchIntegrity, got {result:?}");
+    };
+    assert!(matches!(
+        source,
+        bone_document::SketchEditError::DegenerateEntity(_)
+    ));
+}
+
+fn patch_header(path: &std::path::Path, f: impl FnOnce(&mut DocumentHeader)) {
+    let text = read_file(path);
+    let Ok(mut header) = from_str::<DocumentHeader>(&text) else {
+        panic!("parse header");
+    };
+    f(&mut header);
+    let Ok(out) = to_string(&header) else {
+        panic!("serialise header");
+    };
+    write_file(path, &out);
+}
+
+#[test]
+fn load_refuses_dangling_feature_tree_sketch() {
+    let dir = ok_dir();
+    let folder = DocumentFolder::new(dir.path().join("dangle.bone"));
+    let mut doc = Document::new(document_id(1), "dangle".to_owned());
+    doc.insert_sketch(sketch_id(1), "S".to_owned(), rectangle());
+    assert_save(&doc, &folder);
+
+    patch_header(&folder.document_file(), |h| {
+        h.feature_tree.push_sketch(sketch_id(99));
+    });
+
+    let result = load(&folder).map_err(bone_document::FolderError::into_kind);
+    let Err(bone_document::FolderErrorKind::DanglingTreeSketch { id }) = result else {
+        panic!("expected DanglingTreeSketch, got {result:?}");
+    };
+    assert_eq!(id, sketch_id(99));
+}
+
+#[test]
+fn load_refuses_orphan_registered_sketch() {
+    let dir = ok_dir();
+    let folder = DocumentFolder::new(dir.path().join("orphan.bone"));
+    let mut doc = Document::new(document_id(1), "orphan".to_owned());
+    doc.insert_sketch(sketch_id(1), "S".to_owned(), rectangle());
+    assert_save(&doc, &folder);
+
+    patch_header(&folder.document_file(), |h| {
+        h.feature_tree.remove_sketch(sketch_id(1));
+    });
+
+    let result = load(&folder).map_err(bone_document::FolderError::into_kind);
+    let Err(bone_document::FolderErrorKind::OrphanRegistered { id }) = result else {
+        panic!("expected OrphanRegistered, got {result:?}");
+    };
+    assert_eq!(id, sketch_id(1));
+}
+
+#[test]
+fn blob_path_matches_locked_shape() {
+    let folder = DocumentFolder::new("/tmp/part.bone");
+    let hash = BlobHash::of(b"example");
+    let path = folder.blob_path(hash, BlobKind::STEP);
+    let Ok(suffix) = path.strip_prefix("/tmp/part.bone/blobs") else {
+        panic!("blob under blobs/: {}", path.display());
+    };
+    let components: Vec<String> = suffix
+        .components()
+        .map(|c| c.as_os_str().to_string_lossy().into_owned())
+        .collect();
+    assert_eq!(
+        components.len(),
+        2,
+        "<aa>/<rest>.<kind>: got {components:?}"
+    );
+    assert_eq!(components[0].len(), 2, "fanout is 2 hex chars");
+    let Some(stem) = components[1].strip_suffix(".step") else {
+        panic!("extension is .step: {}", components[1]);
+    };
+    assert_eq!(stem.len(), 30, "remaining 30 hex chars");
+    let hex = hash.truncated_128_hex();
+    assert_eq!(components[0], hex[..2]);
+    assert_eq!(stem, &hex[2..]);
+}
diff --git a/crates/bone-document/tests/folder_snapshots.rs b/crates/bone-document/tests/folder_snapshots.rs
new file mode 100644
index 0000000..9e50f06
--- /dev/null
+++ b/crates/bone-document/tests/folder_snapshots.rs
@@ -0,0 +1,244 @@
+use bone_document::{
+    DimensionKind, Document, DocumentFolder, DocumentHeader, Sketch, SketchDimension, SketchEdit,
+    SketchEntity, SketchFile, SketchRelation, save, to_string,
+};
+use bone_types::{
+    Angle, DocumentId, Length, Point2, Point3, SketchEntityId, SketchId, SketchPlaneBasis,
+    Tolerance, UnitVec3, degree, millimeter,
+};
+use slotmap::KeyData;
+use tempfile::{TempDir, tempdir};
+
+fn plane() -> SketchPlaneBasis {
+    let Ok(basis) = SketchPlaneBasis::new(
+        Point3::origin(),
+        UnitVec3::x_axis(),
+        UnitVec3::y_axis(),
+        Tolerance::new(1e-9),
+    ) else {
+        panic!("xy plane");
+    };
+    basis
+}
+
+fn mm(v: f64) -> Length {
+    Length::new::<millimeter>(v)
+}
+
+fn deg(v: f64) -> Angle {
+    Angle::new::<degree>(v)
+}
+
+fn ok_dir() -> TempDir {
+    let Ok(dir) = tempdir() else {
+        panic!("tempdir");
+    };
+    dir
+}
+
+fn sketch_id(idx: u32) -> SketchId {
+    SketchId::from(KeyData::from_ffi((1u64 << 32) | u64::from(idx)))
+}
+
+fn document_id(idx: u32) -> DocumentId {
+    DocumentId::from(KeyData::from_ffi((1u64 << 32) | u64::from(idx)))
+}
+
+fn point_of(s: &Sketch, i: usize) -> SketchEntityId {
+    s.entity_order()[i]
+}
+
+fn add_entity(s: Sketch, entity: SketchEntity) -> Sketch {
+    let Ok((next, _)) = s.apply(SketchEdit::AddEntity(entity)) else {
+        panic!("add entity");
+    };
+    next
+}
+
+fn add_relation(s: Sketch, relation: SketchRelation) -> Sketch {
+    let Ok((next, _)) = s.apply(SketchEdit::AddRelation(relation)) else {
+        panic!("add relation");
+    };
+    next
+}
+
+fn add_dimension(s: Sketch, dim: SketchDimension) -> Sketch {
+    let Ok((next, _)) = s.apply(SketchEdit::AddDimension(dim)) else {
+        panic!("add dim");
+    };
+    next
+}
+
+fn first_of_kind(s: &Sketch, kind: bone_document::SketchEntityKind) -> SketchEntityId {
+    let Some(&id) = s.entity_order().iter().find(|id| {
+        matches!(
+            s.entities().get(**id).map(bone_document::SketchEntity::kind),
+            Some(k) if k == kind
+        )
+    }) else {
+        panic!("no entity of kind {kind:?}");
+    };
+    id
+}
+
+fn build_full_sketch() -> Sketch {
+    let coords = [
+        Point2::from_mm(0.0, 0.0),
+        Point2::from_mm(10.0, 0.0),
+        Point2::from_mm(10.0, 5.0),
+        Point2::from_mm(0.0, 5.0),
+    ];
+    let with_points = coords.into_iter().fold(Sketch::new(plane()), |s, p| {
+        add_entity(s, SketchEntity::point(p))
+    });
+    let p0 = point_of(&with_points, 0);
+    let p1 = point_of(&with_points, 1);
+    let p2 = point_of(&with_points, 2);
+    let p3 = point_of(&with_points, 3);
+
+    let with_lines = [
+        (p0, p1, false),
+        (p1, p2, false),
+        (p2, p3, false),
+        (p3, p0, true),
+    ]
+    .into_iter()
+    .fold(with_points, |s, (a, b, cons)| {
+        add_entity(s, SketchEntity::line(a, b, cons))
+    });
+    let with_arc = add_entity(with_lines, SketchEntity::arc(p0, p1, p2, false));
+    let with_circle = add_entity(with_arc, SketchEntity::circle(p0, mm(2.5), false));
+
+    let line_ids: Vec<_> = with_circle
+        .entity_order()
+        .iter()
+        .copied()
+        .filter(|id| {
+            matches!(
+                with_circle
+                    .entities()
+                    .get(*id)
+                    .map(bone_document::SketchEntity::kind),
+                Some(bone_document::SketchEntityKind::Line)
+            )
+        })
+        .collect();
+    let arc_id = first_of_kind(&with_circle, bone_document::SketchEntityKind::Arc);
+    let circle_id = first_of_kind(&with_circle, bone_document::SketchEntityKind::Circle);
+
+    let relations = [
+        SketchRelation::Coincident(p0, line_ids[0]),
+        SketchRelation::Horizontal(line_ids[0]),
+        SketchRelation::Vertical(line_ids[1]),
+        SketchRelation::Parallel(line_ids[0], line_ids[2]),
+        SketchRelation::Perpendicular(line_ids[0], line_ids[1]),
+        SketchRelation::Tangent(line_ids[0], circle_id),
+        SketchRelation::Equal(line_ids[0], line_ids[2]),
+        SketchRelation::Concentric(arc_id, circle_id),
+        SketchRelation::Fix(p0),
+    ];
+    let with_relations = relations.into_iter().fold(with_circle, add_relation);
+
+    let dimensions = [
+        SketchDimension::Linear {
+            a: p0,
+            b: p1,
+            value: mm(10.0),
+            kind: DimensionKind::Driving,
+        },
+        SketchDimension::Radius {
+            target: circle_id,
+            value: mm(2.5),
+            kind: DimensionKind::Driven,
+        },
+        SketchDimension::Diameter {
+            target: circle_id,
+            value: mm(5.0),
+            kind: DimensionKind::Driving,
+        },
+        SketchDimension::Angular {
+            a: line_ids[0],
+            b: line_ids[1],
+            value: deg(90.0),
+            kind: DimensionKind::Driving,
+        },
+    ];
+    dimensions.into_iter().fold(with_relations, add_dimension)
+}
+
+fn assert_ron<T: serde::Serialize>(value: &T) -> String {
+    let Ok(ron) = to_string(value) else {
+        panic!("ron");
+    };
+    ron
+}
+
+#[test]
+fn document_header_ron_surface() {
+    let mut doc = Document::new(document_id(1), "demo".to_owned());
+    let sid = sketch_id(7);
+    doc.insert_sketch(sid, "Sketch1".to_owned(), build_full_sketch());
+    doc.set_parameter("width".to_owned(), 10.0);
+
+    let ron = assert_ron(doc.header());
+    insta::assert_snapshot!("document_header", ron);
+}
+
+#[test]
+fn sketch_file_ron_surface() {
+    let file = SketchFile::new(build_full_sketch());
+    let ron = assert_ron(&file);
+    insta::assert_snapshot!("sketch_file", ron);
+}
+
+#[test]
+fn saved_folder_layout_is_stable() {
+    let dir = ok_dir();
+    let folder = DocumentFolder::new(dir.path().join("pinned.bone"));
+    let mut doc = Document::new(document_id(1), "pinned".to_owned());
+    doc.insert_sketch(sketch_id(1), "Rect".to_owned(), build_full_sketch());
+    let Ok(()) = save(&doc, &folder) else {
+        panic!("save");
+    };
+
+    let mut entries: Vec<String> = walk_relative(folder.path()).collect();
+    entries.sort();
+    let listing = entries.join("\n");
+    insta::assert_snapshot!("folder_listing", listing);
+}
+
+fn walk_relative(root: &std::path::Path) -> impl Iterator<Item = String> + '_ {
+    fn walk(root: &std::path::Path, path: &std::path::Path, out: &mut Vec<String>) {
+        let Ok(iter) = std::fs::read_dir(path) else {
+            return;
+        };
+        iter.flatten().for_each(|entry| {
+            let p = entry.path();
+            let rel = p
+                .strip_prefix(root)
+                .unwrap_or(&p)
+                .to_string_lossy()
+                .into_owned();
+            if p.is_dir() {
+                out.push(format!("{rel}/"));
+                walk(root, &p, out);
+            } else {
+                out.push(rel);
+            }
+        });
+    }
+    let mut out = Vec::new();
+    walk(root, root, &mut out);
+    out.into_iter()
+}
+
+#[test]
+fn document_header_roundtrips_through_ron() {
+    let mut doc = Document::new(document_id(1), "rt".to_owned());
+    doc.insert_sketch(sketch_id(1), "S".to_owned(), build_full_sketch());
+    let ron = assert_ron(doc.header());
+    let Ok(parsed) = bone_document::from_str::<DocumentHeader>(&ron) else {
+        panic!("parse");
+    };
+    assert_eq!(&parsed, doc.header());
+}
diff --git a/crates/bone-document/tests/snapshots/folder_snapshots__document_header.snap b/crates/bone-document/tests/snapshots/folder_snapshots__document_header.snap
new file mode 100644
index 0000000..e767b08
--- /dev/null
+++ b/crates/bone-document/tests/snapshots/folder_snapshots__document_header.snap
@@ -0,0 +1,79 @@
+---
+source: crates/bone-document/tests/folder_snapshots.rs
+expression: ron
+---
+#![enable(unwrap_newtypes)]
+#![enable(implicit_some)]
+#![enable(explicit_struct_names)]
+DocumentHeader(
+    schema: SchemaHeader(
+        name: "bone-document",
+        version: SchemaVersion(
+            major: 1,
+            minor: 0,
+        ),
+    ),
+    id: SerKey(
+        idx: 1,
+        version: 1,
+    ),
+    name: "demo",
+    units: Millimetre,
+    parameters: DocumentParameters(
+        order: ["width"],
+        entries: {
+            "width": 10.0,
+        },
+    ),
+    feature_tree: FeatureTree(
+        entries: [FeatureEntry(
+            id: SerKey(
+                idx: 1,
+                version: 1,
+            ),
+            node: Origin,
+        ), FeatureEntry(
+            id: SerKey(
+                idx: 2,
+                version: 1,
+            ),
+            node: PrincipalPlane(Xy),
+        ), FeatureEntry(
+            id: SerKey(
+                idx: 3,
+                version: 1,
+            ),
+            node: PrincipalPlane(Yz),
+        ), FeatureEntry(
+            id: SerKey(
+                idx: 4,
+                version: 1,
+            ),
+            node: PrincipalPlane(Zx),
+        ), FeatureEntry(
+            id: SerKey(
+                idx: 5,
+                version: 1,
+            ),
+            node: Sketch(SerKey(
+                idx: 7,
+                version: 1,
+            )),
+        )],
+    ),
+    sketches: SketchRegistry(
+        order: [SerKey(
+            idx: 7,
+            version: 1,
+        )],
+        entries: {
+            SerKey(
+                idx: 7,
+                version: 1,
+            ): SketchRegistryEntry(
+                label: "Sketch1",
+                filename: "0000000100000007.ron",
+            ),
+        },
+    ),
+)
diff --git a/crates/bone-document/tests/snapshots/folder_snapshots__folder_listing.snap b/crates/bone-document/tests/snapshots/folder_snapshots__folder_listing.snap
new file mode 100644
index 0000000..b05297e
--- /dev/null
+++ b/crates/bone-document/tests/snapshots/folder_snapshots__folder_listing.snap
@@ -0,0 +1,13 @@
+---
+source: crates/bone-document/tests/folder_snapshots.rs
+expression: listing
+---
+.gitattributes
+.gitignore
+blobs/
+caches/
+caches/.gitignore
+caches/CACHEDIR.TAG
+document.ron
+sketches/
+sketches/0000000100000001.ron
diff --git a/crates/bone-document/tests/snapshots/folder_snapshots__sketch_file.snap b/crates/bone-document/tests/snapshots/folder_snapshots__sketch_file.snap
new file mode 100644
index 0000000..852aec5
--- /dev/null
+++ b/crates/bone-document/tests/snapshots/folder_snapshots__sketch_file.snap
@@ -0,0 +1,355 @@
+---
+source: crates/bone-document/tests/folder_snapshots.rs
+expression: ron
+---
+#![enable(unwrap_newtypes)]
+#![enable(implicit_some)]
+#![enable(explicit_struct_names)]
+SketchFile(
+    schema: SchemaHeader(
+        name: "bone-document",
+        version: SchemaVersion(
+            major: 1,
+            minor: 0,
+        ),
+    ),
+    sketch: Sketch(
+        plane: SketchPlaneBasis(
+            origin: Point3(
+                x: 0.0,
+                y: 0.0,
+                z: 0.0,
+            ),
+            x_axis: UnitVec3(
+                x: 1.0,
+                y: 0.0,
+                z: 0.0,
+            ),
+            y_axis: UnitVec3(
+                x: 0.0,
+                y: 1.0,
+                z: 0.0,
+            ),
+        ),
+        entities: [SerdeSlot(
+            value: None,
+            version: 0,
+        ), SerdeSlot(
+            value: Point(PointData(
+                at: Point2(
+                    x: 0.0,
+                    y: 0.0,
+                ),
+            )),
+            version: 1,
+        ), SerdeSlot(
+            value: Point(PointData(
+                at: Point2(
+                    x: 10.0,
+                    y: 0.0,
+                ),
+            )),
+            version: 1,
+        ), SerdeSlot(
+            value: Point(PointData(
+                at: Point2(
+                    x: 10.0,
+                    y: 5.0,
+                ),
+            )),
+            version: 1,
+        ), SerdeSlot(
+            value: Point(PointData(
+                at: Point2(
+                    x: 0.0,
+                    y: 5.0,
+                ),
+            )),
+            version: 1,
+        ), SerdeSlot(
+            value: Line(LineData(
+                a: SerKey(
+                    idx: 1,
+                    version: 1,
+                ),
+                b: SerKey(
+                    idx: 2,
+                    version: 1,
+                ),
+                for_construction: false,
+            )),
+            version: 1,
+        ), SerdeSlot(
+            value: Line(LineData(
+                a: SerKey(
+                    idx: 2,
+                    version: 1,
+                ),
+                b: SerKey(
+                    idx: 3,
+                    version: 1,
+                ),
+                for_construction: false,
+            )),
+            version: 1,
+        ), SerdeSlot(
+            value: Line(LineData(
+                a: SerKey(
+                    idx: 3,
+                    version: 1,
+                ),
+                b: SerKey(
+                    idx: 4,
+                    version: 1,
+                ),
+                for_construction: false,
+            )),
+            version: 1,
+        ), SerdeSlot(
+            value: Line(LineData(
+                a: SerKey(
+                    idx: 4,
+                    version: 1,
+                ),
+                b: SerKey(
+                    idx: 1,
+                    version: 1,
+                ),
+                for_construction: true,
+            )),
+            version: 1,
+        ), SerdeSlot(
+            value: Arc(ArcData(
+                center: SerKey(
+                    idx: 1,
+                    version: 1,
+                ),
+                start: SerKey(
+                    idx: 2,
+                    version: 1,
+                ),
+                end: SerKey(
+                    idx: 3,
+                    version: 1,
+                ),
+                for_construction: false,
+            )),
+            version: 1,
+        ), SerdeSlot(
+            value: Circle(CircleData(
+                center: SerKey(
+                    idx: 1,
+                    version: 1,
+                ),
+                radius: 0.0025,
+                for_construction: false,
+            )),
+            version: 1,
+        )],
+        entity_order: [SerKey(
+            idx: 1,
+            version: 1,
+        ), SerKey(
+            idx: 2,
+            version: 1,
+        ), SerKey(
+            idx: 3,
+            version: 1,
+        ), SerKey(
+            idx: 4,
+            version: 1,
+        ), SerKey(
+            idx: 5,
+            version: 1,
+        ), SerKey(
+            idx: 6,
+            version: 1,
+        ), SerKey(
+            idx: 7,
+            version: 1,
+        ), SerKey(
+            idx: 8,
+            version: 1,
+        ), SerKey(
+            idx: 9,
+            version: 1,
+        ), SerKey(
+            idx: 10,
+            version: 1,
+        )],
+        relations: [SerdeSlot(
+            value: None,
+            version: 0,
+        ), SerdeSlot(
+            value: Coincident(SerKey(
+                idx: 1,
+                version: 1,
+            ), SerKey(
+                idx: 5,
+                version: 1,
+            )),
+            version: 1,
+        ), SerdeSlot(
+            value: Horizontal(SerKey(
+                idx: 5,
+                version: 1,
+            )),
+            version: 1,
+        ), SerdeSlot(
+            value: Vertical(SerKey(
+                idx: 6,
+                version: 1,
+            )),
+            version: 1,
+        ), SerdeSlot(
+            value: Parallel(SerKey(
+                idx: 5,
+                version: 1,
+            ), SerKey(
+                idx: 7,
+                version: 1,
+            )),
+            version: 1,
+        ), SerdeSlot(
+            value: Perpendicular(SerKey(
+                idx: 5,
+                version: 1,
+            ), SerKey(
+                idx: 6,
+                version: 1,
+            )),
+            version: 1,
+        ), SerdeSlot(
+            value: Tangent(SerKey(
+                idx: 5,
+                version: 1,
+            ), SerKey(
+                idx: 10,
+                version: 1,
+            )),
+            version: 1,
+        ), SerdeSlot(
+            value: Equal(SerKey(
+                idx: 5,
+                version: 1,
+            ), SerKey(
+                idx: 7,
+                version: 1,
+            )),
+            version: 1,
+        ), SerdeSlot(
+            value: Concentric(SerKey(
+                idx: 9,
+                version: 1,
+            ), SerKey(
+                idx: 10,
+                version: 1,
+            )),
+            version: 1,
+        ), SerdeSlot(
+            value: Fix(SerKey(
+                idx: 1,
+                version: 1,
+            )),
+            version: 1,
+        )],
+        relation_order: [SerKey(
+            idx: 1,
+            version: 1,
+        ), SerKey(
+            idx: 2,
+            version: 1,
+        ), SerKey(
+            idx: 3,
+            version: 1,
+        ), SerKey(
+            idx: 4,
+            version: 1,
+        ), SerKey(
+            idx: 5,
+            version: 1,
+        ), SerKey(
+            idx: 6,
+            version: 1,
+        ), SerKey(
+            idx: 7,
+            version: 1,
+        ), SerKey(
+            idx: 8,
+            version: 1,
+        ), SerKey(
+            idx: 9,
+            version: 1,
+        )],
+        dimensions: [SerdeSlot(
+            value: None,
+            version: 0,
+        ), SerdeSlot(
+            value: Linear(
+                a: SerKey(
+                    idx: 1,
+                    version: 1,
+                ),
+                b: SerKey(
+                    idx: 2,
+                    version: 1,
+                ),
+                value: 0.01,
+                kind: Driving,
+            ),
+            version: 1,
+        ), SerdeSlot(
+            value: Radius(
+                target: SerKey(
+                    idx: 10,
+                    version: 1,
+                ),
+                value: 0.0025,
+                kind: Driven,
+            ),
+            version: 1,
+        ), SerdeSlot(
+            value: Diameter(
+                target: SerKey(
+                    idx: 10,
+                    version: 1,
+                ),
+                value: 0.005,
+                kind: Driving,
+            ),
+            version: 1,
+        ), SerdeSlot(
+            value: Angular(
+                a: SerKey(
+                    idx: 5,
+                    version: 1,
+                ),
+                b: SerKey(
+                    idx: 6,
+                    version: 1,
+                ),
+                value: 1.5707963267948966,
+                kind: Driving,
+            ),
+            version: 1,
+        )],
+        dimension_order: [SerKey(
+            idx: 1,
+            version: 1,
+        ), SerKey(
+            idx: 2,
+            version: 1,
+        ), SerKey(
+            idx: 3,
+            version: 1,
+        ), SerKey(
+            idx: 4,
+            version: 1,
+        )],
+        parameters: [SerdeSlot(
+            value: None,
+            version: 0,
+        )],
+        parameter_order: [],
+    ),
+)
diff --git a/crates/bone-document/tests/undo.rs b/crates/bone-document/tests/undo.rs
new file mode 100644
index 0000000..27425be
--- /dev/null
+++ b/crates/bone-document/tests/undo.rs
@@ -0,0 +1,150 @@
+use std::num::NonZeroUsize;
+
+use bone_document::{Document, Sketch, UndoStack};
+use bone_types::{DocumentId, Point3, SketchId, SketchPlaneBasis, Tolerance, UnitVec3};
+use slotmap::KeyData;
+
+fn plane() -> SketchPlaneBasis {
+    let Ok(basis) = SketchPlaneBasis::new(
+        Point3::origin(),
+        UnitVec3::x_axis(),
+        UnitVec3::y_axis(),
+        Tolerance::new(1e-9),
+    ) else {
+        panic!("xy plane");
+    };
+    basis
+}
+
+fn sketch_id(idx: u32) -> SketchId {
+    SketchId::from(KeyData::from_ffi((1u64 << 32) | u64::from(idx)))
+}
+
+fn document_id(idx: u32) -> DocumentId {
+    DocumentId::from(KeyData::from_ffi((1u64 << 32) | u64::from(idx)))
+}
+
+fn cap(n: usize) -> NonZeroUsize {
+    let Some(nz) = NonZeroUsize::new(n) else {
+        panic!("capacity");
+    };
+    nz
+}
+
+fn base_doc() -> Document {
+    Document::new(document_id(1), "d".to_owned())
+}
+
+fn with_sketch(mut doc: Document, sid: SketchId) -> Document {
+    doc.insert_sketch(sid, format!("S{sid:?}"), Sketch::new(plane()));
+    doc
+}
+
+#[test]
+fn fresh_stack_has_no_history() {
+    let stack = UndoStack::with_capacity(cap(5));
+    assert!(!stack.can_undo());
+    assert!(!stack.can_redo());
+    assert_eq!(stack.past_len(), 0);
+    assert_eq!(stack.future_len(), 0);
+}
+
+#[test]
+fn undo_swaps_current_with_previous() {
+    let a = base_doc();
+    let mut live = with_sketch(a.clone(), sketch_id(1));
+    let mut stack = UndoStack::with_capacity(cap(5));
+    stack.record(a.clone());
+    assert!(stack.undo(&mut live));
+    assert_eq!(live, a);
+    assert!(!stack.can_undo());
+    assert!(stack.can_redo());
+}
+
+#[test]
+fn undo_on_empty_leaves_current_unchanged() {
+    let mut live = base_doc();
+    let snapshot = live.clone();
+    let mut stack = UndoStack::with_capacity(cap(5));
+    assert!(!stack.undo(&mut live));
+    assert_eq!(live, snapshot);
+    assert_eq!(stack.past_len(), 0);
+    assert_eq!(stack.future_len(), 0);
+}
+
+#[test]
+fn redo_on_empty_leaves_current_unchanged() {
+    let mut live = base_doc();
+    let snapshot = live.clone();
+    let mut stack = UndoStack::with_capacity(cap(5));
+    assert!(!stack.redo(&mut live));
+    assert_eq!(live, snapshot);
+    assert_eq!(stack.past_len(), 0);
+    assert_eq!(stack.future_len(), 0);
+}
+
+#[test]
+fn redo_replays_undone_state() {
+    let a = base_doc();
+    let b = with_sketch(a.clone(), sketch_id(1));
+    let mut live = b.clone();
+    let mut stack = UndoStack::with_capacity(cap(5));
+    stack.record(a.clone());
+    assert!(stack.undo(&mut live));
+    assert!(stack.redo(&mut live));
+    assert_eq!(live, b);
+    assert!(stack.can_undo());
+    assert!(!stack.can_redo());
+}
+
+#[test]
+fn recording_after_undo_clears_redo() {
+    let a = base_doc();
+    let b = with_sketch(a.clone(), sketch_id(1));
+    let c = with_sketch(a.clone(), sketch_id(2));
+    let mut live = b;
+    let mut stack = UndoStack::with_capacity(cap(5));
+    stack.record(a.clone());
+    assert!(stack.undo(&mut live));
+    assert_eq!(live, a);
+    stack.record(live.clone());
+    let _ = c;
+    assert!(stack.can_undo());
+    assert!(!stack.can_redo());
+    assert_eq!(stack.future_len(), 0);
+}
+
+#[test]
+fn capacity_drops_oldest_entries() {
+    let mut stack = UndoStack::with_capacity(cap(2));
+    let snapshots: Vec<Document> = (1..=4)
+        .map(|i| with_sketch(base_doc(), sketch_id(i)))
+        .collect();
+    snapshots.iter().cloned().for_each(|doc| stack.record(doc));
+    assert_eq!(stack.past_len(), 2);
+    let mut live = with_sketch(base_doc(), sketch_id(99));
+    assert!(stack.undo(&mut live));
+    assert_eq!(live, snapshots[3]);
+    assert!(stack.undo(&mut live));
+    assert_eq!(live, snapshots[2]);
+    assert!(!stack.can_undo());
+}
+
+#[test]
+fn undo_redo_cycle_preserves_determinism() {
+    let a = base_doc();
+    let b = with_sketch(a.clone(), sketch_id(1));
+    let c = with_sketch(b.clone(), sketch_id(2));
+    let mut live = c.clone();
+    let mut stack = UndoStack::with_capacity(cap(5));
+    stack.record(a.clone());
+    stack.record(b.clone());
+    assert!(stack.undo(&mut live));
+    assert_eq!(live, b);
+    assert!(stack.undo(&mut live));
+    assert_eq!(live, a);
+    assert!(stack.redo(&mut live));
+    assert_eq!(live, b);
+    assert!(stack.redo(&mut live));
+    assert_eq!(live, c);
+}
diff --git a/crates/bone-kernel/Cargo.toml b/crates/bone-kernel/Cargo.toml
new file mode 100644
index 0000000..4635d86
--- /dev/null
+++ b/crates/bone-kernel/Cargo.toml
@@ -0,0 +1,17 @@
+[package]
+name = "bone-kernel"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+rust-version.workspace = true
+
+[dependencies]
+bone-types = { workspace = true }
+thiserror = { workspace = true }
+uom = { workspace = true }
+
+[dev-dependencies]
+insta = { workspace = true }
+
+[lints]
+workspace = true
diff --git a/crates/bone-kernel/src/aabb.rs b/crates/bone-kernel/src/aabb.rs
new file mode 100644
index 0000000..d087544
--- /dev/null
+++ b/crates/bone-kernel/src/aabb.rs
@@ -0,0 +1,51 @@
+use bone_types::Point2;
+
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub struct Aabb2 {
+    min: Point2,
+    max: Point2,
+}
+
+impl Aabb2 {
+    #[must_use]
+    pub fn from_corners(a: Point2, b: Point2) -> Self {
+        let (ax, ay) = a.coords_mm();
+        let (bx, by) = b.coords_mm();
+        Self {
+            min: Point2::from_mm(ax.min(bx), ay.min(by)),
+            max: Point2::from_mm(ax.max(bx), ay.max(by)),
+        }
+    }
+
+    #[must_use]
+    pub fn min(self) -> Point2 {
+        self.min
+    }
+
+    #[must_use]
+    pub fn max(self) -> Point2 {
+        self.max
+    }
+
+    #[must_use]
+    pub fn extend_point(self, p: Point2) -> Self {
+        let (px, py) = p.coords_mm();
+        let (mnx, mny) = self.min.coords_mm();
+        let (mxx, mxy) = self.max.coords_mm();
+        Self {
+            min: Point2::from_mm(mnx.min(px), mny.min(py)),
+            max: Point2::from_mm(mxx.max(px), mxy.max(py)),
+        }
+    }
+}
+
+impl core::fmt::Display for Aabb2 {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        let (mnx, mny) = self.min.coords_mm();
+        let (mxx, mxy) = self.max.coords_mm();
+        write!(
+            f,
+            "aabb2{{ min=({mnx} mm, {mny} mm), max=({mxx} mm, {mxy} mm) }}"
+        )
+    }
+}
diff --git a/crates/bone-kernel/src/arc2.rs b/crates/bone-kernel/src/arc2.rs
new file mode 100644
index 0000000..9c28784
--- /dev/null
+++ b/crates/bone-kernel/src/arc2.rs
@@ -0,0 +1,332 @@
+use bone_types::{Angle, AngleTolerance, Length, Parameter, Point2, Tolerance, UnitVec2, Vec2};
+use core::f64::consts::{PI, TAU};
+use uom::si::angle::radian;
+use uom::si::length::millimeter;
+
+use crate::KernelError;
+use crate::aabb::Aabb2;
+use crate::circle2::Circle2;
+use crate::closest::ClosestPoint2;
+use crate::curvature::Curvature;
+use crate::curve2::{Curve2, Curve2Kind};
+
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub struct Arc2 {
+    center: Point2,
+    radius: Length,
+    start_angle: Angle,
+    sweep_angle: Angle,
+}
+
+impl Arc2 {
+    pub fn new(
+        center: Point2,
+        radius: Length,
+        start_angle: Angle,
+        sweep_angle: Angle,
+        tolerance: Tolerance,
+    ) -> Result<Self, KernelError> {
+        if radius.get::<millimeter>() < tolerance.value() {
+            return Err(KernelError::DegenerateArc);
+        }
+        let sweep = sweep_angle.get::<radian>();
+        let angle_eps = AngleTolerance::from_arc_length(tolerance, radius).radians();
+        if sweep.abs() < angle_eps {
+            return Err(KernelError::DegenerateArc);
+        }
+        if sweep.abs() > TAU + angle_eps {
+            return Err(KernelError::DegenerateArc);
+        }
+        Ok(Self {
+            center,
+            radius,
+            start_angle,
+            sweep_angle,
+        })
+    }
+
+    #[must_use]
+    pub const fn center(self) -> Point2 {
+        self.center
+    }
+
+    #[must_use]
+    pub const fn radius(self) -> Length {
+        self.radius
+    }
+
+    #[must_use]
+    pub const fn start_angle(self) -> Angle {
+        self.start_angle
+    }
+
+    #[must_use]
+    pub const fn sweep_angle(self) -> Angle {
+        self.sweep_angle
+    }
+
+    #[must_use]
+    pub fn radius_mm(self) -> f64 {
+        self.radius.get::<millimeter>()
+    }
+
+    #[must_use]
+    pub fn start_rad(self) -> f64 {
+        self.start_angle.get::<radian>()
+    }
+
+    #[must_use]
+    pub fn sweep_rad(self) -> f64 {
+        self.sweep_angle.get::<radian>()
+    }
+
+    #[must_use]
+    pub fn as_full_circle(self) -> Circle2 {
+        Circle2::from_raw(self.center, self.radius)
+    }
+
+    #[must_use]
+    pub fn contains_point(self, p: Point2, tolerance: Tolerance) -> bool {
+        let (cx, cy) = self.center.coords_mm();
+        let (px, py) = p.coords_mm();
+        let dx = px - cx;
+        let dy = py - cy;
+        let r = self.radius_mm();
+        let radial = (dx * dx + dy * dy).sqrt();
+        if (radial - r).abs() > tolerance.value() {
+            return false;
+        }
+        let angle_tol = AngleTolerance::from_arc_length(tolerance, self.radius);
+        self.contains_angle_rad(dy.atan2(dx), angle_tol)
+    }
+
+    fn contains_angle_rad(self, theta: f64, tolerance: AngleTolerance) -> bool {
+        let start = self.start_rad();
+        let sweep = self.sweep_rad();
+        let eps = tolerance.radians();
+        let delta = if sweep >= 0.0 {
+            (theta - start).rem_euclid(TAU)
+        } else {
+            (start - theta).rem_euclid(TAU)
+        };
+        let magnitude = sweep.abs();
+        delta <= magnitude + eps || delta >= TAU - eps
+    }
+
+    fn angle_at(self, t: Parameter) -> f64 {
+        self.start_rad() + self.sweep_rad() * t.value()
+    }
+
+    fn clamp_angle_rad(self, theta: f64, tolerance: AngleTolerance) -> f64 {
+        if self.contains_angle_rad(theta, tolerance) {
+            return theta;
+        }
+        let start = self.start_rad();
+        let end = start + self.sweep_rad();
+        let to_start = wrap_delta(theta - start).abs();
+        let to_end = wrap_delta(theta - end).abs();
+        if to_start <= to_end { start } else { end }
+    }
+}
+
+impl Curve2 for Arc2 {
+    fn evaluate(&self, t: Parameter) -> Point2 {
+        let (cx, cy) = self.center.coords_mm();
+        let r = self.radius_mm();
+        let theta = self.angle_at(t);
+        Point2::from_mm(cx + r * theta.cos(), cy + r * theta.sin())
+    }
+
+    fn derivative(&self, t: Parameter) -> Vec2 {
+        let r = self.radius_mm();
+        let sweep = self.sweep_rad();
+        let theta = self.angle_at(t);
+        Vec2::from_mm(-r * sweep * theta.sin(), r * sweep * theta.cos())
+    }
+
+    fn tangent(&self, t: Parameter) -> UnitVec2 {
+        let theta = self.angle_at(t);
+        let sign = self.sweep_rad().signum();
+        UnitVec2::new_unchecked(-sign * theta.sin(), sign * theta.cos())
+    }
+
+    fn curvature(&self, _t: Parameter) -> Curvature {
+        Curvature::from_radius(self.radius).with_sign(self.sweep_rad())
+    }
+
+    fn bounding_box(&self) -> Aabb2 {
+        arc_bounding_box(self.center, self.radius, self.start_angle, self.sweep_angle)
+    }
+
+    fn closest_point(&self, p: Point2, tolerance: Tolerance) -> ClosestPoint2 {
+        let (cx, cy) = self.center.coords_mm();
+        let (px, py) = p.coords_mm();
+        let dx = px - cx;
+        let dy = py - cy;
+        let dist = (dx * dx + dy * dy).sqrt();
+        let r = self.radius_mm();
+        let angle_tol = AngleTolerance::from_arc_length(tolerance, self.radius);
+
+        let theta_unclamped = if dist < tolerance.value() {
+            self.start_rad()
+        } else {
+            dy.atan2(dx)
+        };
+
+        let theta = self.clamp_angle_rad(theta_unclamped, angle_tol);
+        let sweep = self.sweep_rad();
+        let delta = if sweep >= 0.0 {
+            (theta - self.start_rad()).rem_euclid(TAU)
+        } else {
+            (self.start_rad() - theta).rem_euclid(TAU)
+        };
+        let t = (delta / sweep.abs()).clamp(0.0, 1.0);
+        let proj_x = cx + r * theta.cos();
+        let proj_y = cy + r * theta.sin();
+        let projected = Point2::from_mm(proj_x, proj_y);
+        let distance =
+            Length::new::<millimeter>(((px - proj_x).powi(2) + (py - proj_y).powi(2)).sqrt());
+        ClosestPoint2::new(Parameter::new(t), projected, distance)
+    }
+
+    fn as_kind(&self) -> Curve2Kind {
+        Curve2Kind::Arc(*self)
+    }
+}
+
+impl core::fmt::Display for Arc2 {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        write!(
+            f,
+            "arc2{{ c={}, r={} mm, start={} rad, sweep={} rad }}",
+            self.center,
+            self.radius.get::<millimeter>(),
+            self.start_rad(),
+            self.sweep_rad(),
+        )
+    }
+}
+
+pub(crate) fn wrap_delta(a: f64) -> f64 {
+    (a + PI).rem_euclid(TAU) - PI
+}
+
+#[must_use]
+pub fn arc_bounding_box(
+    center: Point2,
+    radius: Length,
+    start_angle: Angle,
+    sweep_angle: Angle,
+) -> Aabb2 {
+    let (cx, cy) = center.coords_mm();
+    let r = radius.get::<millimeter>();
+    let start = start_angle.get::<radian>();
+    let sweep = sweep_angle.get::<radian>();
+    let end = start + sweep;
+    let start_pt = Point2::from_mm(cx + r * start.cos(), cy + r * start.sin());
+    let end_pt = Point2::from_mm(cx + r * end.cos(), cy + r * end.sin());
+    let base = Aabb2::from_corners(start_pt, end_pt);
+    let cardinals: [(f64, Point2); 4] = [
+        (0.0, Point2::from_mm(cx + r, cy)),
+        (PI * 0.5, Point2::from_mm(cx, cy + r)),
+        (PI, Point2::from_mm(cx - r, cy)),
+        (-PI * 0.5, Point2::from_mm(cx, cy - r)),
+    ];
+    cardinals.into_iter().fold(base, |bbox, (theta, extreme)| {
+        if arc_contains_angle_rad(theta, start, sweep) {
+            bbox.extend_point(extreme)
+        } else {
+            bbox
+        }
+    })
+}
+
+fn arc_contains_angle_rad(theta: f64, start: f64, sweep: f64) -> bool {
+    let delta = if sweep >= 0.0 {
+        (theta - start).rem_euclid(TAU)
+    } else {
+        (start - theta).rem_euclid(TAU)
+    };
+    delta <= sweep.abs()
+}
+
+#[cfg(test)]
+mod tests {
+    use super::arc_bounding_box;
+    use bone_types::{Angle, Length, Point2};
+    use core::f64::consts::{FRAC_PI_2, PI, TAU};
+    use uom::si::angle::radian;
+    use uom::si::length::millimeter;
+
+    fn approx(a: f64, b: f64) -> bool {
+        (a - b).abs() < 1e-9
+    }
+
+    fn arc(cx: f64, cy: f64, r_mm: f64, start_rad: f64, sweep_rad: f64) -> (Point2, Point2) {
+        let bbox = arc_bounding_box(
+            Point2::from_mm(cx, cy),
+            Length::new::<millimeter>(r_mm),
+            Angle::new::<radian>(start_rad),
+            Angle::new::<radian>(sweep_rad),
+        );
+        (bbox.min(), bbox.max())
+    }
+
+    #[test]
+    fn full_circle_sweep_spans_all_cardinals() {
+        let (mn, mx) = arc(0.0, 0.0, 2.0, 0.0, TAU);
+        let (mnx, mny) = mn.coords_mm();
+        let (mxx, mxy) = mx.coords_mm();
+        assert!(approx(mnx, -2.0) && approx(mny, -2.0));
+        assert!(approx(mxx, 2.0) && approx(mxy, 2.0));
+    }
+
+    #[test]
+    fn quarter_arc_hugs_its_quadrant() {
+        let (mn, mx) = arc(0.0, 0.0, 1.0, 0.0, FRAC_PI_2);
+        let (mnx, mny) = mn.coords_mm();
+        let (mxx, mxy) = mx.coords_mm();
+        assert!(approx(mnx, 0.0) && approx(mny, 0.0));
+        assert!(approx(mxx, 1.0) && approx(mxy, 1.0));
+    }
+
+    #[test]
+    fn negative_sweep_mirrors_positive() {
+        let (fwd_min, fwd_max) = arc(0.0, 0.0, 1.0, 0.0, FRAC_PI_2);
+        let (rev_min, rev_max) = arc(0.0, 0.0, 1.0, FRAC_PI_2, -FRAC_PI_2);
+        assert_eq!(fwd_min, rev_min);
+        assert_eq!(fwd_max, rev_max);
+    }
+
+    #[test]
+    fn small_sweep_hugs_endpoints_only() {
+        let start = FRAC_PI_2 * 0.5;
+        let sweep = 0.1;
+        let (mn, mx) = arc(0.0, 0.0, 1.0, start, sweep);
+        let (mnx, mny) = mn.coords_mm();
+        let (mxx, mxy) = mx.coords_mm();
+        let end = start + sweep;
+        assert!(approx(mnx, end.cos()) && approx(mny, start.sin()));
+        assert!(approx(mxx, start.cos()) && approx(mxy, end.sin()));
+    }
+
+    #[test]
+    fn wide_sweep_crosses_three_axes() {
+        let start = FRAC_PI_2 * 0.5;
+        let sweep = FRAC_PI_2 * 2.5;
+        let (mn, mx) = arc(0.0, 0.0, 1.0, start, sweep);
+        let (mnx, mny) = mn.coords_mm();
+        let (mxx, mxy) = mx.coords_mm();
+        assert!(approx(mnx, -1.0) && approx(mny, -1.0));
+        assert!(approx(mxx, start.cos()) && approx(mxy, 1.0));
+    }
+
+    #[test]
+    fn translated_center_offsets_the_box() {
+        let (mn, mx) = arc(10.0, -5.0, 3.0, 0.0, PI);
+        let (mnx, mny) = mn.coords_mm();
+        let (mxx, mxy) = mx.coords_mm();
+        assert!(approx(mnx, 7.0) && approx(mny, -5.0));
+        assert!(approx(mxx, 13.0) && approx(mxy, -2.0));
+    }
+}
diff --git a/crates/bone-kernel/src/circle2.rs b/crates/bone-kernel/src/circle2.rs
new file mode 100644
index 0000000..bc02f30
--- /dev/null
+++ b/crates/bone-kernel/src/circle2.rs
@@ -0,0 +1,112 @@
+use bone_types::{Length, Parameter, Point2, Tolerance, UnitVec2, Vec2};
+use core::f64::consts::TAU;
+use uom::si::length::millimeter;
+
+use crate::KernelError;
+use crate::aabb::Aabb2;
+use crate::closest::ClosestPoint2;
+use crate::curvature::Curvature;
+use crate::curve2::{Curve2, Curve2Kind};
+
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub struct Circle2 {
+    center: Point2,
+    radius: Length,
+}
+
+impl Circle2 {
+    pub fn new(center: Point2, radius: Length, tolerance: Tolerance) -> Result<Self, KernelError> {
+        if radius.get::<millimeter>() < tolerance.value() {
+            return Err(KernelError::DegenerateCircle);
+        }
+        Ok(Self { center, radius })
+    }
+
+    #[must_use]
+    pub(crate) const fn from_raw(center: Point2, radius: Length) -> Self {
+        Self { center, radius }
+    }
+
+    #[must_use]
+    pub const fn center(self) -> Point2 {
+        self.center
+    }
+
+    #[must_use]
+    pub const fn radius(self) -> Length {
+        self.radius
+    }
+
+    #[must_use]
+    pub fn radius_mm(self) -> f64 {
+        self.radius.get::<millimeter>()
+    }
+}
+
+impl Curve2 for Circle2 {
+    fn evaluate(&self, t: Parameter) -> Point2 {
+        let (cx, cy) = self.center.coords_mm();
+        let r = self.radius_mm();
+        let theta = TAU * t.value();
+        Point2::from_mm(cx + r * theta.cos(), cy + r * theta.sin())
+    }
+
+    fn derivative(&self, t: Parameter) -> Vec2 {
+        let r = self.radius_mm();
+        let theta = TAU * t.value();
+        Vec2::from_mm(-r * TAU * theta.sin(), r * TAU * theta.cos())
+    }
+
+    fn tangent(&self, t: Parameter) -> UnitVec2 {
+        let theta = TAU * t.value();
+        UnitVec2::new_unchecked(-theta.sin(), theta.cos())
+    }
+
+    fn curvature(&self, _t: Parameter) -> Curvature {
+        Curvature::from_radius(self.radius)
+    }
+
+    fn bounding_box(&self) -> Aabb2 {
+        let (cx, cy) = self.center.coords_mm();
+        let r = self.radius_mm();
+        Aabb2::from_corners(
+            Point2::from_mm(cx - r, cy - r),
+            Point2::from_mm(cx + r, cy + r),
+        )
+    }
+
+    fn closest_point(&self, p: Point2, tolerance: Tolerance) -> ClosestPoint2 {
+        let (cx, cy) = self.center.coords_mm();
+        let (px, py) = p.coords_mm();
+        let dx = px - cx;
+        let dy = py - cy;
+        let dist = (dx * dx + dy * dy).sqrt();
+        let r = self.radius_mm();
+        if dist < tolerance.value() {
+            let projected = Point2::from_mm(cx + r, cy);
+            let distance = Length::new::<millimeter>(r);
+            return ClosestPoint2::new(Parameter::new(0.0), projected, distance);
+        }
+        let theta = dy.atan2(dx);
+        let t_raw = theta / TAU;
+        let t = t_raw.rem_euclid(1.0);
+        let projected = Point2::from_mm(cx + r * theta.cos(), cy + r * theta.sin());
+        let distance = Length::new::<millimeter>((dist - r).abs());
+        ClosestPoint2::new(Parameter::new(t), projected, distance)
+    }
+
+    fn as_kind(&self) -> Curve2Kind {
+        Curve2Kind::Circle(*self)
+    }
+}
+
+impl core::fmt::Display for Circle2 {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        write!(
+            f,
+            "circle2{{ c={}, r={} mm }}",
+            self.center,
+            self.radius.get::<millimeter>(),
+        )
+    }
+}
diff --git a/crates/bone-kernel/src/closest.rs b/crates/bone-kernel/src/closest.rs
new file mode 100644
index 0000000..167044a
--- /dev/null
+++ b/crates/bone-kernel/src/closest.rs
@@ -0,0 +1,47 @@
+use bone_types::{Length, Parameter, Point2};
+
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub struct ClosestPoint2 {
+    parameter: Parameter,
+    point: Point2,
+    distance: Length,
+}
+
+impl ClosestPoint2 {
+    #[must_use]
+    pub const fn new(parameter: Parameter, point: Point2, distance: Length) -> Self {
+        Self {
+            parameter,
+            point,
+            distance,
+        }
+    }
+
+    #[must_use]
+    pub const fn parameter(self) -> Parameter {
+        self.parameter
+    }
+
+    #[must_use]
+    pub const fn point(self) -> Point2 {
+        self.point
+    }
+
+    #[must_use]
+    pub const fn distance(self) -> Length {
+        self.distance
+    }
+}
+
+impl core::fmt::Display for ClosestPoint2 {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        use uom::si::length::millimeter;
+        write!(
+            f,
+            "closest{{ t={}, p={}, d={} mm }}",
+            self.parameter.value(),
+            self.point,
+            self.distance.get::<millimeter>(),
+        )
+    }
+}
diff --git a/crates/bone-kernel/src/curvature.rs b/crates/bone-kernel/src/curvature.rs
new file mode 100644
index 0000000..20fec2a
--- /dev/null
+++ b/crates/bone-kernel/src/curvature.rs
@@ -0,0 +1,30 @@
+use bone_types::Length;
+use uom::si::length::millimeter;
+
+#[derive(Copy, Clone, Debug, PartialEq, PartialOrd)]
+pub struct Curvature(f64);
+
+impl Curvature {
+    pub const ZERO: Self = Self(0.0);
+
+    #[must_use]
+    pub fn from_radius(radius: Length) -> Self {
+        Self(1.0 / radius.get::<millimeter>())
+    }
+
+    #[must_use]
+    pub const fn value_per_mm(self) -> f64 {
+        self.0
+    }
+
+    #[must_use]
+    pub fn with_sign(self, sign: f64) -> Self {
+        Self(self.0 * sign.signum())
+    }
+}
+
+impl core::fmt::Display for Curvature {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        write!(f, "k={}/mm", self.0)
+    }
+}
diff --git a/crates/bone-kernel/src/curve2.rs b/crates/bone-kernel/src/curve2.rs
new file mode 100644
index 0000000..4af4fab
--- /dev/null
+++ b/crates/bone-kernel/src/curve2.rs
@@ -0,0 +1,73 @@
+use bone_types::{Parameter, Point2, Tolerance, UnitVec2, Vec2};
+
+use crate::aabb::Aabb2;
+use crate::arc2::Arc2;
+use crate::circle2::Circle2;
+use crate::closest::ClosestPoint2;
+use crate::curvature::Curvature;
+use crate::line2::Line2;
+
+pub trait Curve2: Copy {
+    fn evaluate(&self, t: Parameter) -> Point2;
+    fn derivative(&self, t: Parameter) -> Vec2;
+    fn tangent(&self, t: Parameter) -> UnitVec2;
+    fn curvature(&self, t: Parameter) -> Curvature;
+    fn bounding_box(&self) -> Aabb2;
+    fn closest_point(&self, p: Point2, tolerance: Tolerance) -> ClosestPoint2;
+    fn as_kind(&self) -> Curve2Kind;
+}
+
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub enum Curve2Kind {
+    Line(Line2),
+    Arc(Arc2),
+    Circle(Circle2),
+}
+
+impl Curve2 for Curve2Kind {
+    fn evaluate(&self, t: Parameter) -> Point2 {
+        match self {
+            Self::Line(c) => c.evaluate(t),
+            Self::Arc(c) => c.evaluate(t),
+            Self::Circle(c) => c.evaluate(t),
+        }
+    }
+    fn derivative(&self, t: Parameter) -> Vec2 {
+        match self {
+            Self::Line(c) => c.derivative(t),
+            Self::Arc(c) => c.derivative(t),
+            Self::Circle(c) => c.derivative(t),
+        }
+    }
+    fn tangent(&self, t: Parameter) -> UnitVec2 {
+        match self {
+            Self::Line(c) => c.tangent(t),
+            Self::Arc(c) => c.tangent(t),
+            Self::Circle(c) => c.tangent(t),
+        }
+    }
+    fn curvature(&self, t: Parameter) -> Curvature {
+        match self {
+            Self::Line(c) => c.curvature(t),
+            Self::Arc(c) => c.curvature(t),
+            Self::Circle(c) => c.curvature(t),
+        }
+    }
+    fn bounding_box(&self) -> Aabb2 {
+        match self {
+            Self::Line(c) => c.bounding_box(),
+            Self::Arc(c) => c.bounding_box(),
+            Self::Circle(c) => c.bounding_box(),
+        }
+    }
+    fn closest_point(&self, p: Point2, tolerance: Tolerance) -> ClosestPoint2 {
+        match self {
+            Self::Line(c) => c.closest_point(p, tolerance),
+            Self::Arc(c) => c.closest_point(p, tolerance),
+            Self::Circle(c) => c.closest_point(p, tolerance),
+        }
+    }
+    fn as_kind(&self) -> Curve2Kind {
+        *self
+    }
+}
diff --git a/crates/bone-kernel/src/intersect.rs b/crates/bone-kernel/src/intersect.rs
new file mode 100644
index 0000000..0fdf576
--- /dev/null
+++ b/crates/bone-kernel/src/intersect.rs
@@ -0,0 +1,305 @@
+use bone_types::{AngleTolerance, Point2, Tolerance};
+use core::f64::consts::TAU;
+
+use crate::arc2::{Arc2, wrap_delta};
+use crate::circle2::Circle2;
+use crate::curve2::Curve2Kind;
+use crate::line2::Line2;
+
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub enum IntersectionSet {
+    Empty,
+    One(Point2),
+    Two(Point2, Point2),
+    Coincident,
+}
+
+impl core::fmt::Display for IntersectionSet {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        match self {
+            Self::Empty => write!(f, "empty"),
+            Self::One(p) => write!(f, "one{{ {p} }}"),
+            Self::Two(a, b) => write!(f, "two{{ {a}, {b} }}"),
+            Self::Coincident => write!(f, "coincident"),
+        }
+    }
+}
+
+#[must_use]
+pub fn intersect_curves(a: &Curve2Kind, b: &Curve2Kind, tolerance: Tolerance) -> IntersectionSet {
+    match (a, b) {
+        (Curve2Kind::Line(l), Curve2Kind::Line(m)) => line_line(l, m, tolerance),
+        (Curve2Kind::Line(l), Curve2Kind::Circle(c))
+        | (Curve2Kind::Circle(c), Curve2Kind::Line(l)) => line_circle(l, c, tolerance),
+        (Curve2Kind::Line(l), Curve2Kind::Arc(a)) | (Curve2Kind::Arc(a), Curve2Kind::Line(l)) => {
+            line_arc(l, a, tolerance)
+        }
+        (Curve2Kind::Circle(c), Curve2Kind::Circle(d)) => circle_circle(c, d, tolerance),
+        (Curve2Kind::Circle(c), Curve2Kind::Arc(a))
+        | (Curve2Kind::Arc(a), Curve2Kind::Circle(c)) => circle_arc(c, a, tolerance),
+        (Curve2Kind::Arc(a), Curve2Kind::Arc(b)) => arc_arc(a, b, tolerance),
+    }
+}
+
+fn line_line(a: &Line2, b: &Line2, tolerance: Tolerance) -> IntersectionSet {
+    let (ax, ay) = a.start().coords_mm();
+    let (ux, uy) = a.unit_direction().components();
+    let la = a.length_mm();
+    let (cx, cy) = b.start().coords_mm();
+    let (vx, vy) = b.unit_direction().components();
+    let lb = b.length_mm();
+    let eps = tolerance.value();
+
+    let det = ux * vy - uy * vx;
+    let angle_eps = eps / la.max(lb);
+
+    if det.abs() < angle_eps {
+        let dx = cx - ax;
+        let dy = cy - ay;
+        let perp = dx * uy - dy * ux;
+        if perp.abs() > eps {
+            return IntersectionSet::Empty;
+        }
+        let (ex, ey) = b.end().coords_mm();
+        let t_c = dx * ux + dy * uy;
+        let t_d = (ex - ax) * ux + (ey - ay) * uy;
+        let (t_lo, t_hi) = if t_c <= t_d { (t_c, t_d) } else { (t_d, t_c) };
+        let overlap_lo = t_lo.max(0.0);
+        let overlap_hi = t_hi.min(la);
+        let width = overlap_hi - overlap_lo;
+        if width < -eps {
+            return IntersectionSet::Empty;
+        }
+        if width <= eps {
+            let t = f64::midpoint(overlap_lo, overlap_hi);
+            return IntersectionSet::One(Point2::from_mm(ax + t * ux, ay + t * uy));
+        }
+        return IntersectionSet::Coincident;
+    }
+
+    let t = ((cx - ax) * vy - (cy - ay) * vx) / det;
+    let s = ((cx - ax) * uy - (cy - ay) * ux) / det;
+    if !in_segment(t, la, eps) || !in_segment(s, lb, eps) {
+        return IntersectionSet::Empty;
+    }
+    IntersectionSet::One(Point2::from_mm(ax + t * ux, ay + t * uy))
+}
+
+fn line_circle(line: &Line2, circle: &Circle2, tolerance: Tolerance) -> IntersectionSet {
+    let (ax, ay) = line.start().coords_mm();
+    let (ux, uy) = line.unit_direction().components();
+    let la = line.length_mm();
+    let (cx, cy) = circle.center().coords_mm();
+    let r = circle.radius_mm();
+    let eps = tolerance.value();
+
+    let fx = ax - cx;
+    let fy = ay - cy;
+    let b_coef = 2.0 * (fx * ux + fy * uy);
+    let c_coef = fx * fx + fy * fy - r * r;
+    let disc = b_coef * b_coef - 4.0 * c_coef;
+    if disc < -eps * eps {
+        return IntersectionSet::Empty;
+    }
+    let sqrt_disc = disc.max(0.0).sqrt();
+    let t1 = f64::midpoint(-b_coef, -sqrt_disc);
+    let t2 = f64::midpoint(-b_coef, sqrt_disc);
+
+    let p = |t: f64| Point2::from_mm(ax + t * ux, ay + t * uy);
+    let in1 = in_segment(t1, la, eps);
+    let in2 = in_segment(t2, la, eps);
+    if sqrt_disc < eps {
+        if in1 {
+            IntersectionSet::One(p(t1))
+        } else {
+            IntersectionSet::Empty
+        }
+    } else {
+        match (in1, in2) {
+            (true, true) => IntersectionSet::Two(p(t1), p(t2)),
+            (true, false) => IntersectionSet::One(p(t1)),
+            (false, true) => IntersectionSet::One(p(t2)),
+            (false, false) => IntersectionSet::Empty,
+        }
+    }
+}
+
+fn line_arc(line: &Line2, arc: &Arc2, tolerance: Tolerance) -> IntersectionSet {
+    let circle = arc.as_full_circle();
+    match line_circle(line, &circle, tolerance) {
+        IntersectionSet::Empty => IntersectionSet::Empty,
+        IntersectionSet::One(p) => {
+            if arc.contains_point(p, tolerance) {
+                IntersectionSet::One(p)
+            } else {
+                IntersectionSet::Empty
+            }
+        }
+        IntersectionSet::Two(p, q) => filter_two_by_arc(p, q, arc, tolerance),
+        IntersectionSet::Coincident => unreachable!("line_circle never returns Coincident"),
+    }
+}
+
+fn circle_circle(a: &Circle2, b: &Circle2, tolerance: Tolerance) -> IntersectionSet {
+    let (ax, ay) = a.center().coords_mm();
+    let (bx, by) = b.center().coords_mm();
+    let ra = a.radius_mm();
+    let rb = b.radius_mm();
+    let eps = tolerance.value();
+
+    let dx = bx - ax;
+    let dy = by - ay;
+    let d_sq = dx * dx + dy * dy;
+    let d = d_sq.sqrt();
+
+    if d < eps && (ra - rb).abs() < eps {
+        return IntersectionSet::Coincident;
+    }
+    if d > ra + rb + eps {
+        return IntersectionSet::Empty;
+    }
+    if d + ra.min(rb) < ra.max(rb) - eps {
+        return IntersectionSet::Empty;
+    }
+    if d < eps {
+        return IntersectionSet::Empty;
+    }
+
+    let a_proj = (d_sq + ra * ra - rb * rb) / (2.0 * d);
+    let h_sq = ra * ra - a_proj * a_proj;
+    let mx = ax + a_proj * dx / d;
+    let my = ay + a_proj * dy / d;
+
+    if h_sq.abs() < eps * eps {
+        return IntersectionSet::One(Point2::from_mm(mx, my));
+    }
+    if h_sq < 0.0 {
+        return IntersectionSet::Empty;
+    }
+    let h = h_sq.sqrt();
+    let px = -dy / d * h;
+    let py = dx / d * h;
+    IntersectionSet::Two(
+        Point2::from_mm(mx + px, my + py),
+        Point2::from_mm(mx - px, my - py),
+    )
+}
+
+fn circle_arc(circle: &Circle2, arc: &Arc2, tolerance: Tolerance) -> IntersectionSet {
+    let arc_full = arc.as_full_circle();
+    match circle_circle(circle, &arc_full, tolerance) {
+        IntersectionSet::Empty => IntersectionSet::Empty,
+        IntersectionSet::Coincident => IntersectionSet::Coincident,
+        IntersectionSet::One(p) => {
+            if arc.contains_point(p, tolerance) {
+                IntersectionSet::One(p)
+            } else {
+                IntersectionSet::Empty
+            }
+        }
+        IntersectionSet::Two(p, q) => filter_two_by_arc(p, q, arc, tolerance),
+    }
+}
+
+fn arc_arc(a: &Arc2, b: &Arc2, tolerance: Tolerance) -> IntersectionSet {
+    let a_full = a.as_full_circle();
+    let b_full = b.as_full_circle();
+    match circle_circle(&a_full, &b_full, tolerance) {
+        IntersectionSet::Empty => IntersectionSet::Empty,
+        IntersectionSet::Coincident => same_circle_arc_overlap(a, b, tolerance),
+        IntersectionSet::One(p) => {
+            if a.contains_point(p, tolerance) && b.contains_point(p, tolerance) {
+                IntersectionSet::One(p)
+            } else {
+                IntersectionSet::Empty
+            }
+        }
+        IntersectionSet::Two(p, q) => {
+            let p_ok = a.contains_point(p, tolerance) && b.contains_point(p, tolerance);
+            let q_ok = a.contains_point(q, tolerance) && b.contains_point(q, tolerance);
+            match (p_ok, q_ok) {
+                (true, true) => IntersectionSet::Two(p, q),
+                (true, false) => IntersectionSet::One(p),
+                (false, true) => IntersectionSet::One(q),
+                (false, false) => IntersectionSet::Empty,
+            }
+        }
+    }
+}
+
+fn same_circle_arc_overlap(a: &Arc2, b: &Arc2, tolerance: Tolerance) -> IntersectionSet {
+    let eps_angle = AngleTolerance::from_arc_length(tolerance, a.radius()).radians();
+    let (lo_a, mag_a) = canonical_range(a);
+    let (lo_b, mag_b) = canonical_range(b);
+
+    if mag_a >= TAU - eps_angle || mag_b >= TAU - eps_angle {
+        return IntersectionSet::Coincident;
+    }
+
+    let hi_a = lo_a + mag_a;
+
+    let overlaps: [Option<(f64, f64)>; 3] = [-TAU, 0.0, TAU].map(|shift| {
+        let lo_b_s = lo_b + shift;
+        let hi_b_s = lo_b_s + mag_b;
+        let o_lo = lo_a.max(lo_b_s);
+        let o_hi = hi_a.min(hi_b_s);
+        (o_hi + eps_angle >= o_lo).then_some((o_lo, o_hi))
+    });
+
+    let has_1d = overlaps
+        .iter()
+        .any(|o| matches!(o, Some((s, e)) if e - s > eps_angle));
+    if has_1d {
+        return IntersectionSet::Coincident;
+    }
+
+    let points: Vec<f64> = overlaps
+        .iter()
+        .filter_map(|o| o.map(|(s, e)| (0.5 * (s + e)).rem_euclid(TAU)))
+        .fold(Vec::new(), |mut acc, t| {
+            if !acc.iter().any(|&p: &f64| angle_close(p, t, eps_angle)) {
+                acc.push(t);
+            }
+            acc
+        });
+
+    match points.as_slice() {
+        [] => IntersectionSet::Empty,
+        [t] => IntersectionSet::One(arc_point_at_angle(a, *t)),
+        [t1, t2, ..] => {
+            IntersectionSet::Two(arc_point_at_angle(a, *t1), arc_point_at_angle(a, *t2))
+        }
+    }
+}
+
+fn canonical_range(arc: &Arc2) -> (f64, f64) {
+    let start = arc.start_rad();
+    let sweep = arc.sweep_rad();
+    let lo = if sweep >= 0.0 { start } else { start + sweep };
+    (lo.rem_euclid(TAU), sweep.abs())
+}
+
+fn arc_point_at_angle(arc: &Arc2, theta: f64) -> Point2 {
+    let (cx, cy) = arc.center().coords_mm();
+    let r = arc.radius_mm();
+    Point2::from_mm(cx + r * theta.cos(), cy + r * theta.sin())
+}
+
+fn angle_close(a: f64, b: f64, eps: f64) -> bool {
+    wrap_delta(a - b).abs() < eps
+}
+
+fn filter_two_by_arc(p: Point2, q: Point2, arc: &Arc2, tolerance: Tolerance) -> IntersectionSet {
+    let p_in = arc.contains_point(p, tolerance);
+    let q_in = arc.contains_point(q, tolerance);
+    match (p_in, q_in) {
+        (true, true) => IntersectionSet::Two(p, q),
+        (true, false) => IntersectionSet::One(p),
+        (false, true) => IntersectionSet::One(q),
+        (false, false) => IntersectionSet::Empty,
+    }
+}
+
+fn in_segment(t: f64, length: f64, eps: f64) -> bool {
+    t >= -eps && t <= length + eps
+}
diff --git a/crates/bone-kernel/src/lib.rs b/crates/bone-kernel/src/lib.rs
new file mode 100644
index 0000000..eb2292d
--- /dev/null
+++ b/crates/bone-kernel/src/lib.rs
@@ -0,0 +1,29 @@
+pub mod aabb;
+pub mod arc2;
+pub mod circle2;
+pub mod closest;
+pub mod curvature;
+pub mod curve2;
+pub mod intersect;
+pub mod line2;
+
+pub use aabb::Aabb2;
+pub use arc2::{Arc2, arc_bounding_box};
+pub use circle2::Circle2;
+pub use closest::ClosestPoint2;
+pub use curvature::Curvature;
+pub use curve2::{Curve2, Curve2Kind};
+pub use intersect::{IntersectionSet, intersect_curves};
+pub use line2::Line2;
+
+#[derive(Debug, thiserror::Error)]
+pub enum KernelError {
+    #[error("line endpoints coincide within tolerance")]
+    DegenerateLine,
+    #[error("arc sweep is within tolerance of zero or exceeds 2π")]
+    DegenerateArc,
+    #[error("circle radius is within tolerance of zero")]
+    DegenerateCircle,
+}
+
+pub type Result<T, E = KernelError> = core::result::Result<T, E>;
diff --git a/crates/bone-kernel/src/line2.rs b/crates/bone-kernel/src/line2.rs
new file mode 100644
index 0000000..ee9c95e
--- /dev/null
+++ b/crates/bone-kernel/src/line2.rs
@@ -0,0 +1,95 @@
+use bone_types::{Length, Parameter, Point2, Tolerance, UnitVec2, Vec2};
+use uom::si::length::millimeter;
+
+use crate::KernelError;
+use crate::aabb::Aabb2;
+use crate::closest::ClosestPoint2;
+use crate::curvature::Curvature;
+use crate::curve2::{Curve2, Curve2Kind};
+
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub struct Line2 {
+    start: Point2,
+    end: Point2,
+}
+
+impl Line2 {
+    pub fn new(start: Point2, end: Point2, tolerance: Tolerance) -> Result<Self, KernelError> {
+        if (end - start).norm_mm() < tolerance.value() {
+            return Err(KernelError::DegenerateLine);
+        }
+        Ok(Self { start, end })
+    }
+
+    #[must_use]
+    pub const fn start(self) -> Point2 {
+        self.start
+    }
+
+    #[must_use]
+    pub const fn end(self) -> Point2 {
+        self.end
+    }
+
+    #[must_use]
+    pub fn length(self) -> Length {
+        (self.end - self.start).norm()
+    }
+
+    #[must_use]
+    pub fn length_mm(self) -> f64 {
+        (self.end - self.start).norm_mm()
+    }
+
+    #[must_use]
+    pub fn unit_direction(self) -> UnitVec2 {
+        let (dx, dy) = (self.end - self.start).coords_mm();
+        let len = (dx * dx + dy * dy).sqrt();
+        UnitVec2::new_unchecked(dx / len, dy / len)
+    }
+}
+
+impl Curve2 for Line2 {
+    fn evaluate(&self, t: Parameter) -> Point2 {
+        let (sx, sy) = self.start.coords_mm();
+        let (ex, ey) = self.end.coords_mm();
+        let u = t.value();
+        Point2::from_mm(sx + u * (ex - sx), sy + u * (ey - sy))
+    }
+
+    fn derivative(&self, _t: Parameter) -> Vec2 {
+        self.end - self.start
+    }
+
+    fn tangent(&self, _t: Parameter) -> UnitVec2 {
+        self.unit_direction()
+    }
+
+    fn curvature(&self, _t: Parameter) -> Curvature {
+        Curvature::ZERO
+    }
+
+    fn bounding_box(&self) -> Aabb2 {
+        Aabb2::from_corners(self.start, self.end)
+    }
+
+    fn closest_point(&self, p: Point2, _tolerance: Tolerance) -> ClosestPoint2 {
+        let d = self.end - self.start;
+        let v = p - self.start;
+        let len_sq = d.norm_squared_mm2();
+        let t = (v.dot_mm2(d) / len_sq).clamp(0.0, 1.0);
+        let point = self.evaluate(Parameter::new(t));
+        let dist_mm = (p - point).norm_mm();
+        ClosestPoint2::new(Parameter::new(t), point, Length::new::<millimeter>(dist_mm))
+    }
+
+    fn as_kind(&self) -> Curve2Kind {
+        Curve2Kind::Line(*self)
+    }
+}
+
+impl core::fmt::Display for Line2 {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        write!(f, "line2{{ {} -> {} }}", self.start, self.end)
+    }
+}
diff --git a/crates/bone-kernel/tests/curves.rs b/crates/bone-kernel/tests/curves.rs
new file mode 100644
index 0000000..672ed28
--- /dev/null
+++ b/crates/bone-kernel/tests/curves.rs
@@ -0,0 +1,155 @@
+use bone_kernel::{Arc2, Circle2, Curve2, Line2};
+use bone_types::{Angle, Length, Parameter, Point2, Tolerance};
+use uom::si::angle::radian;
+use uom::si::length::millimeter;
+
+const TOL: Tolerance = Tolerance::new(1e-9);
+
+fn samples() -> [Parameter; 5] {
+    [
+        Parameter::new(0.0),
+        Parameter::new(0.25),
+        Parameter::new(0.5),
+        Parameter::new(0.75),
+        Parameter::new(1.0),
+    ]
+}
+
+fn fmt_point(p: Point2) -> String {
+    let (x, y) = p.coords_mm();
+    format!("({x:.6}, {y:.6})")
+}
+
+fn fmt_curve<C: Curve2 + core::fmt::Display + core::fmt::Debug>(name: &str, c: &C) -> String {
+    let samples = samples();
+    let points = samples
+        .iter()
+        .map(|t| {
+            format!(
+                "t={:.2} -> p={} v={}",
+                t.value(),
+                fmt_point(c.evaluate(*t)),
+                c.derivative(*t)
+            )
+        })
+        .collect::<Vec<_>>()
+        .join("\n    ");
+    let k_start = c.curvature(Parameter::new(0.0));
+    let k_mid = c.curvature(Parameter::new(0.5));
+    let tangent_mid = c.tangent(Parameter::new(0.5));
+    let bbox = c.bounding_box();
+    let closest = c.closest_point(Point2::from_mm(0.0, 0.0), TOL);
+    format!(
+        "{name}_display = {c}\n\
+         {name}_debug   = {c:?}\n\
+         {name}_samples:\n    {points}\n\
+         {name}_tangent_mid = {tangent_mid}\n\
+         {name}_curvature_start = {k_start}\n\
+         {name}_curvature_mid   = {k_mid}\n\
+         {name}_bbox    = {bbox}\n\
+         {name}_closest_origin = {closest}",
+    )
+}
+
+#[test]
+fn line2_surface() {
+    let Ok(line) = Line2::new(Point2::from_mm(1.0, 2.0), Point2::from_mm(7.0, 6.0), TOL) else {
+        panic!("endpoints distinct");
+    };
+    insta::assert_snapshot!(fmt_curve("line2", &line));
+}
+
+#[test]
+fn circle2_surface() {
+    let Ok(circle) = Circle2::new(
+        Point2::from_mm(2.0, -1.0),
+        Length::new::<millimeter>(3.0),
+        TOL,
+    ) else {
+        panic!("radius positive");
+    };
+    insta::assert_snapshot!(fmt_curve("circle2", &circle));
+}
+
+#[test]
+fn arc2_quarter_ccw_surface() {
+    let Ok(arc) = Arc2::new(
+        Point2::from_mm(0.0, 0.0),
+        Length::new::<millimeter>(5.0),
+        Angle::new::<radian>(0.0),
+        Angle::new::<radian>(core::f64::consts::FRAC_PI_2),
+        TOL,
+    ) else {
+        panic!("sweep nonzero");
+    };
+    insta::assert_snapshot!(fmt_curve("arc2_q", &arc));
+}
+
+#[test]
+fn arc2_half_cw_surface() {
+    let Ok(arc) = Arc2::new(
+        Point2::from_mm(1.0, 1.0),
+        Length::new::<millimeter>(2.0),
+        Angle::new::<radian>(core::f64::consts::PI),
+        Angle::new::<radian>(-core::f64::consts::PI),
+        TOL,
+    ) else {
+        panic!("sweep nonzero");
+    };
+    insta::assert_snapshot!(fmt_curve("arc2_h", &arc));
+}
+
+#[test]
+fn line2_rejects_degenerate_endpoints() {
+    let err = Line2::new(
+        Point2::from_mm(1.0, 1.0),
+        Point2::from_mm(1.0, 1.0 + 1e-12),
+        TOL,
+    );
+    assert!(err.is_err());
+}
+
+#[test]
+fn circle2_rejects_zero_radius() {
+    let err = Circle2::new(
+        Point2::from_mm(0.0, 0.0),
+        Length::new::<millimeter>(0.0),
+        TOL,
+    );
+    assert!(err.is_err());
+}
+
+#[test]
+fn arc2_rejects_zero_sweep() {
+    let err = Arc2::new(
+        Point2::from_mm(0.0, 0.0),
+        Length::new::<millimeter>(1.0),
+        Angle::new::<radian>(0.0),
+        Angle::new::<radian>(0.0),
+        TOL,
+    );
+    assert!(err.is_err());
+}
+
+#[test]
+fn arc2_closest_point_wide_sweep_returns_midarc_t() {
+    let Ok(arc) = Arc2::new(
+        Point2::from_mm(0.0, 0.0),
+        Length::new::<millimeter>(1.0),
+        Angle::new::<radian>(0.0),
+        Angle::new::<radian>(3.0 * core::f64::consts::FRAC_PI_2),
+        TOL,
+    ) else {
+        panic!("3π/2 sweep within bounds");
+    };
+    let px = -0.7_f64;
+    let py = -0.8_f64;
+    let closest = arc.closest_point(Point2::from_mm(px, py), TOL);
+    let theta = py.atan2(px).rem_euclid(core::f64::consts::TAU);
+    let expected_t = theta / (3.0 * core::f64::consts::FRAC_PI_2);
+    let got_t = closest.parameter().value();
+    assert!(
+        (got_t - expected_t).abs() < 1e-12,
+        "wide-sweep closest-point: expected t≈{expected_t}, got {got_t}"
+    );
+}
diff --git a/crates/bone-kernel/tests/intersect.rs b/crates/bone-kernel/tests/intersect.rs
new file mode 100644
index 0000000..e0d5a58
--- /dev/null
+++ b/crates/bone-kernel/tests/intersect.rs
@@ -0,0 +1,143 @@
+use bone_kernel::{Arc2, Circle2, Curve2, Curve2Kind, Line2, intersect_curves};
+use bone_types::{Angle, Length, Point2, Tolerance};
+use uom::si::angle::radian;
+use uom::si::length::millimeter;
+
+const TOL: Tolerance = Tolerance::new(1e-9);
+
+fn line(ax: f64, ay: f64, bx: f64, by: f64) -> Curve2Kind {
+    let Ok(l) = Line2::new(Point2::from_mm(ax, ay), Point2::from_mm(bx, by), TOL) else {
+        panic!("line endpoints distinct");
+    };
+    l.as_kind()
+}
+
+fn circle(cx: f64, cy: f64, r: f64) -> Curve2Kind {
+    let Ok(c) = Circle2::new(Point2::from_mm(cx, cy), Length::new::<millimeter>(r), TOL) else {
+        panic!("radius positive");
+    };
+    c.as_kind()
+}
+
+fn arc(cx: f64, cy: f64, r: f64, start: f64, sweep: f64) -> Curve2Kind {
+    let Ok(a) = Arc2::new(
+        Point2::from_mm(cx, cy),
+        Length::new::<millimeter>(r),
+        Angle::new::<radian>(start),
+        Angle::new::<radian>(sweep),
+        TOL,
+    ) else {
+        panic!("sweep nonzero");
+    };
+    a.as_kind()
+}
+
+fn row(label: &str, a: &Curve2Kind, b: &Curve2Kind) -> String {
+    format!("{label} = {}", intersect_curves(a, b, TOL))
+}
+
+#[test]
+fn intersection_matrix_surface() {
+    let line_diag = line(-5.0, -5.0, 5.0, 5.0);
+    let line_cross = line(-5.0, 5.0, 5.0, -5.0);
+    let line_parallel = line(-5.0, -4.0, 5.0, 6.0);
+    let line_coincident = line(-2.0, -2.0, 2.0, 2.0);
+    let line_collinear_touch = line(5.0, 5.0, 10.0, 10.0);
+    let line_miss = line(10.0, 10.0, 11.0, 11.0);
+    let line_tangent = line(-5.0, 3.0, 5.0, 3.0);
+    let line_through_circle = line(-5.0, 0.0, 5.0, 0.0);
+
+    let circle_unit = circle(0.0, 0.0, 3.0);
+    let circle_shift = circle(4.0, 0.0, 3.0);
+    let circle_contain = circle(0.0, 0.0, 1.0);
+    let circle_dup = circle(0.0, 0.0, 3.0);
+    let circle_far = circle(20.0, 0.0, 1.0);
+
+    let arc_right = arc(
+        0.0,
+        0.0,
+        3.0,
+        -core::f64::consts::FRAC_PI_2,
+        core::f64::consts::PI,
+    );
+    let arc_upper = arc(0.0, 0.0, 3.0, 0.0, core::f64::consts::PI);
+    let arc_other = arc(
+        4.0,
+        0.0,
+        3.0,
+        core::f64::consts::FRAC_PI_2,
+        core::f64::consts::PI,
+    );
+    let line_above_arc = line(-5.0, 5.0, 5.0, 5.0);
+
+    let arc_same_disjoint_q1 = arc(0.0, 0.0, 3.0, 0.0, core::f64::consts::FRAC_PI_2);
+    let arc_same_disjoint_q3 = arc(
+        0.0,
+        0.0,
+        3.0,
+        core::f64::consts::PI,
+        core::f64::consts::FRAC_PI_2,
+    );
+    let arc_same_touch_left = arc(0.0, 0.0, 3.0, 0.0, core::f64::consts::PI);
+    let arc_same_touch_right = arc(0.0, 0.0, 3.0, core::f64::consts::PI, core::f64::consts::PI);
+    let arc_same_partial_b = arc(
+        0.0,
+        0.0,
+        3.0,
+        core::f64::consts::FRAC_PI_2,
+        core::f64::consts::PI,
+    );
+    let arc_same_single_touch = arc(
+        0.0,
+        0.0,
+        3.0,
+        core::f64::consts::FRAC_PI_2,
+        core::f64::consts::FRAC_PI_2,
+    );
+
+    let rows = [
+        row("line_line_cross", &line_diag, &line_cross),
+        row("line_line_parallel_miss", &line_diag, &line_parallel),
+        row("line_line_coincident_overlap", &line_diag, &line_coincident),
+        row(
+            "line_line_collinear_touch",
+            &line_diag,
+            &line_collinear_touch,
+        ),
+        row("line_line_segment_miss", &line_diag, &line_miss),
+        row("line_circle_tangent", &line_tangent, &circle_unit),
+        row("line_circle_secant", &line_through_circle, &circle_unit),
+        row("line_arc_right_hit", &line_through_circle, &arc_right),
+        row("line_arc_upper_endpoints", &line_through_circle, &arc_upper),
+        row("line_arc_above_miss", &line_above_arc, &arc_upper),
+        row("circle_circle_two_points", &circle_unit, &circle_shift),
+        row("circle_circle_contained", &circle_unit, &circle_contain),
+        row("circle_circle_coincident", &circle_unit, &circle_dup),
+        row("circle_circle_disjoint", &circle_unit, &circle_far),
+        row("circle_arc_partial", &circle_shift, &arc_upper),
+        row("arc_arc_upper_vs_other", &arc_upper, &arc_other),
+        row(
+            "arc_arc_same_circle_disjoint",
+            &arc_same_disjoint_q1,
+            &arc_same_disjoint_q3,
+        ),
+        row(
+            "arc_arc_same_circle_two_touches",
+            &arc_same_touch_left,
+            &arc_same_touch_right,
+        ),
+        row(
+            "arc_arc_same_circle_partial_overlap",
+            &arc_upper,
+            &arc_same_partial_b,
+        ),
+        row(
+            "arc_arc_same_circle_single_touch",
+            &arc_same_disjoint_q1,
+            &arc_same_single_touch,
+        ),
+    ];
+
+    let surface = rows.join("\n");
+    insta::assert_snapshot!(surface);
+}
diff --git a/crates/bone-kernel/tests/snapshots/curves__arc2_half_cw_surface.snap b/crates/bone-kernel/tests/snapshots/curves__arc2_half_cw_surface.snap
new file mode 100644
index 0000000..0894289
--- /dev/null
+++ b/crates/bone-kernel/tests/snapshots/curves__arc2_half_cw_surface.snap
@@ -0,0 +1,17 @@
+---
+source: crates/bone-kernel/tests/curves.rs
+expression: "fmt_curve(\"arc2_h\", &arc)"
+---
+arc2_h_display = arc2{ c=(1 mm, 1 mm), r=2 mm, start=3.141592653589793 rad, sweep=-3.141592653589793 rad }
+arc2_h_debug   = Arc2 { center: Point2(1 mm, 1 mm), radius: 0.002 m^1, start_angle: 3.141592653589793, sweep_angle: -3.141592653589793 }
+arc2_h_samples:
+    t=0.00 -> p=(-1.000000, 1.000000) v=<0.0000000000000007694682774887159 mm, 6.283185307179586 mm>
+    t=0.25 -> p=(-0.414214, 2.414214) v=<4.442882938158366 mm, 4.442882938158366 mm>
+    t=0.50 -> p=(1.000000, 3.000000) v=<6.283185307179586 mm, -0.00000000000000038473413874435795 mm>
+    t=0.75 -> p=(2.414214, 2.414214) v=<4.442882938158366 mm, -4.442882938158366 mm>
+    t=1.00 -> p=(3.000000, 1.000000) v=<0 mm, -6.283185307179586 mm>
+arc2_h_tangent_mid = [1, -0.00000000000000006123233995736766]
+arc2_h_curvature_start = k=-0.5/mm
+arc2_h_curvature_mid   = k=-0.5/mm
+arc2_h_bbox    = aabb2{ min=(-1 mm, 1 mm), max=(3 mm, 3 mm) }
+arc2_h_closest_origin = closest{ t=0, p=(-1 mm, 1.0000000000000002 mm), d=1.4142135623730951 mm }
diff --git a/crates/bone-kernel/tests/snapshots/curves__arc2_quarter_ccw_surface.snap b/crates/bone-kernel/tests/snapshots/curves__arc2_quarter_ccw_surface.snap
new file mode 100644
index 0000000..b192efb
--- /dev/null
+++ b/crates/bone-kernel/tests/snapshots/curves__arc2_quarter_ccw_surface.snap
@@ -0,0 +1,17 @@
+---
+source: crates/bone-kernel/tests/curves.rs
+expression: "fmt_curve(\"arc2_q\", &arc)"
+---
+arc2_q_display = arc2{ c=(0 mm, 0 mm), r=5 mm, start=0 rad, sweep=1.5707963267948966 rad }
+arc2_q_debug   = Arc2 { center: Point2(0 mm, 0 mm), radius: 0.005 m^1, start_angle: 0.0, sweep_angle: 1.5707963267948966 }
+arc2_q_samples:
+    t=0.00 -> p=(5.000000, 0.000000) v=<-0 mm, 7.853981633974483 mm>
+    t=0.25 -> p=(4.619398, 1.913417) v=<-3.0055886494217314 mm, 7.256132880348577 mm>
+    t=0.50 -> p=(3.535534, 3.535534) v=<-5.553603672697957 mm, 5.553603672697958 mm>
+    t=0.75 -> p=(1.913417, 4.619398) v=<-7.256132880348577 mm, 3.005588649421732 mm>
+    t=1.00 -> p=(0.000000, 5.000000) v=<-7.853981633974483 mm, 0.0000000000000004809176734304475 mm>
+arc2_q_tangent_mid = [-0.7071067811865475, 0.7071067811865476]
+arc2_q_curvature_start = k=0.2/mm
+arc2_q_curvature_mid   = k=0.2/mm
+arc2_q_bbox    = aabb2{ min=(0 mm, 0 mm), max=(5 mm, 5 mm) }
+arc2_q_closest_origin = closest{ t=0, p=(5 mm, 0 mm), d=5 mm }
diff --git a/crates/bone-kernel/tests/snapshots/curves__circle2_surface.snap b/crates/bone-kernel/tests/snapshots/curves__circle2_surface.snap
new file mode 100644
index 0000000..4b127f6
--- /dev/null
+++ b/crates/bone-kernel/tests/snapshots/curves__circle2_surface.snap
@@ -0,0 +1,17 @@
+---
+source: crates/bone-kernel/tests/curves.rs
+expression: "fmt_curve(\"circle2\", &circle)"
+---
+circle2_display = circle2{ c=(2 mm, -1 mm), r=3 mm }
+circle2_debug   = Circle2 { center: Point2(2 mm, -1 mm), radius: 0.003 m^1 }
+circle2_samples:
+    t=0.00 -> p=(5.000000, -1.000000) v=<-0 mm, 18.84955592153876 mm>
+    t=0.25 -> p=(2.000000, 2.000000) v=<-18.84955592153876 mm, 0.0000000000000011542024162330739 mm>
+    t=0.50 -> p=(-1.000000, -1.000000) v=<-0.0000000000000023084048324661477 mm, -18.84955592153876 mm>
+    t=0.75 -> p=(2.000000, -4.000000) v=<18.84955592153876 mm, -0.0000000000000034626072486992214 mm>
+    t=1.00 -> p=(5.000000, -1.000000) v=<0.0000000000000046168096649322955 mm, 18.84955592153876 mm>
+circle2_tangent_mid = [-0.00000000000000012246467991473532, -1]
+circle2_curvature_start = k=0.3333333333333333/mm
+circle2_curvature_mid   = k=0.3333333333333333/mm
+circle2_bbox    = aabb2{ min=(-1 mm, -4 mm), max=(5 mm, 2 mm) }
+circle2_closest_origin = closest{ t=0.42620819117478337, p=(-0.6832815729997477 mm, 0.3416407864998743 mm), d=0.7639320225002102 mm }
diff --git a/crates/bone-kernel/tests/snapshots/curves__line2_surface.snap b/crates/bone-kernel/tests/snapshots/curves__line2_surface.snap
new file mode 100644
index 0000000..4b5ebe7
--- /dev/null
+++ b/crates/bone-kernel/tests/snapshots/curves__line2_surface.snap
@@ -0,0 +1,17 @@
+---
+source: crates/bone-kernel/tests/curves.rs
+expression: "fmt_curve(\"line2\", &line)"
+---
+line2_display = line2{ (1 mm, 2 mm) -> (7 mm, 6 mm) }
+line2_debug   = Line2 { start: Point2(1 mm, 2 mm), end: Point2(7 mm, 6 mm) }
+line2_samples:
+    t=0.00 -> p=(1.000000, 2.000000) v=<6 mm, 4 mm>
+    t=0.25 -> p=(2.500000, 3.000000) v=<6 mm, 4 mm>
+    t=0.50 -> p=(4.000000, 4.000000) v=<6 mm, 4 mm>
+    t=0.75 -> p=(5.500000, 5.000000) v=<6 mm, 4 mm>
+    t=1.00 -> p=(7.000000, 6.000000) v=<6 mm, 4 mm>
+line2_tangent_mid = [0.8320502943378437, 0.5547001962252291]
+line2_curvature_start = k=0/mm
+line2_curvature_mid   = k=0/mm
+line2_bbox    = aabb2{ min=(1 mm, 2 mm), max=(7 mm, 6 mm) }
+line2_closest_origin = closest{ t=0, p=(1 mm, 2 mm), d=2.23606797749979 mm }
diff --git a/crates/bone-kernel/tests/snapshots/intersect__intersection_matrix_surface.snap b/crates/bone-kernel/tests/snapshots/intersect__intersection_matrix_surface.snap
new file mode 100644
index 0000000..40e0e65
--- /dev/null
+++ b/crates/bone-kernel/tests/snapshots/intersect__intersection_matrix_surface.snap
@@ -0,0 +1,24 @@
+---
+source: crates/bone-kernel/tests/intersect.rs
+expression: surface
+---
+line_line_cross = one{ (0 mm, 0 mm) }
+line_line_parallel_miss = empty
+line_line_coincident_overlap = coincident
+line_line_collinear_touch = one{ (5 mm, 5 mm) }
+line_line_segment_miss = empty
+line_circle_tangent = one{ (0 mm, 3 mm) }
+line_circle_secant = two{ (-3 mm, 0 mm), (3 mm, 0 mm) }
+line_arc_right_hit = one{ (3 mm, 0 mm) }
+line_arc_upper_endpoints = two{ (-3 mm, 0 mm), (3 mm, 0 mm) }
+line_arc_above_miss = empty
+circle_circle_two_points = two{ (2 mm, 2.23606797749979 mm), (2 mm, -2.23606797749979 mm) }
+circle_circle_contained = empty
+circle_circle_coincident = coincident
+circle_circle_disjoint = empty
+circle_arc_partial = one{ (2 mm, 2.23606797749979 mm) }
+arc_arc_upper_vs_other = one{ (2 mm, 2.23606797749979 mm) }
+arc_arc_same_circle_disjoint = empty
+arc_arc_same_circle_two_touches = two{ (3 mm, 0 mm), (-3 mm, 0.00000000000000036739403974420594 mm) }
+arc_arc_same_circle_partial_overlap = coincident
+arc_arc_same_circle_single_touch = one{ (0.00000000000000018369701987210297 mm, 3 mm) }
diff --git a/crates/bone-render/Cargo.toml b/crates/bone-render/Cargo.toml
new file mode 100644
index 0000000..eccbb98
--- /dev/null
+++ b/crates/bone-render/Cargo.toml
@@ -0,0 +1,25 @@
+[package]
+name = "bone-render"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+rust-version.workspace = true
+
+[dependencies]
+bone-document = { workspace = true }
+bone-kernel = { workspace = true }
+bone-types = { workspace = true }
+bytemuck = { workspace = true }
+png = { workspace = true }
+slotmap = { workspace = true }
+thiserror = { workspace = true }
+tracing = { workspace = true }
+uom = { workspace = true }
+wgpu = { workspace = true }
+
+[dev-dependencies]
+pollster = { workspace = true }
+proptest = { workspace = true }
+
+[lints]
+workspace = true
diff --git a/crates/bone-render/src/camera.rs b/crates/bone-render/src/camera.rs
new file mode 100644
index 0000000..a66d042
--- /dev/null
+++ b/crates/bone-render/src/camera.rs
@@ -0,0 +1,330 @@
+use bone_types::{Length, Vec2};
+use uom::si::length::millimeter;
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct ViewportPx(u32);
+
+impl ViewportPx {
+    #[must_use]
+    pub const fn new(value: u32) -> Self {
+        Self(value)
+    }
+
+    #[must_use]
+    pub const fn value(self) -> u32 {
+        self.0
+    }
+}
+
+impl core::fmt::Display for ViewportPx {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        write!(f, "{}px", self.0)
+    }
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
+pub struct ViewportExtent {
+    width: ViewportPx,
+    height: ViewportPx,
+}
+
+impl ViewportExtent {
+    #[must_use]
+    pub const fn new(width: ViewportPx, height: ViewportPx) -> Self {
+        Self { width, height }
+    }
+
+    #[must_use]
+    pub const fn square(side: ViewportPx) -> Self {
+        Self {
+            width: side,
+            height: side,
+        }
+    }
+
+    #[must_use]
+    pub const fn width(self) -> ViewportPx {
+        self.width
+    }
+
+    #[must_use]
+    pub const fn height(self) -> ViewportPx {
+        self.height
+    }
+
+    #[must_use]
+    pub fn pixel_count(self) -> u64 {
+        u64::from(self.width.value()) * u64::from(self.height.value())
+    }
+}
+
+impl core::fmt::Display for ViewportExtent {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        write!(f, "{}x{}", self.width, self.height)
+    }
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, PartialOrd)]
+pub struct PixelsPerMm(f64);
+
+impl PixelsPerMm {
+    pub const DEFAULT: Self = Self(10.0);
+
+    #[must_use]
+    pub fn new(value: f64) -> Self {
+        assert!(
+            value.is_finite() && value > 0.0,
+            "PixelsPerMm requires a positive, finite value: got {value}",
+        );
+        Self(value)
+    }
+
+    #[must_use]
+    pub const fn value(self) -> f64 {
+        self.0
+    }
+}
+
+impl core::fmt::Display for PixelsPerMm {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        write!(f, "{} px/mm", self.0)
+    }
+}
+
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub struct Camera2 {
+    extent: ViewportExtent,
+    pan_mm: Vec2,
+    zoom: PixelsPerMm,
+}
+
+impl Camera2 {
+    #[must_use]
+    pub fn new(extent: ViewportExtent) -> Self {
+        Self {
+            extent,
+            pan_mm: Vec2::zero(),
+            zoom: PixelsPerMm::DEFAULT,
+        }
+    }
+
+    #[must_use]
+    pub const fn extent(self) -> ViewportExtent {
+        self.extent
+    }
+
+    #[must_use]
+    pub const fn pan_mm(self) -> Vec2 {
+        self.pan_mm
+    }
+
+    #[must_use]
+    pub const fn zoom(self) -> PixelsPerMm {
+        self.zoom
+    }
+
+    #[must_use]
+    pub fn with_extent(self, extent: ViewportExtent) -> Self {
+        Self { extent, ..self }
+    }
+
+    #[must_use]
+    pub fn with_pan(self, pan_mm: Vec2) -> Self {
+        Self { pan_mm, ..self }
+    }
+
+    #[must_use]
+    pub fn with_zoom(self, zoom: PixelsPerMm) -> Self {
+        Self { zoom, ..self }
+    }
+
+    #[must_use]
+    pub fn world_mm_per_pixel(self) -> f64 {
+        1.0 / self.zoom.value()
+    }
+
+    #[must_use]
+    #[allow(clippy::cast_possible_truncation)]
+    pub fn clip_from_world_mm(self) -> [f32; 16] {
+        let (sx, sy) = self.scale();
+        let (px, py) = self.pan_mm.coords_mm();
+        let tx = -px * sx;
+        let ty = -py * sy;
+        column_major(sx as f32, sy as f32, tx as f32, ty as f32)
+    }
+
+    #[must_use]
+    #[allow(clippy::cast_possible_truncation)]
+    pub fn world_mm_from_clip(self) -> [f32; 16] {
+        let (sx, sy) = self.scale();
+        let inv_x = 1.0 / sx;
+        let inv_y = 1.0 / sy;
+        let (px, py) = self.pan_mm.coords_mm();
+        column_major(inv_x as f32, inv_y as f32, px as f32, py as f32)
+    }
+
+    fn scale(self) -> (f64, f64) {
+        let w = f64::from(self.extent.width.value());
+        let h = f64::from(self.extent.height.value());
+        let z = self.zoom.value();
+        (2.0 * z / w, 2.0 * z / h)
+    }
+}
+
+fn column_major(sx: f32, sy: f32, tx: f32, ty: f32) -> [f32; 16] {
+    [
+        sx, 0.0, 0.0, 0.0, 0.0, sy, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0, tx, ty, 0.0, 1.0,
+    ]
+}
+
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub struct GridSpacing {
+    minor: Length,
+    major_every: u32,
+}
+
+impl GridSpacing {
+    #[must_use]
+    pub fn minor(self) -> Length {
+        self.minor
+    }
+
+    #[must_use]
+    pub const fn major_every(self) -> u32 {
+        self.major_every
+    }
+
+    #[must_use]
+    pub fn major(self) -> Length {
+        self.minor * f64::from(self.major_every)
+    }
+
+    #[must_use]
+    pub fn from_zoom(zoom: PixelsPerMm, target_minor_px: f32) -> Self {
+        assert!(
+            target_minor_px.is_finite() && target_minor_px > 0.0,
+            "GridSpacing::from_zoom requires a positive, finite target: got {target_minor_px}",
+        );
+        let desired_mm = f64::from(target_minor_px) / zoom.value();
+        let k = desired_mm.log10().floor();
+        let frac = desired_mm / 10f64.powf(k);
+        let (multiplier, major_every, k_bump) = if frac < f64::sqrt(2.0) {
+            (1.0, 10, 0.0)
+        } else if frac < f64::sqrt(10.0) {
+            (2.0, 5, 0.0)
+        } else if frac < f64::sqrt(50.0) {
+            (5.0, 2, 0.0)
+        } else {
+            (1.0, 10, 1.0)
+        };
+        let minor_mm = multiplier * 10f64.powf(k + k_bump);
+        Self {
+            minor: Length::new::<millimeter>(minor_mm),
+            major_every,
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn extent(w: u32, h: u32) -> ViewportExtent {
+        ViewportExtent::new(ViewportPx::new(w), ViewportPx::new(h))
+    }
+
+    fn approx_eq(a: f32, b: f32) -> bool {
+        (a - b).abs() < 1e-6
+    }
+
+    #[test]
+    fn default_camera_maps_origin_to_clip_center() {
+        let cam = Camera2::new(extent(200, 100));
+        let m = cam.clip_from_world_mm();
+        assert!(approx_eq(m[12], 0.0));
+        assert!(approx_eq(m[13], 0.0));
+    }
+
+    #[test]
+    fn clip_from_world_is_column_major() {
+        let cam = Camera2::new(extent(100, 100)).with_pan(Vec2::from_mm(2.0, 3.0));
+        let m = cam.clip_from_world_mm();
+        assert!(approx_eq(m[3], 0.0));
+        assert!(approx_eq(m[7], 0.0));
+        assert!(approx_eq(m[11], 0.0));
+        assert!(approx_eq(m[15], 1.0));
+    }
+
+    #[test]
+    fn inverse_round_trips_origin() {
+        let cam = Camera2::new(extent(256, 128))
+            .with_pan(Vec2::from_mm(-5.0, 4.0))
+            .with_zoom(PixelsPerMm::new(8.0));
+        let fwd = cam.clip_from_world_mm();
+        let inv = cam.world_mm_from_clip();
+        let rt = mul4(&inv, &fwd);
+        (0..16).for_each(|idx| {
+            let (col, row) = (idx / 4, idx % 4);
+            let want = if col == row { 1.0 } else { 0.0 };
+            assert!(
+                approx_eq(rt[idx], want),
+                "round trip broke at col={col},row={row}: {}",
+                rt[idx]
+            );
+        });
+    }
+
+    fn mul4(a: &[f32; 16], b: &[f32; 16]) -> [f32; 16] {
+        core::array::from_fn(|idx| {
+            let (col, row) = (idx / 4, idx % 4);
+            (0..4).map(|k| a[k * 4 + row] * b[col * 4 + k]).sum()
+        })
+    }
+
+    #[test]
+    fn grid_spacing_selects_one_two_five_in_decade() {
+        [1.0_f64, 10.0, 100.0]
+            .into_iter()
+            .map(PixelsPerMm::new)
+            .map(|z| GridSpacing::from_zoom(z, 16.0))
+            .for_each(|s| {
+                let minor_mm = s.minor().get::<millimeter>();
+                let k = minor_mm.log10().floor();
+                let decade = 10f64.powf(k);
+                let mult = minor_mm / decade;
+                let snapped = [1.0, 2.0, 5.0]
+                    .into_iter()
+                    .any(|m: f64| (mult - m).abs() < 1e-9);
+                assert!(snapped, "minor {minor_mm} mm not in {{1, 2, 5}} x decade");
+                assert!([2, 5, 10].contains(&s.major_every()));
+            });
+    }
+
+    #[test]
+    fn grid_spacing_rolls_over_decade_past_sqrt_fifty() {
+        let zoom = PixelsPerMm::new(2.0);
+        let s = GridSpacing::from_zoom(zoom, 16.0);
+        let minor_mm = s.minor().get::<millimeter>();
+        assert!(
+            (minor_mm - 10.0).abs() < 1e-9,
+            "expected rollover to 10 mm, got {minor_mm}",
+        );
+        assert_eq!(s.major_every(), 10);
+    }
+
+    #[test]
+    fn clip_x_of_pan_point_is_zero() {
+        let cam = Camera2::new(extent(320, 240))
+            .with_pan(Vec2::from_mm(7.0, -3.0))
+            .with_zoom(PixelsPerMm::new(5.0));
+        let m = cam.clip_from_world_mm();
+        let world = [7.0_f32, -3.0_f32, 0.0_f32, 1.0_f32];
+        let clip = apply(&m, world);
+        assert!(approx_eq(clip[0], 0.0));
+        assert!(approx_eq(clip[1], 0.0));
+    }
+
+    fn apply(m: &[f32; 16], v: [f32; 4]) -> [f32; 4] {
+        core::array::from_fn(|row| (0..4).map(|col| m[col * 4 + row] * v[col]).sum())
+    }
+}
diff --git a/crates/bone-render/src/diff.rs b/crates/bone-render/src/diff.rs
new file mode 100644
index 0000000..6812272
--- /dev/null
+++ b/crates/bone-render/src/diff.rs
@@ -0,0 +1,280 @@
+use bone_types::Tolerance;
+
+use crate::camera::ViewportExtent;
+use crate::snapshot::SnapshotFrame;
+
+#[derive(Copy, Clone, Debug, PartialEq, PartialOrd)]
+pub struct PixelDiffThreshold(f64);
+
+impl PixelDiffThreshold {
+    pub const EXACT: Self = Self(0.0);
+
+    #[must_use]
+    pub fn new(channel_fraction: f64) -> Self {
+        Self(channel_fraction.clamp(0.0, 1.0))
+    }
+
+    #[must_use]
+    pub const fn value(self) -> f64 {
+        self.0
+    }
+
+    #[must_use]
+    #[allow(clippy::cast_possible_truncation, clippy::cast_sign_loss)]
+    pub fn as_u8(self) -> u8 {
+        (self.0.clamp(0.0, 1.0) * 255.0).round() as u8
+    }
+}
+
+impl From<Tolerance> for PixelDiffThreshold {
+    fn from(t: Tolerance) -> Self {
+        Self::new(t.value())
+    }
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+pub struct PixelMismatch {
+    x: u32,
+    y: u32,
+    channel_delta: [u8; 4],
+}
+
+impl PixelMismatch {
+    #[must_use]
+    pub const fn x(self) -> u32 {
+        self.x
+    }
+
+    #[must_use]
+    pub const fn y(self) -> u32 {
+        self.y
+    }
+
+    #[must_use]
+    pub const fn channel_delta(self) -> [u8; 4] {
+        self.channel_delta
+    }
+
+    #[must_use]
+    pub fn max_delta(self) -> u8 {
+        self.channel_delta.into_iter().max().unwrap_or(0)
+    }
+}
+
+#[derive(Clone, Debug, Default)]
+pub struct PixelDiffReport {
+    worst: Option<PixelMismatch>,
+    over_threshold: u32,
+}
+
+impl PixelDiffReport {
+    #[must_use]
+    pub const fn worst(&self) -> Option<PixelMismatch> {
+        self.worst
+    }
+
+    #[must_use]
+    pub const fn over_threshold(&self) -> u32 {
+        self.over_threshold
+    }
+
+    #[must_use]
+    pub const fn is_clean(&self) -> bool {
+        self.over_threshold == 0
+    }
+}
+
+#[derive(Debug, thiserror::Error)]
+pub enum PixelDiffError {
+    #[error(
+        "pixel buffer length mismatch: actual={actual}, expected={expected}, required={required}"
+    )]
+    ShapeMismatch {
+        actual: usize,
+        expected: usize,
+        required: usize,
+    },
+    #[error("viewport dimension is zero")]
+    ZeroExtent,
+    #[error("pixel count {pixels} exceeds u32::MAX")]
+    ExtentOverflow { pixels: u64 },
+}
+
+pub struct PixelDiff;
+
+impl PixelDiff {
+    pub fn compare_bytes(
+        extent: ViewportExtent,
+        actual: &[u8],
+        expected: &[u8],
+        threshold: PixelDiffThreshold,
+    ) -> core::result::Result<PixelDiffReport, PixelDiffError> {
+        let width = extent.width().value();
+        let height = extent.height().value();
+        if width == 0 || height == 0 {
+            return Err(PixelDiffError::ZeroExtent);
+        }
+        let pixel_count = extent.pixel_count();
+        if pixel_count > u64::from(u32::MAX) {
+            return Err(PixelDiffError::ExtentOverflow {
+                pixels: pixel_count,
+            });
+        }
+        let Ok(required) = usize::try_from(pixel_count * 4) else {
+            return Err(PixelDiffError::ExtentOverflow {
+                pixels: pixel_count,
+            });
+        };
+        if actual.len() != required || expected.len() != required {
+            return Err(PixelDiffError::ShapeMismatch {
+                actual: actual.len(),
+                expected: expected.len(),
+                required,
+            });
+        }
+        let limit = threshold.as_u8();
+        let report = actual
+            .chunks_exact(4)
+            .zip(expected.chunks_exact(4))
+            .zip(0_u32..)
+            .filter_map(|((a, e), idx)| {
+                let delta: [u8; 4] = [
+                    a[0].abs_diff(e[0]),
+                    a[1].abs_diff(e[1]),
+                    a[2].abs_diff(e[2]),
+                    a[3].abs_diff(e[3]),
+                ];
+                let max = delta[0].max(delta[1]).max(delta[2]).max(delta[3]);
+                (max > limit).then_some((idx, delta))
+            })
+            .fold(PixelDiffReport::default(), |acc, (idx, delta)| {
+                let m = PixelMismatch {
+                    x: idx % width,
+                    y: idx / width,
+                    channel_delta: delta,
+                };
+                let worst = match acc.worst {
+                    Some(prev) if prev.max_delta() >= m.max_delta() => Some(prev),
+                    _ => Some(m),
+                };
+                PixelDiffReport {
+                    worst,
+                    over_threshold: acc.over_threshold + 1,
+                }
+            });
+        Ok(report)
+    }
+
+    pub fn compare(
+        actual: &SnapshotFrame,
+        expected: &[u8],
+        threshold: PixelDiffThreshold,
+    ) -> core::result::Result<PixelDiffReport, PixelDiffError> {
+        Self::compare_bytes(actual.extent(), actual.rgba(), expected, threshold)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::camera::{ViewportExtent, ViewportPx};
+
+    fn extent(w: u32, h: u32) -> ViewportExtent {
+        ViewportExtent::new(ViewportPx::new(w), ViewportPx::new(h))
+    }
+
+    fn run(
+        extent: ViewportExtent,
+        actual: &[u8],
+        expected: &[u8],
+        threshold: PixelDiffThreshold,
+    ) -> PixelDiffReport {
+        let Ok(report) = PixelDiff::compare_bytes(extent, actual, expected, threshold) else {
+            panic!("compare_bytes rejected inputs");
+        };
+        report
+    }
+
+    #[test]
+    fn exact_match_is_clean() {
+        let buf = vec![10_u8, 20, 30, 255, 10, 20, 30, 255];
+        let report = run(extent(2, 1), &buf, &buf, PixelDiffThreshold::EXACT);
+        assert!(report.is_clean());
+        assert!(report.worst().is_none());
+    }
+
+    #[test]
+    fn single_channel_delta_under_threshold_is_clean() {
+        let a = vec![10_u8, 20, 30, 255];
+        let b = vec![12_u8, 20, 30, 255];
+        let report = run(extent(1, 1), &a, &b, PixelDiffThreshold::new(4.0 / 255.0));
+        assert!(report.is_clean());
+    }
+
+    #[test]
+    fn over_threshold_reports_worst_pixel_coords() {
+        let a = vec![0_u8, 0, 0, 255, 200, 0, 0, 255];
+        let b = vec![0_u8, 0, 0, 255, 50, 0, 0, 255];
+        let report = run(extent(2, 1), &a, &b, PixelDiffThreshold::EXACT);
+        let Some(worst) = report.worst() else {
+            panic!("worst pixel present");
+        };
+        assert_eq!(worst.x(), 1);
+        assert_eq!(worst.y(), 0);
+        assert_eq!(worst.max_delta(), 150);
+        assert_eq!(report.over_threshold(), 1);
+    }
+
+    #[test]
+    fn tolerance_maps_to_threshold() {
+        let t = Tolerance::new(8.0 / 255.0);
+        let th: PixelDiffThreshold = t.into();
+        assert_eq!(th.as_u8(), 8);
+    }
+
+    #[test]
+    fn coords_index_by_row_major() {
+        let a = vec![0_u8; 32];
+        let mut b = a.clone();
+        let pixel = 5_usize;
+        b[pixel * 4] = 200;
+        let report = run(extent(4, 2), &a, &b, PixelDiffThreshold::EXACT);
+        let Some(worst) = report.worst() else {
+            panic!("worst present");
+        };
+        assert_eq!(worst.x(), 1);
+        assert_eq!(worst.y(), 1);
+    }
+
+    #[test]
+    fn shape_mismatch_is_rejected() {
+        let a = vec![0_u8; 12];
+        let b = vec![0_u8; 8];
+        let Err(err) = PixelDiff::compare_bytes(extent(2, 1), &a, &b, PixelDiffThreshold::EXACT)
+        else {
+            panic!("expected ShapeMismatch");
+        };
+        let PixelDiffError::ShapeMismatch {
+            actual,
+            expected,
+            required,
+        } = err
+        else {
+            panic!("wrong error variant: {err:?}");
+        };
+        assert_eq!(actual, 12);
+        assert_eq!(expected, 8);
+        assert_eq!(required, 8);
+    }
+
+    #[test]
+    fn zero_extent_is_rejected() {
+        let buf: Vec<u8> = Vec::new();
+        let Err(err) =
+            PixelDiff::compare_bytes(extent(0, 4), &buf, &buf, PixelDiffThreshold::EXACT)
+        else {
+            panic!("expected ZeroExtent");
+        };
+        assert!(matches!(err, PixelDiffError::ZeroExtent));
+    }
+}
diff --git a/crates/bone-render/src/gpu.rs b/crates/bone-render/src/gpu.rs
new file mode 100644
index 0000000..0cc2000
--- /dev/null
+++ b/crates/bone-render/src/gpu.rs
@@ -0,0 +1,372 @@
+use crate::camera::{ViewportExtent, ViewportPx};
+use crate::pick::{PickId, PickIndex, Picker};
+use crate::snapshot::{SnapshotFrame, Style};
+use crate::{RenderError, Result};
+
+pub(crate) const COLOR_FORMAT: wgpu::TextureFormat = wgpu::TextureFormat::Rgba8Unorm;
+pub(crate) const PICK_FORMAT: wgpu::TextureFormat = wgpu::TextureFormat::R32Uint;
+pub(crate) const BYTES_PER_PIXEL: u32 = 4;
+pub(crate) const PICK_BYTES_PER_PIXEL: u32 = 4;
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+pub struct BackendTag(wgpu::Backend);
+
+impl BackendTag {
+    #[cfg(test)]
+    pub(crate) const fn from_backend(backend: wgpu::Backend) -> Self {
+        Self(backend)
+    }
+
+    #[must_use]
+    pub fn backend(self) -> wgpu::Backend {
+        self.0
+    }
+}
+
+impl core::fmt::Display for BackendTag {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        write!(f, "{:?}", self.0)
+    }
+}
+
+#[derive(Clone, Debug)]
+pub struct Capabilities {
+    adapter_limits: wgpu::Limits,
+    backend: BackendTag,
+    adapter_name: String,
+}
+
+impl Capabilities {
+    pub(crate) fn probe(adapter: &wgpu::Adapter) -> Self {
+        let info = adapter.get_info();
+        Self {
+            adapter_limits: adapter.limits(),
+            backend: BackendTag(info.backend),
+            adapter_name: info.name,
+        }
+    }
+
+    #[must_use]
+    pub fn adapter_limits(&self) -> &wgpu::Limits {
+        &self.adapter_limits
+    }
+
+    #[must_use]
+    pub fn backend(&self) -> BackendTag {
+        self.backend
+    }
+
+    #[must_use]
+    pub fn adapter_name(&self) -> &str {
+        &self.adapter_name
+    }
+}
+
+pub struct Gpu {
+    device: wgpu::Device,
+    queue: wgpu::Queue,
+    capabilities: Capabilities,
+}
+
+impl Gpu {
+    pub(crate) fn from_parts(
+        device: wgpu::Device,
+        queue: wgpu::Queue,
+        capabilities: Capabilities,
+    ) -> Self {
+        Self {
+            device,
+            queue,
+            capabilities,
+        }
+    }
+
+    #[must_use]
+    pub fn device(&self) -> &wgpu::Device {
+        &self.device
+    }
+
+    #[must_use]
+    pub fn queue(&self) -> &wgpu::Queue {
+        &self.queue
+    }
+
+    #[must_use]
+    pub fn capabilities(&self) -> &Capabilities {
+        &self.capabilities
+    }
+}
+
+pub struct OffscreenContext {
+    gpu: Gpu,
+    color: wgpu::Texture,
+    pick: wgpu::Texture,
+    pick_staging: wgpu::Buffer,
+    extent: ViewportExtent,
+}
+
+impl OffscreenContext {
+    pub async fn new(extent: ViewportExtent) -> Result<Self> {
+        if extent.width().value() == 0 || extent.height().value() == 0 {
+            return Err(RenderError::ZeroExtent);
+        }
+        let instance =
+            wgpu::Instance::new(wgpu::InstanceDescriptor::new_without_display_handle_from_env());
+        let adapter = instance
+            .request_adapter(&wgpu::RequestAdapterOptions {
+                power_preference: wgpu::PowerPreference::LowPower,
+                force_fallback_adapter: false,
+                compatible_surface: None,
+            })
+            .await?;
+        let (device, queue) = adapter
+            .request_device(&wgpu::DeviceDescriptor {
+                label: Some("bone-render:offscreen"),
+                required_features: wgpu::Features::empty(),
+                required_limits: wgpu::Limits::downlevel_defaults(),
+                experimental_features: wgpu::ExperimentalFeatures::default(),
+                memory_hints: wgpu::MemoryHints::default(),
+                trace: wgpu::Trace::Off,
+            })
+            .await?;
+        let capabilities = Capabilities::probe(&adapter);
+        let color = device.create_texture(&wgpu::TextureDescriptor {
+            label: Some("bone-render:offscreen-color"),
+            size: texture_size(extent),
+            mip_level_count: 1,
+            sample_count: 1,
+            dimension: wgpu::TextureDimension::D2,
+            format: COLOR_FORMAT,
+            usage: wgpu::TextureUsages::RENDER_ATTACHMENT | wgpu::TextureUsages::COPY_SRC,
+            view_formats: &[],
+        });
+        let pick = device.create_texture(&wgpu::TextureDescriptor {
+            label: Some("bone-render:offscreen-pick"),
+            size: texture_size(extent),
+            mip_level_count: 1,
+            sample_count: 1,
+            dimension: wgpu::TextureDimension::D2,
+            format: PICK_FORMAT,
+            usage: wgpu::TextureUsages::RENDER_ATTACHMENT | wgpu::TextureUsages::COPY_SRC,
+            view_formats: &[],
+        });
+        let pick_staging = device.create_buffer(&wgpu::BufferDescriptor {
+            label: Some("bone-render:pick-readback"),
+            size: u64::from(wgpu::COPY_BYTES_PER_ROW_ALIGNMENT),
+            usage: wgpu::BufferUsages::COPY_DST | wgpu::BufferUsages::MAP_READ,
+            mapped_at_creation: false,
+        });
+        Ok(Self {
+            gpu: Gpu {
+                device,
+                queue,
+                capabilities,
+            },
+            color,
+            pick,
+            pick_staging,
+            extent,
+        })
+    }
+
+    #[must_use]
+    pub fn gpu(&self) -> &Gpu {
+        &self.gpu
+    }
+
+    #[must_use]
+    pub fn extent(&self) -> ViewportExtent {
+        self.extent
+    }
+
+    #[must_use]
+    pub const fn color_format(&self) -> wgpu::TextureFormat {
+        COLOR_FORMAT
+    }
+
+    #[must_use]
+    pub fn color_view(&self) -> wgpu::TextureView {
+        self.color
+            .create_view(&wgpu::TextureViewDescriptor::default())
+    }
+
+    #[must_use]
+    pub fn pick_view(&self) -> wgpu::TextureView {
+        self.pick
+            .create_view(&wgpu::TextureViewDescriptor::default())
+    }
+
+    #[must_use]
+    pub fn picker(&self, index: PickIndex) -> Picker<'_> {
+        Picker::new(
+            &self.gpu,
+            &self.pick,
+            &self.pick_staging,
+            self.extent,
+            index,
+        )
+    }
+
+    pub fn render_clear(&self, style: &Style) -> Result<SnapshotFrame> {
+        self.render(|encoder, color_view, pick_view| {
+            let _pass = encoder.begin_render_pass(&wgpu::RenderPassDescriptor {
+                label: Some("bone-render:clear"),
+                color_attachments: &[
+                    Some(wgpu::RenderPassColorAttachment {
+                        view: color_view,
+                        resolve_target: None,
+                        depth_slice: None,
+                        ops: wgpu::Operations {
+                            load: wgpu::LoadOp::Clear(style.background().into()),
+                            store: wgpu::StoreOp::Store,
+                        },
+                    }),
+                    Some(wgpu::RenderPassColorAttachment {
+                        view: pick_view,
+                        resolve_target: None,
+                        depth_slice: None,
+                        ops: wgpu::Operations {
+                            load: wgpu::LoadOp::Clear(pick_clear_color()),
+                            store: wgpu::StoreOp::Store,
+                        },
+                    }),
+                ],
+                depth_stencil_attachment: None,
+                timestamp_writes: None,
+                occlusion_query_set: None,
+                multiview_mask: None,
+            });
+        })
+    }
+
+    pub(crate) fn render<F>(&self, mut build_passes: F) -> Result<SnapshotFrame>
+    where
+        F: FnMut(&mut wgpu::CommandEncoder, &wgpu::TextureView, &wgpu::TextureView),
+    {
+        let mut encoder = self
+            .gpu
+            .device
+            .create_command_encoder(&wgpu::CommandEncoderDescriptor {
+                label: Some("bone-render:encoder"),
+            });
+        let color_view = self.color_view();
+        let pick_view = self.pick_view();
+        build_passes(&mut encoder, &color_view, &pick_view);
+        let padded_bpr = padded_bytes_per_row(self.extent.width());
+        let staging = self.gpu.device.create_buffer(&wgpu::BufferDescriptor {
+            label: Some("bone-render:readback"),
+            size: u64::from(padded_bpr) * u64::from(self.extent.height().value()),
+            usage: wgpu::BufferUsages::COPY_DST | wgpu::BufferUsages::MAP_READ,
+            mapped_at_creation: false,
+        });
+        encoder.copy_texture_to_buffer(
+            wgpu::TexelCopyTextureInfo {
+                texture: &self.color,
+                mip_level: 0,
+                origin: wgpu::Origin3d::ZERO,
+                aspect: wgpu::TextureAspect::All,
+            },
+            wgpu::TexelCopyBufferInfo {
+                buffer: &staging,
+                layout: wgpu::TexelCopyBufferLayout {
+                    offset: 0,
+                    bytes_per_row: Some(padded_bpr),
+                    rows_per_image: Some(self.extent.height().value()),
+                },
+            },
+            texture_size(self.extent),
+        );
+        self.gpu.queue.submit(Some(encoder.finish()));
+        let rgba = read_staging(&self.gpu.device, &staging, self.extent, padded_bpr)?;
+        Ok(SnapshotFrame::new(
+            self.extent,
+            rgba,
+            self.gpu.capabilities.backend(),
+        ))
+    }
+}
+
+pub(crate) fn clear_pick_attachment(
+    encoder: &mut wgpu::CommandEncoder,
+    pick_view: &wgpu::TextureView,
+) {
+    let _pass = encoder.begin_render_pass(&wgpu::RenderPassDescriptor {
+        label: Some("bone-render:pick-clear"),
+        color_attachments: &[Some(wgpu::RenderPassColorAttachment {
+            view: pick_view,
+            resolve_target: None,
+            depth_slice: None,
+            ops: wgpu::Operations {
+                load: wgpu::LoadOp::Clear(pick_clear_color()),
+                store: wgpu::StoreOp::Store,
+            },
+        })],
+        depth_stencil_attachment: None,
+        timestamp_writes: None,
+        occlusion_query_set: None,
+        multiview_mask: None,
+    });
+}
+
+fn pick_clear_color() -> wgpu::Color {
+    wgpu::Color {
+        r: f64::from(PickId::NONE.raw()),
+        g: 0.0,
+        b: 0.0,
+        a: 0.0,
+    }
+}
+
+fn texture_size(extent: ViewportExtent) -> wgpu::Extent3d {
+    wgpu::Extent3d {
+        width: extent.width().value(),
+        height: extent.height().value(),
+        depth_or_array_layers: 1,
+    }
+}
+
+pub(crate) fn padded_bytes_per_row(width: ViewportPx) -> u32 {
+    let align = wgpu::COPY_BYTES_PER_ROW_ALIGNMENT;
+    let w = width.value();
+    debug_assert!(
+        w <= u32::MAX / BYTES_PER_PIXEL,
+        "viewport width too large for padded row calc"
+    );
+    let raw = w * BYTES_PER_PIXEL;
+    raw.div_ceil(align) * align
+}
+
+fn read_staging(
+    device: &wgpu::Device,
+    buffer: &wgpu::Buffer,
+    extent: ViewportExtent,
+    padded_bpr: u32,
+) -> Result<Vec<u8>> {
+    let slice = buffer.slice(..);
+    let (tx, rx) =
+        std::sync::mpsc::sync_channel::<core::result::Result<(), wgpu::BufferAsyncError>>(1);
+    slice.map_async(wgpu::MapMode::Read, move |res| {
+        let _ = tx.send(res);
+    });
+    device
+        .poll(wgpu::PollType::wait_indefinitely())
+        .map_err(RenderError::Poll)?;
+    match rx.try_recv() {
+        Ok(Ok(())) => {}
+        Ok(Err(e)) => return Err(RenderError::Map(e)),
+        Err(_) => return Err(RenderError::MapMissing),
+    }
+    let row_bytes = extent.width().value() * BYTES_PER_PIXEL;
+    let rgba: Vec<u8> = {
+        let view = slice.get_mapped_range();
+        (0..extent.height().value())
+            .flat_map(|y| {
+                let start = (y * padded_bpr) as usize;
+                let end = start + row_bytes as usize;
+                view[start..end].iter().copied()
+            })
+            .collect()
+    };
+    buffer.unmap();
+    Ok(rgba)
+}
diff --git a/crates/bone-render/src/lib.rs b/crates/bone-render/src/lib.rs
new file mode 100644
index 0000000..de04dad
--- /dev/null
+++ b/crates/bone-render/src/lib.rs
@@ -0,0 +1,105 @@
+pub mod camera;
+pub mod diff;
+pub mod gpu;
+pub mod pick;
+pub mod pipelines;
+pub mod scene;
+pub mod snapshot;
+pub mod surface;
+
+pub use camera::{Camera2, GridSpacing, PixelsPerMm, ViewportExtent, ViewportPx};
+pub use diff::{PixelDiff, PixelDiffError, PixelDiffReport, PixelDiffThreshold, PixelMismatch};
+pub use gpu::{BackendTag, Capabilities, Gpu, OffscreenContext};
+pub use pick::{EntityKindTag, PickId, PickIdError, PickIndex, PickQuery, PickedItem, Picker};
+pub use pipelines::{ArcPipeline, GridPipeline, LinesPipeline};
+pub use scene::{SceneArc, SceneCircle, SceneLine, ScenePoint, SketchScene};
+pub use snapshot::{
+    ClearColor, GridStyle, SnapshotFrame, StrokeStyle, Style, decode_png, encode_png,
+};
+pub use surface::{SurfaceContext, SurfaceError};
+
+#[derive(Debug, thiserror::Error)]
+pub enum RenderError {
+    #[error("no wgpu adapter matched the offscreen request: {0}")]
+    NoAdapter(#[from] wgpu::RequestAdapterError),
+    #[error("wgpu device request failed: {0}")]
+    Device(#[from] wgpu::RequestDeviceError),
+    #[error("wgpu poll failed: {0}")]
+    Poll(wgpu::PollError),
+    #[error("wgpu buffer map failed: {0}")]
+    Map(wgpu::BufferAsyncError),
+    #[error("wgpu buffer map callback did not fire after poll")]
+    MapMissing,
+    #[error("png encode failed: {0}")]
+    PngEncode(#[from] png::EncodingError),
+    #[error("png decode failed: {0}")]
+    PngDecode(#[from] png::DecodingError),
+    #[error("png format unsupported: color={color_type:?}, depth={bit_depth:?}, require rgba8")]
+    PngFormat {
+        color_type: png::ColorType,
+        bit_depth: png::BitDepth,
+    },
+    #[error("viewport dimension is zero")]
+    ZeroExtent,
+    #[error("pick id construction failed: {0}")]
+    PickId(#[from] PickIdError),
+    #[error("pick query {query} outside viewport {extent}")]
+    PickOutOfBounds {
+        query: PickQuery,
+        extent: ViewportExtent,
+    },
+}
+
+pub type Result<T, E = RenderError> = core::result::Result<T, E>;
+
+#[derive(Debug)]
+pub struct SketchRenderer {
+    grid: GridPipeline,
+    arcs: ArcPipeline,
+    lines: LinesPipeline,
+}
+
+impl SketchRenderer {
+    #[must_use]
+    pub fn new(gpu: &Gpu, color_format: wgpu::TextureFormat) -> Self {
+        Self {
+            grid: GridPipeline::new(gpu, color_format),
+            arcs: ArcPipeline::new(gpu, color_format),
+            lines: LinesPipeline::new(gpu, color_format),
+        }
+    }
+
+    pub fn encode_passes(
+        &self,
+        encoder: &mut wgpu::CommandEncoder,
+        color_view: &wgpu::TextureView,
+        pick_view: &wgpu::TextureView,
+        scene: &SketchScene,
+        camera: Camera2,
+        style: &Style,
+    ) {
+        self.grid.draw(encoder, color_view, camera, style);
+        gpu::clear_pick_attachment(encoder, pick_view);
+        self.arcs
+            .draw(encoder, color_view, pick_view, camera, style, scene);
+        self.lines
+            .draw(encoder, color_view, pick_view, camera, style, scene);
+    }
+
+    pub fn render(
+        &self,
+        ctx: &OffscreenContext,
+        scene: &SketchScene,
+        camera: Camera2,
+        style: &Style,
+    ) -> Result<SnapshotFrame> {
+        debug_assert_eq!(
+            camera.extent(),
+            ctx.extent(),
+            "camera extent must match offscreen context extent",
+        );
+        ctx.render(|encoder, color_view, pick_view| {
+            self.encode_passes(encoder, color_view, pick_view, scene, camera, style);
+        })
+    }
+}
diff --git a/crates/bone-render/src/pick.rs b/crates/bone-render/src/pick.rs
new file mode 100644
index 0000000..16fcc92
--- /dev/null
+++ b/crates/bone-render/src/pick.rs
@@ -0,0 +1,670 @@
+use bone_types::{SketchDimensionId, SketchEntityId, SketchRelationId};
+use slotmap::Key;
+use std::collections::HashMap;
+use std::collections::hash_map::Entry;
+
+use crate::camera::{ViewportExtent, ViewportPx};
+use crate::gpu::{Gpu, PICK_BYTES_PER_PIXEL};
+use crate::{RenderError, Result};
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
+#[repr(u8)]
+pub enum EntityKindTag {
+    Point = 0,
+    Line = 1,
+    Arc = 2,
+    Circle = 3,
+    Relation = 4,
+    Dimension = 5,
+}
+
+impl EntityKindTag {
+    #[must_use]
+    pub const fn bits(self) -> u8 {
+        self as u8
+    }
+
+    #[must_use]
+    pub const fn from_bits(bits: u8) -> Option<Self> {
+        match bits {
+            0 => Some(Self::Point),
+            1 => Some(Self::Line),
+            2 => Some(Self::Arc),
+            3 => Some(Self::Circle),
+            4 => Some(Self::Relation),
+            5 => Some(Self::Dimension),
+            _ => None,
+        }
+    }
+}
+
+impl core::fmt::Display for EntityKindTag {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        let name = match self {
+            Self::Point => "point",
+            Self::Line => "line",
+            Self::Arc => "arc",
+            Self::Circle => "circle",
+            Self::Relation => "relation",
+            Self::Dimension => "dimension",
+        };
+        f.write_str(name)
+    }
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
+pub enum PickedItem {
+    Point(SketchEntityId),
+    Line(SketchEntityId),
+    Arc(SketchEntityId),
+    Circle(SketchEntityId),
+    Relation(SketchRelationId),
+    Dimension(SketchDimensionId),
+}
+
+impl PickedItem {
+    #[must_use]
+    pub const fn tag(self) -> EntityKindTag {
+        match self {
+            Self::Point(_) => EntityKindTag::Point,
+            Self::Line(_) => EntityKindTag::Line,
+            Self::Arc(_) => EntityKindTag::Arc,
+            Self::Circle(_) => EntityKindTag::Circle,
+            Self::Relation(_) => EntityKindTag::Relation,
+            Self::Dimension(_) => EntityKindTag::Dimension,
+        }
+    }
+}
+
+#[derive(Debug, thiserror::Error)]
+pub enum PickIdError {
+    #[error("slotmap slot {slot:#x} exceeds {limit:#x}, the 28-bit PickId index field")]
+    SlotIndexOverflow { slot: u32, limit: u32 },
+    #[error("two entries collapse to packed slot index {0:#x} in the PickIndex")]
+    SlotIndexCollision(u32),
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
+pub struct PickId(u32);
+
+impl PickId {
+    pub const TAG_SHIFT: u32 = 28;
+    pub const INDEX_MASK: u32 = 0x0FFF_FFFF;
+    pub const TAG_MASK: u32 = 0xF000_0000;
+    pub const NONE: Self = Self(u32::MAX);
+
+    #[must_use]
+    pub const fn from_raw(bits: u32) -> Self {
+        Self(bits)
+    }
+
+    #[must_use]
+    pub const fn raw(self) -> u32 {
+        self.0
+    }
+
+    #[must_use]
+    #[allow(clippy::cast_possible_truncation)]
+    pub const fn tag(self) -> Option<EntityKindTag> {
+        EntityKindTag::from_bits((self.0 >> Self::TAG_SHIFT) as u8)
+    }
+
+    #[must_use]
+    pub const fn index(self) -> u32 {
+        self.0 & Self::INDEX_MASK
+    }
+
+    pub fn point(id: SketchEntityId) -> Result<Self, PickIdError> {
+        Self::from_keyed(EntityKindTag::Point, id)
+    }
+
+    pub fn line(id: SketchEntityId) -> Result<Self, PickIdError> {
+        Self::from_keyed(EntityKindTag::Line, id)
+    }
+
+    pub fn arc(id: SketchEntityId) -> Result<Self, PickIdError> {
+        Self::from_keyed(EntityKindTag::Arc, id)
+    }
+
+    pub fn circle(id: SketchEntityId) -> Result<Self, PickIdError> {
+        Self::from_keyed(EntityKindTag::Circle, id)
+    }
+
+    pub fn relation(id: SketchRelationId) -> Result<Self, PickIdError> {
+        Self::from_keyed(EntityKindTag::Relation, id)
+    }
+
+    pub fn dimension(id: SketchDimensionId) -> Result<Self, PickIdError> {
+        Self::from_keyed(EntityKindTag::Dimension, id)
+    }
+
+    #[must_use]
+    pub fn unpack(self, index: &PickIndex) -> Option<PickedItem> {
+        let tag = self.tag()?;
+        let slot = self.index();
+        match tag {
+            EntityKindTag::Point => index.lookup_entity(slot, tag).map(PickedItem::Point),
+            EntityKindTag::Line => index.lookup_entity(slot, tag).map(PickedItem::Line),
+            EntityKindTag::Arc => index.lookup_entity(slot, tag).map(PickedItem::Arc),
+            EntityKindTag::Circle => index.lookup_entity(slot, tag).map(PickedItem::Circle),
+            EntityKindTag::Relation => index
+                .relations
+                .get(&slot)
+                .copied()
+                .map(PickedItem::Relation),
+            EntityKindTag::Dimension => index
+                .dimensions
+                .get(&slot)
+                .copied()
+                .map(PickedItem::Dimension),
+        }
+    }
+
+    fn from_keyed<K: Key>(tag: EntityKindTag, id: K) -> Result<Self, PickIdError> {
+        slot_index(id).map(|idx| Self::pack(tag, idx))
+    }
+
+    const fn pack(tag: EntityKindTag, idx: u32) -> Self {
+        debug_assert!(idx <= Self::INDEX_MASK);
+        Self(((tag.bits() as u32) << Self::TAG_SHIFT) | idx)
+    }
+}
+
+const _: () = assert!(PickId::TAG_MASK | PickId::INDEX_MASK == u32::MAX);
+const _: () = assert!(PickId::TAG_MASK & PickId::INDEX_MASK == 0);
+const _: () = assert!(PickId::TAG_MASK == 0xFu32 << PickId::TAG_SHIFT);
+const _: () = assert!(PickId::NONE.tag().is_none());
+
+impl core::fmt::Display for PickId {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        match self.tag() {
+            Some(tag) => write!(f, "{tag}#{}", self.index()),
+            None => write!(f, "invalid#{:08x}", self.0),
+        }
+    }
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
+pub struct PickQuery {
+    x: ViewportPx,
+    y: ViewportPx,
+}
+
+impl PickQuery {
+    #[must_use]
+    pub const fn new(x: ViewportPx, y: ViewportPx) -> Self {
+        Self { x, y }
+    }
+
+    #[must_use]
+    pub const fn x(self) -> ViewportPx {
+        self.x
+    }
+
+    #[must_use]
+    pub const fn y(self) -> ViewportPx {
+        self.y
+    }
+}
+
+impl core::fmt::Display for PickQuery {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        write!(f, "({}, {})", self.x, self.y)
+    }
+}
+
+pub struct Picker<'a> {
+    gpu: &'a Gpu,
+    pick: &'a wgpu::Texture,
+    staging: &'a wgpu::Buffer,
+    extent: ViewportExtent,
+    index: PickIndex,
+}
+
+impl<'a> Picker<'a> {
+    pub(crate) fn new(
+        gpu: &'a Gpu,
+        pick: &'a wgpu::Texture,
+        staging: &'a wgpu::Buffer,
+        extent: ViewportExtent,
+        index: PickIndex,
+    ) -> Self {
+        Self {
+            gpu,
+            pick,
+            staging,
+            extent,
+            index,
+        }
+    }
+
+    pub fn raw_at(&self, query: PickQuery) -> Result<PickId> {
+        let width = self.extent.width().value();
+        let height = self.extent.height().value();
+        if query.x.value() >= width || query.y.value() >= height {
+            return Err(RenderError::PickOutOfBounds {
+                query,
+                extent: self.extent,
+            });
+        }
+        let device = self.gpu.device();
+        let queue = self.gpu.queue();
+        let padded_bpr = wgpu::COPY_BYTES_PER_ROW_ALIGNMENT;
+        let mut encoder = device.create_command_encoder(&wgpu::CommandEncoderDescriptor {
+            label: Some("bone-render:pick-encoder"),
+        });
+        encoder.copy_texture_to_buffer(
+            wgpu::TexelCopyTextureInfo {
+                texture: self.pick,
+                mip_level: 0,
+                origin: wgpu::Origin3d {
+                    x: query.x.value(),
+                    y: query.y.value(),
+                    z: 0,
+                },
+                aspect: wgpu::TextureAspect::All,
+            },
+            wgpu::TexelCopyBufferInfo {
+                buffer: self.staging,
+                layout: wgpu::TexelCopyBufferLayout {
+                    offset: 0,
+                    bytes_per_row: Some(padded_bpr),
+                    rows_per_image: Some(1),
+                },
+            },
+            wgpu::Extent3d {
+                width: 1,
+                height: 1,
+                depth_or_array_layers: 1,
+            },
+        );
+        queue.submit(Some(encoder.finish()));
+
+        let slice = self.staging.slice(..);
+        let (tx, rx) =
+            std::sync::mpsc::sync_channel::<core::result::Result<(), wgpu::BufferAsyncError>>(1);
+        slice.map_async(wgpu::MapMode::Read, move |res| {
+            let _ = tx.send(res);
+        });
+        device
+            .poll(wgpu::PollType::wait_indefinitely())
+            .map_err(RenderError::Poll)?;
+        match rx.try_recv() {
+            Ok(Ok(())) => {}
+            Ok(Err(e)) => return Err(RenderError::Map(e)),
+            Err(_) => return Err(RenderError::MapMissing),
+        }
+        let raw = {
+            let view = slice.get_mapped_range();
+            debug_assert!(view.len() >= PICK_BYTES_PER_PIXEL as usize);
+            u32::from_le_bytes([view[0], view[1], view[2], view[3]])
+        };
+        self.staging.unmap();
+        Ok(PickId::from_raw(raw))
+    }
+
+    pub fn at(&self, query: PickQuery) -> Result<Option<PickedItem>> {
+        let pid = self.raw_at(query)?;
+        Ok(pid.unpack(&self.index))
+    }
+}
+
+impl core::fmt::Debug for Picker<'_> {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        f.debug_struct("Picker")
+            .field("extent", &self.extent)
+            .finish_non_exhaustive()
+    }
+}
+
+#[derive(Debug, Default, Clone)]
+pub struct PickIndex {
+    entities: HashMap<u32, (SketchEntityId, EntityKindTag)>,
+    relations: HashMap<u32, SketchRelationId>,
+    dimensions: HashMap<u32, SketchDimensionId>,
+}
+
+impl PickIndex {
+    pub fn build<EI, RI, DI>(
+        entities: EI,
+        relations: RI,
+        dimensions: DI,
+    ) -> Result<Self, PickIdError>
+    where
+        EI: IntoIterator<Item = (SketchEntityId, EntityKindTag)>,
+        RI: IntoIterator<Item = SketchRelationId>,
+        DI: IntoIterator<Item = SketchDimensionId>,
+    {
+        Ok(Self {
+            entities: try_collect_keyed(entities, |(id, _)| slot_index(*id))?,
+            relations: try_collect_keyed(relations, |id| slot_index(*id))?,
+            dimensions: try_collect_keyed(dimensions, |id| slot_index(*id))?,
+        })
+    }
+
+    fn lookup_entity(&self, slot: u32, expected: EntityKindTag) -> Option<SketchEntityId> {
+        self.entities
+            .get(&slot)
+            .filter(|(_, kind)| *kind == expected)
+            .map(|(id, _)| *id)
+    }
+}
+
+#[allow(clippy::cast_possible_truncation)]
+fn slot_index<K: Key>(key: K) -> Result<u32, PickIdError> {
+    let slot = (key.data().as_ffi() & 0xFFFF_FFFF_u64) as u32;
+    if slot > PickId::INDEX_MASK {
+        return Err(PickIdError::SlotIndexOverflow {
+            slot,
+            limit: PickId::INDEX_MASK,
+        });
+    }
+    Ok(slot)
+}
+
+fn try_collect_keyed<I, V, F>(iter: I, slot_of: F) -> Result<HashMap<u32, V>, PickIdError>
+where
+    I: IntoIterator<Item = V>,
+    F: Fn(&V) -> Result<u32, PickIdError>,
+{
+    iter.into_iter().try_fold(HashMap::new(), |mut acc, item| {
+        let slot = slot_of(&item)?;
+        match acc.entry(slot) {
+            Entry::Vacant(v) => {
+                v.insert(item);
+                Ok(acc)
+            }
+            Entry::Occupied(_) => Err(PickIdError::SlotIndexCollision(slot)),
+        }
+    })
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use proptest::prelude::*;
+    use slotmap::{KeyData, SlotMap};
+
+    #[derive(Copy, Clone, Debug)]
+    enum TagKind {
+        Point,
+        Line,
+        Arc,
+        Circle,
+        Relation,
+        Dimension,
+    }
+
+    impl TagKind {
+        fn entity_kind(self) -> EntityKindTag {
+            match self {
+                Self::Point | Self::Relation | Self::Dimension => EntityKindTag::Point,
+                Self::Line => EntityKindTag::Line,
+                Self::Arc => EntityKindTag::Arc,
+                Self::Circle => EntityKindTag::Circle,
+            }
+        }
+    }
+
+    fn arb_tag() -> impl Strategy<Value = TagKind> {
+        prop_oneof![
+            Just(TagKind::Point),
+            Just(TagKind::Line),
+            Just(TagKind::Arc),
+            Just(TagKind::Circle),
+            Just(TagKind::Relation),
+            Just(TagKind::Dimension),
+        ]
+    }
+
+    type TestSlots = (
+        SlotMap<SketchEntityId, EntityKindTag>,
+        SlotMap<SketchRelationId, ()>,
+        SlotMap<SketchDimensionId, ()>,
+    );
+
+    fn populate_uniform(
+        n: usize,
+        entity_kind: EntityKindTag,
+    ) -> (
+        Vec<SketchEntityId>,
+        Vec<SketchRelationId>,
+        Vec<SketchDimensionId>,
+        TestSlots,
+    ) {
+        let mut entities = SlotMap::with_key();
+        let mut relations = SlotMap::with_key();
+        let mut dimensions = SlotMap::with_key();
+        let e: Vec<_> = (0..n).map(|_| entities.insert(entity_kind)).collect();
+        let r: Vec<_> = (0..n).map(|_| relations.insert(())).collect();
+        let d: Vec<_> = (0..n).map(|_| dimensions.insert(())).collect();
+        (e, r, d, (entities, relations, dimensions))
+    }
+
+    fn ok<T>(result: Result<T, PickIdError>) -> T {
+        let Ok(value) = result else {
+            panic!("test fixture should fit in PickId field");
+        };
+        value
+    }
+
+    fn build_index(slots: &TestSlots) -> PickIndex {
+        ok(PickIndex::build(
+            slots.0.iter().map(|(k, kind)| (k, *kind)),
+            slots.1.keys(),
+            slots.2.keys(),
+        ))
+    }
+
+    fn build_case(
+        tag: TagKind,
+        e: &[SketchEntityId],
+        r: &[SketchRelationId],
+        d: &[SketchDimensionId],
+        target: usize,
+    ) -> (PickId, PickedItem) {
+        match tag {
+            TagKind::Point => (ok(PickId::point(e[target])), PickedItem::Point(e[target])),
+            TagKind::Line => (ok(PickId::line(e[target])), PickedItem::Line(e[target])),
+            TagKind::Arc => (ok(PickId::arc(e[target])), PickedItem::Arc(e[target])),
+            TagKind::Circle => (ok(PickId::circle(e[target])), PickedItem::Circle(e[target])),
+            TagKind::Relation => (
+                ok(PickId::relation(r[target])),
+                PickedItem::Relation(r[target]),
+            ),
+            TagKind::Dimension => (
+                ok(PickId::dimension(d[target])),
+                PickedItem::Dimension(d[target]),
+            ),
+        }
+    }
+
+    fn forget(slots: &mut TestSlots, item: PickedItem) {
+        match item {
+            PickedItem::Point(k)
+            | PickedItem::Line(k)
+            | PickedItem::Arc(k)
+            | PickedItem::Circle(k) => {
+                slots.0.remove(k);
+            }
+            PickedItem::Relation(k) => {
+                slots.1.remove(k);
+            }
+            PickedItem::Dimension(k) => {
+                slots.2.remove(k);
+            }
+        }
+    }
+
+    #[test]
+    fn tag_bits_roundtrip() {
+        (0u8..6).for_each(|bits| {
+            let Some(tag) = EntityKindTag::from_bits(bits) else {
+                panic!("tag {bits} should decode");
+            };
+            assert_eq!(tag.bits(), bits);
+        });
+    }
+
+    #[test]
+    fn invalid_tag_bits_return_none() {
+        (6u8..16).for_each(|bits| {
+            assert!(EntityKindTag::from_bits(bits).is_none());
+        });
+    }
+
+    #[test]
+    fn raw_roundtrip() {
+        let mut entities: SlotMap<SketchEntityId, ()> = SlotMap::with_key();
+        let k = entities.insert(());
+        let pid = ok(PickId::point(k));
+        assert_eq!(PickId::from_raw(pid.raw()), pid);
+    }
+
+    #[test]
+    fn tag_and_index_split_cleanly() {
+        let mut entities: SlotMap<SketchEntityId, ()> = SlotMap::with_key();
+        let k = entities.insert(());
+        let pid = ok(PickId::line(k));
+        assert_eq!(pid.tag(), Some(EntityKindTag::Line));
+        assert_eq!(pid.raw() & PickId::TAG_MASK, 1u32 << PickId::TAG_SHIFT);
+        assert_eq!(pid.index(), pid.raw() & PickId::INDEX_MASK);
+    }
+
+    #[test]
+    fn display_renders_tag_and_index() {
+        let mut entities: SlotMap<SketchEntityId, ()> = SlotMap::with_key();
+        let k = entities.insert(());
+        let pid = ok(PickId::arc(k));
+        assert!(format!("{pid}").starts_with("arc#"));
+    }
+
+    #[test]
+    fn display_renders_invalid_tag() {
+        let raw = 0xF000_0000u32;
+        let pid = PickId::from_raw(raw);
+        assert_eq!(pid.tag(), None);
+        assert_eq!(format!("{pid}"), "invalid#f0000000");
+    }
+
+    #[test]
+    fn max_index_roundtrip() {
+        let raw =
+            (u32::from(EntityKindTag::Point.bits()) << PickId::TAG_SHIFT) | PickId::INDEX_MASK;
+        let pid = PickId::from_raw(raw);
+        assert_eq!(pid.tag(), Some(EntityKindTag::Point));
+        assert_eq!(pid.index(), PickId::INDEX_MASK);
+        assert_eq!(pid.raw(), raw);
+    }
+
+    #[test]
+    fn slot_overflow_is_rejected_by_constructor() {
+        let bad_slot: u32 = PickId::INDEX_MASK + 1;
+        let key = SketchEntityId::from(KeyData::from_ffi((1u64 << 32) | u64::from(bad_slot)));
+        let result = PickId::point(key);
+        assert!(matches!(
+            result,
+            Err(PickIdError::SlotIndexOverflow { slot, limit })
+                if slot == bad_slot && limit == PickId::INDEX_MASK
+        ));
+    }
+
+    #[test]
+    fn slot_overflow_is_rejected_by_pick_index_build() {
+        let bad_slot: u32 = PickId::INDEX_MASK + 1;
+        let key = SketchEntityId::from(KeyData::from_ffi((1u64 << 32) | u64::from(bad_slot)));
+        let result = PickIndex::build(
+            [(key, EntityKindTag::Point)],
+            std::iter::empty::<SketchRelationId>(),
+            std::iter::empty::<SketchDimensionId>(),
+        );
+        assert!(matches!(result, Err(PickIdError::SlotIndexOverflow { .. })));
+    }
+
+    #[test]
+    fn none_sentinel_is_invalid() {
+        assert_eq!(PickId::NONE.raw(), u32::MAX);
+        assert_eq!(PickId::NONE.tag(), None);
+        assert_eq!(PickId::NONE.unpack(&PickIndex::default()), None);
+    }
+
+    #[test]
+    fn collision_in_pick_index_build_is_rejected() {
+        let mut entities: SlotMap<SketchEntityId, EntityKindTag> = SlotMap::with_key();
+        let k = entities.insert(EntityKindTag::Point);
+        let result = PickIndex::build(
+            [(k, EntityKindTag::Point), (k, EntityKindTag::Line)],
+            std::iter::empty::<SketchRelationId>(),
+            std::iter::empty::<SketchDimensionId>(),
+        );
+        assert!(matches!(result, Err(PickIdError::SlotIndexCollision(_))));
+    }
+
+    fn build_entity_index(entities: &SlotMap<SketchEntityId, EntityKindTag>) -> PickIndex {
+        ok(PickIndex::build(
+            entities.iter().map(|(k, kind)| (k, *kind)),
+            std::iter::empty::<SketchRelationId>(),
+            std::iter::empty::<SketchDimensionId>(),
+        ))
+    }
+
+    #[test]
+    fn slot_reuse_with_same_kind_returns_new_key() {
+        let mut entities: SlotMap<SketchEntityId, EntityKindTag> = SlotMap::with_key();
+        let k1 = entities.insert(EntityKindTag::Point);
+        let pid = ok(PickId::point(k1));
+        entities.remove(k1);
+        let k2 = entities.insert(EntityKindTag::Point);
+        assert_ne!(k1, k2);
+        let index = build_entity_index(&entities);
+        assert_eq!(pid.unpack(&index), Some(PickedItem::Point(k2)));
+    }
+
+    #[test]
+    fn slot_reuse_with_different_kind_returns_none() {
+        let mut entities: SlotMap<SketchEntityId, EntityKindTag> = SlotMap::with_key();
+        let k1 = entities.insert(EntityKindTag::Point);
+        let pid = ok(PickId::point(k1));
+        entities.remove(k1);
+        let _k2 = entities.insert(EntityKindTag::Line);
+        let index = build_entity_index(&entities);
+        assert_eq!(pid.unpack(&index), None);
+    }
+
+    proptest! {
+        #[test]
+        fn unpack_roundtrip_across_all_populated(
+            tag in arb_tag(),
+            n in 1usize..32,
+            target in 0usize..32,
+        ) {
+            let target = target % n;
+            let (e, r, d, slots) = populate_uniform(n, tag.entity_kind());
+            let (pid, expected) = build_case(tag, &e, &r, &d, target);
+            let index = build_index(&slots);
+            prop_assert_eq!(pid.unpack(&index), Some(expected));
+            prop_assert_eq!(pid.tag(), Some(expected.tag()));
+        }
+
+        #[test]
+        fn removed_slot_unpacks_to_none(
+            tag in arb_tag(),
+            n in 1usize..32,
+            target in 0usize..32,
+        ) {
+            let target = target % n;
+            let (e, r, d, mut slots) = populate_uniform(n, tag.entity_kind());
+            let (pid, item) = build_case(tag, &e, &r, &d, target);
+            forget(&mut slots, item);
+            let index = build_index(&slots);
+            prop_assert!(pid.unpack(&index).is_none());
+        }
+
+        #[test]
+        fn invalid_tag_bits_unpack_to_none(bits in 6u8..16, idx in 0u32..=PickId::INDEX_MASK) {
+            let raw = (u32::from(bits) << PickId::TAG_SHIFT) | (idx & PickId::INDEX_MASK);
+            let pid = PickId::from_raw(raw);
+            prop_assert!(pid.unpack(&PickIndex::default()).is_none());
+        }
+    }
+}
diff --git a/crates/bone-render/src/pipelines/arc.rs b/crates/bone-render/src/pipelines/arc.rs
new file mode 100644
index 0000000..f4e4949
--- /dev/null
+++ b/crates/bone-render/src/pipelines/arc.rs
@@ -0,0 +1,317 @@
+use bone_types::{Angle, Length, Point2};
+use uom::si::angle::radian;
+use uom::si::length::millimeter;
+use wgpu::util::DeviceExt;
+
+use crate::camera::Camera2;
+use crate::gpu::{Gpu, PICK_FORMAT};
+use crate::pipelines::{CONSTRUCTION_BIT, FRAME_UNIFORM_SIZE, build_frame_uniform};
+use crate::scene::{SceneArc, SceneCircle, SketchScene};
+use crate::snapshot::Style;
+
+#[repr(C)]
+#[derive(Copy, Clone, bytemuck::Pod, bytemuck::Zeroable)]
+struct ArcInstance {
+    center: [f32; 2],
+    radius_mm: f32,
+    half_width_px: f32,
+    start_rad: f32,
+    sweep_rad: f32,
+    aabb_min_mm: [f32; 2],
+    aabb_max_mm: [f32; 2],
+    pick_id: u32,
+    style_bits: u32,
+}
+
+const INSTANCE_STRIDE: u64 = core::mem::size_of::<ArcInstance>() as u64;
+
+pub struct ArcPipeline {
+    device: wgpu::Device,
+    queue: wgpu::Queue,
+    pipeline: wgpu::RenderPipeline,
+    uniform_buffer: wgpu::Buffer,
+    bind_group: wgpu::BindGroup,
+}
+
+impl ArcPipeline {
+    #[must_use]
+    pub fn new(gpu: &Gpu, color_format: wgpu::TextureFormat) -> Self {
+        let device = gpu.device().clone();
+        let queue = gpu.queue().clone();
+        let shader = device.create_shader_module(wgpu::ShaderModuleDescriptor {
+            label: Some("bone-render:arc-shader"),
+            source: wgpu::ShaderSource::Wgsl(include_str!("arc.wgsl").into()),
+        });
+        let bind_group_layout = device.create_bind_group_layout(&wgpu::BindGroupLayoutDescriptor {
+            label: Some("bone-render:arc-bgl"),
+            entries: &[wgpu::BindGroupLayoutEntry {
+                binding: 0,
+                visibility: wgpu::ShaderStages::VERTEX_FRAGMENT,
+                ty: wgpu::BindingType::Buffer {
+                    ty: wgpu::BufferBindingType::Uniform,
+                    has_dynamic_offset: false,
+                    min_binding_size: wgpu::BufferSize::new(FRAME_UNIFORM_SIZE),
+                },
+                count: None,
+            }],
+        });
+        let pipeline_layout = device.create_pipeline_layout(&wgpu::PipelineLayoutDescriptor {
+            label: Some("bone-render:arc-layout"),
+            bind_group_layouts: &[Some(&bind_group_layout)],
+            immediate_size: 0,
+        });
+        let pipeline = device.create_render_pipeline(&wgpu::RenderPipelineDescriptor {
+            label: Some("bone-render:arc-pipeline"),
+            layout: Some(&pipeline_layout),
+            vertex: wgpu::VertexState {
+                module: &shader,
+                entry_point: Some("vs"),
+                compilation_options: wgpu::PipelineCompilationOptions::default(),
+                buffers: &[wgpu::VertexBufferLayout {
+                    array_stride: INSTANCE_STRIDE,
+                    step_mode: wgpu::VertexStepMode::Instance,
+                    attributes: &INSTANCE_ATTRS,
+                }],
+            },
+            fragment: Some(wgpu::FragmentState {
+                module: &shader,
+                entry_point: Some("fs"),
+                compilation_options: wgpu::PipelineCompilationOptions::default(),
+                targets: &[
+                    Some(wgpu::ColorTargetState {
+                        format: color_format,
+                        blend: Some(wgpu::BlendState::PREMULTIPLIED_ALPHA_BLENDING),
+                        write_mask: wgpu::ColorWrites::ALL,
+                    }),
+                    Some(wgpu::ColorTargetState {
+                        format: PICK_FORMAT,
+                        blend: None,
+                        write_mask: wgpu::ColorWrites::ALL,
+                    }),
+                ],
+            }),
+            primitive: wgpu::PrimitiveState {
+                topology: wgpu::PrimitiveTopology::TriangleList,
+                strip_index_format: None,
+                front_face: wgpu::FrontFace::Ccw,
+                cull_mode: None,
+                polygon_mode: wgpu::PolygonMode::Fill,
+                conservative: false,
+                unclipped_depth: false,
+            },
+            depth_stencil: None,
+            multisample: wgpu::MultisampleState::default(),
+            multiview_mask: None,
+            cache: None,
+        });
+        let uniform_buffer = device.create_buffer(&wgpu::BufferDescriptor {
+            label: Some("bone-render:arc-uniform"),
+            size: FRAME_UNIFORM_SIZE,
+            usage: wgpu::BufferUsages::UNIFORM | wgpu::BufferUsages::COPY_DST,
+            mapped_at_creation: false,
+        });
+        let bind_group = device.create_bind_group(&wgpu::BindGroupDescriptor {
+            label: Some("bone-render:arc-bg"),
+            layout: &bind_group_layout,
+            entries: &[wgpu::BindGroupEntry {
+                binding: 0,
+                resource: uniform_buffer.as_entire_binding(),
+            }],
+        });
+        Self {
+            device,
+            queue,
+            pipeline,
+            uniform_buffer,
+            bind_group,
+        }
+    }
+
+    pub fn draw(
+        &self,
+        encoder: &mut wgpu::CommandEncoder,
+        color_view: &wgpu::TextureView,
+        pick_view: &wgpu::TextureView,
+        camera: Camera2,
+        style: &Style,
+        scene: &SketchScene,
+    ) {
+        let instances = build_instances(scene, style);
+        if instances.is_empty() {
+            return;
+        }
+        let uniform = build_frame_uniform(camera, style);
+        self.queue
+            .write_buffer(&self.uniform_buffer, 0, bytemuck::bytes_of(&uniform));
+        let instance_buffer = self
+            .device
+            .create_buffer_init(&wgpu::util::BufferInitDescriptor {
+                label: Some("bone-render:arc-instances"),
+                contents: bytemuck::cast_slice(&instances),
+                usage: wgpu::BufferUsages::VERTEX,
+            });
+        let mut pass = encoder.begin_render_pass(&wgpu::RenderPassDescriptor {
+            label: Some("bone-render:arc-pass"),
+            color_attachments: &[
+                Some(wgpu::RenderPassColorAttachment {
+                    view: color_view,
+                    resolve_target: None,
+                    depth_slice: None,
+                    ops: wgpu::Operations {
+                        load: wgpu::LoadOp::Load,
+                        store: wgpu::StoreOp::Store,
+                    },
+                }),
+                Some(wgpu::RenderPassColorAttachment {
+                    view: pick_view,
+                    resolve_target: None,
+                    depth_slice: None,
+                    ops: wgpu::Operations {
+                        load: wgpu::LoadOp::Load,
+                        store: wgpu::StoreOp::Store,
+                    },
+                }),
+            ],
+            depth_stencil_attachment: None,
+            timestamp_writes: None,
+            occlusion_query_set: None,
+            multiview_mask: None,
+        });
+        pass.set_pipeline(&self.pipeline);
+        pass.set_bind_group(0, &self.bind_group, &[]);
+        pass.set_vertex_buffer(0, instance_buffer.slice(..));
+        let len = instances.len();
+        let Ok(count) = u32::try_from(len) else {
+            panic!("arc instance count {len} exceeds u32::MAX");
+        };
+        pass.draw(0..6, 0..count);
+    }
+}
+
+impl core::fmt::Debug for ArcPipeline {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        f.debug_struct("ArcPipeline").finish_non_exhaustive()
+    }
+}
+
+const INSTANCE_ATTRS: [wgpu::VertexAttribute; 9] = wgpu::vertex_attr_array![
+    0 => Float32x2,
+    1 => Float32,
+    2 => Float32,
+    3 => Float32,
+    4 => Float32,
+    5 => Float32x2,
+    6 => Float32x2,
+    7 => Uint32,
+    8 => Uint32,
+];
+
+const FULL_CIRCLE_SWEEP: f32 = core::f32::consts::TAU;
+
+fn build_instances(scene: &SketchScene, style: &Style) -> Vec<ArcInstance> {
+    let arcs = scene.arcs().iter().map(|a| arc_instance(*a, style));
+    let circles = scene.circles().iter().map(|c| circle_instance(*c, style));
+    arcs.chain(circles).collect()
+}
+
+#[allow(clippy::cast_possible_truncation)]
+fn arc_instance(arc: SceneArc, style: &Style) -> ArcInstance {
+    let (cx, cy) = arc.center().coords_mm();
+    let radius_mm = arc.radius().get::<millimeter>() as f32;
+    let start_rad = arc.start_angle().get::<radian>() as f32;
+    let sweep_rad = arc.sweep_angle().get::<radian>() as f32;
+    let (aabb_min_mm, aabb_max_mm) = arc_aabb_offsets_mm(
+        arc.center(),
+        arc.radius(),
+        arc.start_angle(),
+        arc.sweep_angle(),
+    );
+    let bits = if arc.for_construction() {
+        CONSTRUCTION_BIT
+    } else {
+        0
+    };
+    ArcInstance {
+        center: [cx as f32, cy as f32],
+        radius_mm,
+        half_width_px: style.strokes().stroke_width_px() * 0.5,
+        start_rad,
+        sweep_rad,
+        aabb_min_mm,
+        aabb_max_mm,
+        pick_id: arc.pick().raw(),
+        style_bits: bits,
+    }
+}
+
+#[allow(clippy::cast_possible_truncation)]
+fn circle_instance(circle: SceneCircle, style: &Style) -> ArcInstance {
+    let (cx, cy) = circle.center().coords_mm();
+    let radius_mm = circle.radius().get::<millimeter>() as f32;
+    let bits = if circle.for_construction() {
+        CONSTRUCTION_BIT
+    } else {
+        0
+    };
+    ArcInstance {
+        center: [cx as f32, cy as f32],
+        radius_mm,
+        half_width_px: style.strokes().stroke_width_px() * 0.5,
+        start_rad: 0.0,
+        sweep_rad: FULL_CIRCLE_SWEEP,
+        aabb_min_mm: [-radius_mm, -radius_mm],
+        aabb_max_mm: [radius_mm, radius_mm],
+        pick_id: circle.pick().raw(),
+        style_bits: bits,
+    }
+}
+
+#[allow(clippy::cast_possible_truncation)]
+fn arc_aabb_offsets_mm(
+    center: Point2,
+    radius: Length,
+    start: Angle,
+    sweep: Angle,
+) -> ([f32; 2], [f32; 2]) {
+    let aabb = bone_kernel::arc_bounding_box(center, radius, start, sweep);
+    let (cx, cy) = center.coords_mm();
+    let (mnx, mny) = aabb.min().coords_mm();
+    let (mxx, mxy) = aabb.max().coords_mm();
+    (
+        [(mnx - cx) as f32, (mny - cy) as f32],
+        [(mxx - cx) as f32, (mxy - cy) as f32],
+    )
+}
+
+#[cfg(test)]
+mod tests {
+    use super::arc_aabb_offsets_mm;
+    use bone_types::{Angle, Length, Point2};
+    use core::f64::consts::FRAC_PI_2;
+    use uom::si::angle::radian;
+    use uom::si::length::millimeter;
+
+    #[test]
+    fn quarter_arc_at_origin_offsets_to_unit_quadrant() {
+        let (min, max) = arc_aabb_offsets_mm(
+            Point2::from_mm(0.0, 0.0),
+            Length::new::<millimeter>(1.0),
+            Angle::new::<radian>(0.0),
+            Angle::new::<radian>(FRAC_PI_2),
+        );
+        assert!(min[0].abs() < 1e-5 && min[1].abs() < 1e-5);
+        assert!((max[0] - 1.0).abs() < 1e-5 && (max[1] - 1.0).abs() < 1e-5);
+    }
+
+    #[test]
+    fn translated_center_yields_the_same_relative_offsets() {
+        let (min, max) = arc_aabb_offsets_mm(
+            Point2::from_mm(10.0, -5.0),
+            Length::new::<millimeter>(1.0),
+            Angle::new::<radian>(0.0),
+            Angle::new::<radian>(FRAC_PI_2),
+        );
+        assert!(min[0].abs() < 1e-5 && min[1].abs() < 1e-5);
+        assert!((max[0] - 1.0).abs() < 1e-5 && (max[1] - 1.0).abs() < 1e-5);
+    }
+}
diff --git a/crates/bone-render/src/pipelines/arc.wgsl b/crates/bone-render/src/pipelines/arc.wgsl
new file mode 100644
index 0000000..c7f8f5d
--- /dev/null
+++ b/crates/bone-render/src/pipelines/arc.wgsl
@@ -0,0 +1,141 @@
+struct Frame {
+    clip_from_world:  mat4x4<f32>,
+    stroke_color:     vec4<f32>,
+    construction:     vec4<f32>,
+    pixels_per_mm:    f32,
+    dash_period_px:   f32,
+    dash_on_ratio:    f32,
+    _pad:             f32,
+};
+
+struct Instance {
+    @location(0) center:        vec2<f32>,
+    @location(1) radius_mm:     f32,
+    @location(2) half_width_px: f32,
+    @location(3) start_rad:     f32,
+    @location(4) sweep_rad:     f32,
+    @location(5) aabb_min_mm:   vec2<f32>,
+    @location(6) aabb_max_mm:   vec2<f32>,
+    @location(7) pick_id:       u32,
+    @location(8) style_bits:    u32,
+};
+
+struct VsOut {
+    @builtin(position) clip:        vec4<f32>,
+    @location(0) local_px:          vec2<f32>,
+    @location(1) radius_px:         f32,
+    @location(2) half_width_px:     f32,
+    @location(3) start_rad:         f32,
+    @location(4) sweep_rad:         f32,
+    @location(5) @interpolate(flat) style_bits: u32,
+    @location(6) @interpolate(flat) pick_id:    u32,
+};
+
+struct FsOut {
+    @location(0) color:   vec4<f32>,
+    @location(1) pick_id: u32,
+};
+
+@group(0) @binding(0) var<uniform> u: Frame;
+
+const CORNERS: array<vec2<f32>, 6> = array<vec2<f32>, 6>(
+    vec2<f32>(0.0, 0.0),
+    vec2<f32>(1.0, 0.0),
+    vec2<f32>(0.0, 1.0),
+    vec2<f32>(0.0, 1.0),
+    vec2<f32>(1.0, 0.0),
+    vec2<f32>(1.0, 1.0),
+);
+
+const TWO_PI: f32 = 6.283185307179586;
+const CONSTRUCTION_BIT: u32 = 1u;
+
+@vertex
+fn vs(@builtin(vertex_index) vid: u32, inst: Instance) -> VsOut {
+    var corners = CORNERS;
+    let corner = corners[vid];
+    let expand_mm = (inst.half_width_px + 1.0) / u.pixels_per_mm;
+    let bbox_min = inst.aabb_min_mm - vec2<f32>(expand_mm, expand_mm);
+    let bbox_max = inst.aabb_max_mm + vec2<f32>(expand_mm, expand_mm);
+    let offset_mm = mix(bbox_min, bbox_max, corner);
+    let world_mm = inst.center + offset_mm;
+    let clip = u.clip_from_world * vec4<f32>(world_mm, 0.0, 1.0);
+
+    var out: VsOut;
+    out.clip = clip;
+    out.local_px = offset_mm * u.pixels_per_mm;
+    out.radius_px = inst.radius_mm * u.pixels_per_mm;
+    out.half_width_px = inst.half_width_px;
+    out.start_rad = inst.start_rad;
+    out.sweep_rad = inst.sweep_rad;
+    out.style_bits = inst.style_bits;
+    out.pick_id = inst.pick_id;
+    return out;
+}
+
+struct ArcPoint {
+    distance_px: f32,
+    along_px:    f32,
+};
+
+fn arc_sample(p: vec2<f32>, radius_px: f32, start_rad: f32, sweep_rad: f32) -> ArcPoint {
+    let r_px = length(p);
+    let full_circle = sweep_rad >= TWO_PI - 1.0e-4;
+    if (full_circle) {
+        let theta = atan2(p.y, p.x);
+        var delta = theta - start_rad;
+        delta = delta - floor(delta / TWO_PI) * TWO_PI;
+        var out: ArcPoint;
+        out.distance_px = abs(r_px - radius_px);
+        out.along_px = delta * radius_px;
+        return out;
+    }
+    let theta = atan2(p.y, p.x);
+    var delta = theta - start_rad;
+    delta = delta - floor(delta / TWO_PI) * TWO_PI;
+    var theta_nearest: f32;
+    var along_rad: f32;
+    if (delta <= sweep_rad) {
+        theta_nearest = theta;
+        along_rad = delta;
+    } else {
+        let to_end = delta - sweep_rad;
+        let to_start = TWO_PI - delta;
+        if (to_end < to_start) {
+            theta_nearest = start_rad + sweep_rad;
+            along_rad = sweep_rad;
+        } else {
+            theta_nearest = start_rad;
+            along_rad = 0.0;
+        }
+    }
+    let nearest = vec2<f32>(cos(theta_nearest), sin(theta_nearest)) * radius_px;
+    var out: ArcPoint;
+    out.distance_px = length(p - nearest);
+    out.along_px = along_rad * radius_px;
+    return out;
+}
+
+@fragment
+fn fs(in: VsOut) -> FsOut {
+    let sample = arc_sample(in.local_px, in.radius_px, in.start_rad, in.sweep_rad);
+    let aa = 0.5;
+    let coverage = 1.0 - smoothstep(in.half_width_px - aa, in.half_width_px + aa, sample.distance_px);
+    if (coverage <= 0.0) {
+        discard;
+    }
+
+    let is_construction = (in.style_bits & CONSTRUCTION_BIT) != 0u;
+    let dash_visible = !is_construction
+        || u.dash_period_px <= 0.0
+        || fract(sample.along_px / u.dash_period_px) <= u.dash_on_ratio;
+    let color_coverage = select(0.0, coverage, dash_visible);
+
+    let base = select(u.stroke_color, u.construction, is_construction);
+    let a = base.a * color_coverage;
+
+    var out: FsOut;
+    out.color = vec4<f32>(base.rgb * a, a);
+    out.pick_id = in.pick_id;
+    return out;
+}
diff --git a/crates/bone-render/src/pipelines/grid.rs b/crates/bone-render/src/pipelines/grid.rs
new file mode 100644
index 0000000..c9b4a52
--- /dev/null
+++ b/crates/bone-render/src/pipelines/grid.rs
@@ -0,0 +1,176 @@
+use uom::si::length::millimeter;
+
+use crate::camera::{Camera2, GridSpacing};
+use crate::gpu::Gpu;
+use crate::snapshot::Style;
+
+#[repr(C, align(16))]
+#[derive(Copy, Clone, bytemuck::Pod, bytemuck::Zeroable)]
+struct GridUniform {
+    world_from_clip: [f32; 16],
+    minor: [f32; 4],
+    major: [f32; 4],
+    axis_x: [f32; 4],
+    axis_y: [f32; 4],
+    origin: [f32; 4],
+    viewport: [f32; 2],
+    minor_spacing: f32,
+    major_every: f32,
+    line_width_px: f32,
+    axis_width_px: f32,
+    origin_radius_px: f32,
+    pixels_per_mm: f32,
+}
+
+const UNIFORM_SIZE: u64 = core::mem::size_of::<GridUniform>() as u64;
+
+pub struct GridPipeline {
+    pipeline: wgpu::RenderPipeline,
+    uniform_buffer: wgpu::Buffer,
+    bind_group: wgpu::BindGroup,
+    queue: wgpu::Queue,
+}
+
+impl GridPipeline {
+    #[must_use]
+    pub fn new(gpu: &Gpu, color_format: wgpu::TextureFormat) -> Self {
+        let device = gpu.device();
+        let shader = device.create_shader_module(wgpu::ShaderModuleDescriptor {
+            label: Some("bone-render:grid-shader"),
+            source: wgpu::ShaderSource::Wgsl(include_str!("grid.wgsl").into()),
+        });
+        let bind_group_layout = device.create_bind_group_layout(&wgpu::BindGroupLayoutDescriptor {
+            label: Some("bone-render:grid-bgl"),
+            entries: &[wgpu::BindGroupLayoutEntry {
+                binding: 0,
+                visibility: wgpu::ShaderStages::VERTEX_FRAGMENT,
+                ty: wgpu::BindingType::Buffer {
+                    ty: wgpu::BufferBindingType::Uniform,
+                    has_dynamic_offset: false,
+                    min_binding_size: wgpu::BufferSize::new(UNIFORM_SIZE),
+                },
+                count: None,
+            }],
+        });
+        let pipeline_layout = device.create_pipeline_layout(&wgpu::PipelineLayoutDescriptor {
+            label: Some("bone-render:grid-layout"),
+            bind_group_layouts: &[Some(&bind_group_layout)],
+            immediate_size: 0,
+        });
+        let pipeline = device.create_render_pipeline(&wgpu::RenderPipelineDescriptor {
+            label: Some("bone-render:grid-pipeline"),
+            layout: Some(&pipeline_layout),
+            vertex: wgpu::VertexState {
+                module: &shader,
+                entry_point: Some("vs"),
+                compilation_options: wgpu::PipelineCompilationOptions::default(),
+                buffers: &[],
+            },
+            fragment: Some(wgpu::FragmentState {
+                module: &shader,
+                entry_point: Some("fs"),
+                compilation_options: wgpu::PipelineCompilationOptions::default(),
+                targets: &[Some(wgpu::ColorTargetState {
+                    format: color_format,
+                    blend: Some(wgpu::BlendState::PREMULTIPLIED_ALPHA_BLENDING),
+                    write_mask: wgpu::ColorWrites::ALL,
+                })],
+            }),
+            primitive: wgpu::PrimitiveState {
+                topology: wgpu::PrimitiveTopology::TriangleList,
+                strip_index_format: None,
+                front_face: wgpu::FrontFace::Ccw,
+                cull_mode: None,
+                polygon_mode: wgpu::PolygonMode::Fill,
+                conservative: false,
+                unclipped_depth: false,
+            },
+            depth_stencil: None,
+            multisample: wgpu::MultisampleState::default(),
+            multiview_mask: None,
+            cache: None,
+        });
+        let uniform_buffer = device.create_buffer(&wgpu::BufferDescriptor {
+            label: Some("bone-render:grid-uniform"),
+            size: UNIFORM_SIZE,
+            usage: wgpu::BufferUsages::UNIFORM | wgpu::BufferUsages::COPY_DST,
+            mapped_at_creation: false,
+        });
+        let bind_group = device.create_bind_group(&wgpu::BindGroupDescriptor {
+            label: Some("bone-render:grid-bg"),
+            layout: &bind_group_layout,
+            entries: &[wgpu::BindGroupEntry {
+                binding: 0,
+                resource: uniform_buffer.as_entire_binding(),
+            }],
+        });
+        Self {
+            pipeline,
+            uniform_buffer,
+            bind_group,
+            queue: gpu.queue().clone(),
+        }
+    }
+
+    pub fn draw(
+        &self,
+        encoder: &mut wgpu::CommandEncoder,
+        view: &wgpu::TextureView,
+        camera: Camera2,
+        style: &Style,
+    ) {
+        let spacing = GridSpacing::from_zoom(camera.zoom(), style.grid().minor_spacing_target_px());
+        let uniform = build_uniform(camera, spacing, style);
+        self.queue
+            .write_buffer(&self.uniform_buffer, 0, bytemuck::bytes_of(&uniform));
+        let mut pass = encoder.begin_render_pass(&wgpu::RenderPassDescriptor {
+            label: Some("bone-render:grid-pass"),
+            color_attachments: &[Some(wgpu::RenderPassColorAttachment {
+                view,
+                resolve_target: None,
+                depth_slice: None,
+                ops: wgpu::Operations {
+                    load: wgpu::LoadOp::Clear(style.background().into()),
+                    store: wgpu::StoreOp::Store,
+                },
+            })],
+            depth_stencil_attachment: None,
+            timestamp_writes: None,
+            occlusion_query_set: None,
+            multiview_mask: None,
+        });
+        pass.set_pipeline(&self.pipeline);
+        pass.set_bind_group(0, &self.bind_group, &[]);
+        pass.draw(0..3, 0..1);
+    }
+}
+
+impl core::fmt::Debug for GridPipeline {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        f.debug_struct("GridPipeline").finish_non_exhaustive()
+    }
+}
+
+#[allow(clippy::cast_possible_truncation, clippy::cast_precision_loss)]
+fn build_uniform(camera: Camera2, spacing: GridSpacing, style: &Style) -> GridUniform {
+    let grid = style.grid();
+    let extent = camera.extent();
+    GridUniform {
+        world_from_clip: camera.world_mm_from_clip(),
+        minor: grid.minor().to_rgba_array(),
+        major: grid.major().to_rgba_array(),
+        axis_x: grid.axis_x().to_rgba_array(),
+        axis_y: grid.axis_y().to_rgba_array(),
+        origin: grid.origin().to_rgba_array(),
+        viewport: [
+            extent.width().value() as f32,
+            extent.height().value() as f32,
+        ],
+        minor_spacing: spacing.minor().get::<millimeter>() as f32,
+        major_every: spacing.major_every() as f32,
+        line_width_px: grid.line_width_px(),
+        axis_width_px: grid.axis_width_px(),
+        origin_radius_px: grid.origin_radius_px(),
+        pixels_per_mm: camera.zoom().value() as f32,
+    }
+}
diff --git a/crates/bone-render/src/pipelines/grid.wgsl b/crates/bone-render/src/pipelines/grid.wgsl
new file mode 100644
index 0000000..0e3250a
--- /dev/null
+++ b/crates/bone-render/src/pipelines/grid.wgsl
@@ -0,0 +1,76 @@
+struct Grid {
+    world_from_clip: mat4x4<f32>,
+    minor:            vec4<f32>,
+    major:            vec4<f32>,
+    axis_x:           vec4<f32>,
+    axis_y:           vec4<f32>,
+    origin:           vec4<f32>,
+    viewport:         vec2<f32>,
+    minor_spacing:    f32,
+    major_every:      f32,
+    line_width_px:    f32,
+    axis_width_px:    f32,
+    origin_radius_px: f32,
+    pixels_per_mm:    f32,
+};
+
+@group(0) @binding(0) var<uniform> u: Grid;
+
+@vertex
+fn vs(@builtin(vertex_index) vid: u32) -> @builtin(position) vec4<f32> {
+    var positions = array<vec2<f32>, 3>(
+        vec2<f32>(-1.0, -1.0),
+        vec2<f32>( 3.0, -1.0),
+        vec2<f32>(-1.0,  3.0),
+    );
+    return vec4<f32>(positions[vid], 0.0, 1.0);
+}
+
+fn line_distance_world(coord: f32, spacing: f32) -> f32 {
+    let nearest = round(coord / spacing) * spacing;
+    return abs(coord - nearest);
+}
+
+fn line_alpha(dist_px: f32, half_width_px: f32) -> f32 {
+    let aa = min(0.5, half_width_px);
+    return 1.0 - smoothstep(half_width_px - aa, half_width_px + aa, dist_px);
+}
+
+fn over(dst_pma: vec4<f32>, src: vec4<f32>, coverage: f32) -> vec4<f32> {
+    let a = src.a * coverage;
+    let src_pma = vec4<f32>(src.rgb * a, a);
+    return src_pma + dst_pma * (1.0 - a);
+}
+
+@fragment
+fn fs(@builtin(position) pos: vec4<f32>) -> @location(0) vec4<f32> {
+    let ndc = (pos.xy / u.viewport) * vec2<f32>(2.0, -2.0) + vec2<f32>(-1.0, 1.0);
+    let world4 = u.world_from_clip * vec4<f32>(ndc, 0.0, 1.0);
+    let world = world4.xy / world4.w;
+
+    var acc = vec4<f32>(0.0, 0.0, 0.0, 0.0);
+
+    let half_line = u.line_width_px * 0.5;
+    let dx_minor = line_distance_world(world.x, u.minor_spacing) * u.pixels_per_mm;
+    let dy_minor = line_distance_world(world.y, u.minor_spacing) * u.pixels_per_mm;
+    let minor_cov = max(line_alpha(dx_minor, half_line), line_alpha(dy_minor, half_line));
+    acc = over(acc, u.minor, minor_cov);
+
+    let major_spacing = u.minor_spacing * u.major_every;
+    let dx_major = line_distance_world(world.x, major_spacing) * u.pixels_per_mm;
+    let dy_major = line_distance_world(world.y, major_spacing) * u.pixels_per_mm;
+    let major_cov = max(line_alpha(dx_major, half_line), line_alpha(dy_major, half_line));
+    acc = over(acc, u.major, major_cov);
+
+    let axis_half = u.axis_width_px * 0.5;
+    let ax_y_cov = line_alpha(abs(world.x) * u.pixels_per_mm, axis_half);
+    acc = over(acc, u.axis_y, ax_y_cov);
+    let ax_x_cov = line_alpha(abs(world.y) * u.pixels_per_mm, axis_half);
+    acc = over(acc, u.axis_x, ax_x_cov);
+
+    let d_origin_px = length(world) * u.pixels_per_mm;
+    let origin_cov = 1.0 - smoothstep(u.origin_radius_px - 1.0, u.origin_radius_px + 1.0, d_origin_px);
+    acc = over(acc, u.origin, origin_cov);
+
+    return acc;
+}
diff --git a/crates/bone-render/src/pipelines/lines.rs b/crates/bone-render/src/pipelines/lines.rs
new file mode 100644
index 0000000..f8e7a8e
--- /dev/null
+++ b/crates/bone-render/src/pipelines/lines.rs
@@ -0,0 +1,233 @@
+use wgpu::util::DeviceExt;
+
+use crate::camera::Camera2;
+use crate::gpu::{Gpu, PICK_FORMAT};
+use crate::pipelines::{CONSTRUCTION_BIT, FRAME_UNIFORM_SIZE, build_frame_uniform};
+use crate::scene::{SceneLine, ScenePoint, SketchScene};
+use crate::snapshot::Style;
+
+#[repr(C)]
+#[derive(Copy, Clone, bytemuck::Pod, bytemuck::Zeroable)]
+struct LineInstance {
+    a: [f32; 2],
+    b: [f32; 2],
+    half_width_px: f32,
+    pick_id: u32,
+    style_bits: u32,
+}
+
+const INSTANCE_STRIDE: u64 = core::mem::size_of::<LineInstance>() as u64;
+
+pub struct LinesPipeline {
+    device: wgpu::Device,
+    queue: wgpu::Queue,
+    pipeline: wgpu::RenderPipeline,
+    uniform_buffer: wgpu::Buffer,
+    bind_group: wgpu::BindGroup,
+}
+
+impl LinesPipeline {
+    #[must_use]
+    pub fn new(gpu: &Gpu, color_format: wgpu::TextureFormat) -> Self {
+        let device = gpu.device().clone();
+        let queue = gpu.queue().clone();
+        let shader = device.create_shader_module(wgpu::ShaderModuleDescriptor {
+            label: Some("bone-render:lines-shader"),
+            source: wgpu::ShaderSource::Wgsl(include_str!("lines.wgsl").into()),
+        });
+        let bind_group_layout = device.create_bind_group_layout(&wgpu::BindGroupLayoutDescriptor {
+            label: Some("bone-render:lines-bgl"),
+            entries: &[wgpu::BindGroupLayoutEntry {
+                binding: 0,
+                visibility: wgpu::ShaderStages::VERTEX_FRAGMENT,
+                ty: wgpu::BindingType::Buffer {
+                    ty: wgpu::BufferBindingType::Uniform,
+                    has_dynamic_offset: false,
+                    min_binding_size: wgpu::BufferSize::new(FRAME_UNIFORM_SIZE),
+                },
+                count: None,
+            }],
+        });
+        let pipeline_layout = device.create_pipeline_layout(&wgpu::PipelineLayoutDescriptor {
+            label: Some("bone-render:lines-layout"),
+            bind_group_layouts: &[Some(&bind_group_layout)],
+            immediate_size: 0,
+        });
+        let pipeline = device.create_render_pipeline(&wgpu::RenderPipelineDescriptor {
+            label: Some("bone-render:lines-pipeline"),
+            layout: Some(&pipeline_layout),
+            vertex: wgpu::VertexState {
+                module: &shader,
+                entry_point: Some("vs"),
+                compilation_options: wgpu::PipelineCompilationOptions::default(),
+                buffers: &[wgpu::VertexBufferLayout {
+                    array_stride: INSTANCE_STRIDE,
+                    step_mode: wgpu::VertexStepMode::Instance,
+                    attributes: &INSTANCE_ATTRS,
+                }],
+            },
+            fragment: Some(wgpu::FragmentState {
+                module: &shader,
+                entry_point: Some("fs"),
+                compilation_options: wgpu::PipelineCompilationOptions::default(),
+                targets: &[
+                    Some(wgpu::ColorTargetState {
+                        format: color_format,
+                        blend: Some(wgpu::BlendState::PREMULTIPLIED_ALPHA_BLENDING),
+                        write_mask: wgpu::ColorWrites::ALL,
+                    }),
+                    Some(wgpu::ColorTargetState {
+                        format: PICK_FORMAT,
+                        blend: None,
+                        write_mask: wgpu::ColorWrites::ALL,
+                    }),
+                ],
+            }),
+            primitive: wgpu::PrimitiveState {
+                topology: wgpu::PrimitiveTopology::TriangleList,
+                strip_index_format: None,
+                front_face: wgpu::FrontFace::Ccw,
+                cull_mode: None,
+                polygon_mode: wgpu::PolygonMode::Fill,
+                conservative: false,
+                unclipped_depth: false,
+            },
+            depth_stencil: None,
+            multisample: wgpu::MultisampleState::default(),
+            multiview_mask: None,
+            cache: None,
+        });
+        let uniform_buffer = device.create_buffer(&wgpu::BufferDescriptor {
+            label: Some("bone-render:lines-uniform"),
+            size: FRAME_UNIFORM_SIZE,
+            usage: wgpu::BufferUsages::UNIFORM | wgpu::BufferUsages::COPY_DST,
+            mapped_at_creation: false,
+        });
+        let bind_group = device.create_bind_group(&wgpu::BindGroupDescriptor {
+            label: Some("bone-render:lines-bg"),
+            layout: &bind_group_layout,
+            entries: &[wgpu::BindGroupEntry {
+                binding: 0,
+                resource: uniform_buffer.as_entire_binding(),
+            }],
+        });
+        Self {
+            device,
+            queue,
+            pipeline,
+            uniform_buffer,
+            bind_group,
+        }
+    }
+
+    pub fn draw(
+        &self,
+        encoder: &mut wgpu::CommandEncoder,
+        color_view: &wgpu::TextureView,
+        pick_view: &wgpu::TextureView,
+        camera: Camera2,
+        style: &Style,
+        scene: &SketchScene,
+    ) {
+        let instances = build_instances(scene, style);
+        if instances.is_empty() {
+            return;
+        }
+        let uniform = build_frame_uniform(camera, style);
+        self.queue
+            .write_buffer(&self.uniform_buffer, 0, bytemuck::bytes_of(&uniform));
+        let instance_buffer = self
+            .device
+            .create_buffer_init(&wgpu::util::BufferInitDescriptor {
+                label: Some("bone-render:lines-instances"),
+                contents: bytemuck::cast_slice(&instances),
+                usage: wgpu::BufferUsages::VERTEX,
+            });
+        let mut pass = encoder.begin_render_pass(&wgpu::RenderPassDescriptor {
+            label: Some("bone-render:lines-pass"),
+            color_attachments: &[
+                Some(wgpu::RenderPassColorAttachment {
+                    view: color_view,
+                    resolve_target: None,
+                    depth_slice: None,
+                    ops: wgpu::Operations {
+                        load: wgpu::LoadOp::Load,
+                        store: wgpu::StoreOp::Store,
+                    },
+                }),
+                Some(wgpu::RenderPassColorAttachment {
+                    view: pick_view,
+                    resolve_target: None,
+                    depth_slice: None,
+                    ops: wgpu::Operations {
+                        load: wgpu::LoadOp::Load,
+                        store: wgpu::StoreOp::Store,
+                    },
+                }),
+            ],
+            depth_stencil_attachment: None,
+            timestamp_writes: None,
+            occlusion_query_set: None,
+            multiview_mask: None,
+        });
+        pass.set_pipeline(&self.pipeline);
+        pass.set_bind_group(0, &self.bind_group, &[]);
+        pass.set_vertex_buffer(0, instance_buffer.slice(..));
+        let len = instances.len();
+        let Ok(count) = u32::try_from(len) else {
+            panic!("line instance count {len} exceeds u32::MAX");
+        };
+        pass.draw(0..6, 0..count);
+    }
+}
+
+impl core::fmt::Debug for LinesPipeline {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        f.debug_struct("LinesPipeline").finish_non_exhaustive()
+    }
+}
+
+const INSTANCE_ATTRS: [wgpu::VertexAttribute; 5] = wgpu::vertex_attr_array![
+    0 => Float32x2,
+    1 => Float32x2,
+    2 => Float32,
+    3 => Uint32,
+    4 => Uint32,
+];
+
+fn build_instances(scene: &SketchScene, style: &Style) -> Vec<LineInstance> {
+    let lines = scene.lines().iter().map(|l| line_instance(*l, style));
+    let points = scene.points().iter().map(|p| point_instance(*p, style));
+    lines.chain(points).collect()
+}
+
+#[allow(clippy::cast_possible_truncation)]
+fn line_instance(line: SceneLine, style: &Style) -> LineInstance {
+    let (ax, ay) = line.a().coords_mm();
+    let (bx, by) = line.b().coords_mm();
+    let bits = if line.for_construction() {
+        CONSTRUCTION_BIT
+    } else {
+        0
+    };
+    LineInstance {
+        a: [ax as f32, ay as f32],
+        b: [bx as f32, by as f32],
+        half_width_px: style.strokes().stroke_width_px() * 0.5,
+        pick_id: line.pick().raw(),
+        style_bits: bits,
+    }
+}
+
+#[allow(clippy::cast_possible_truncation)]
+fn point_instance(point: ScenePoint, style: &Style) -> LineInstance {
+    let (x, y) = point.at().coords_mm();
+    let xy = [x as f32, y as f32];
+    LineInstance {
+        a: xy,
+        b: xy,
+        half_width_px: style.strokes().point_radius_px(),
+        pick_id: point.pick().raw(),
+        style_bits: 0,
+    }
+}
diff --git a/crates/bone-render/src/pipelines/lines.wgsl b/crates/bone-render/src/pipelines/lines.wgsl
new file mode 100644
index 0000000..7002bd5
--- /dev/null
+++ b/crates/bone-render/src/pipelines/lines.wgsl
@@ -0,0 +1,98 @@
+struct Frame {
+    clip_from_world:  mat4x4<f32>,
+    stroke_color:     vec4<f32>,
+    construction:     vec4<f32>,
+    pixels_per_mm:    f32,
+    dash_period_px:   f32,
+    dash_on_ratio:    f32,
+    _pad:             f32,
+};
+
+struct Instance {
+    @location(0) a:             vec2<f32>,
+    @location(1) b:             vec2<f32>,
+    @location(2) half_width_px: f32,
+    @location(3) pick_id:       u32,
+    @location(4) style_bits:    u32,
+};
+
+struct VsOut {
+    @builtin(position) clip:    vec4<f32>,
+    @location(0) local_px:      vec2<f32>,
+    @location(1) length_px:     f32,
+    @location(2) half_width_px: f32,
+    @location(3) @interpolate(flat) style_bits: u32,
+    @location(4) @interpolate(flat) pick_id:    u32,
+};
+
+struct FsOut {
+    @location(0) color:   vec4<f32>,
+    @location(1) pick_id: u32,
+};
+
+@group(0) @binding(0) var<uniform> u: Frame;
+
+const CORNERS: array<vec2<f32>, 6> = array<vec2<f32>, 6>(
+    vec2<f32>(0.0, 0.0),
+    vec2<f32>(1.0, 0.0),
+    vec2<f32>(0.0, 1.0),
+    vec2<f32>(0.0, 1.0),
+    vec2<f32>(1.0, 0.0),
+    vec2<f32>(1.0, 1.0),
+);
+
+const CONSTRUCTION_BIT: u32 = 1u;
+
+@vertex
+fn vs(@builtin(vertex_index) vid: u32, inst: Instance) -> VsOut {
+    var corners = CORNERS;
+    let corner = corners[vid];
+    let d_mm = inst.b - inst.a;
+    let len_mm = length(d_mm);
+    let safe = max(len_mm, 1.0e-9);
+    let t_hat = select(vec2<f32>(1.0, 0.0), d_mm / safe, len_mm > 1.0e-9);
+    let n_hat = vec2<f32>(-t_hat.y, t_hat.x);
+
+    let expand_mm = (inst.half_width_px + 1.0) / u.pixels_per_mm;
+    let along_mm = mix(-expand_mm, len_mm + expand_mm, corner.x);
+    let perp_mm = mix(-expand_mm, expand_mm, corner.y);
+
+    let world_mm = inst.a + along_mm * t_hat + perp_mm * n_hat;
+    let clip = u.clip_from_world * vec4<f32>(world_mm, 0.0, 1.0);
+
+    var out: VsOut;
+    out.clip = clip;
+    out.local_px = vec2<f32>(along_mm, perp_mm) * u.pixels_per_mm;
+    out.length_px = len_mm * u.pixels_per_mm;
+    out.half_width_px = inst.half_width_px;
+    out.style_bits = inst.style_bits;
+    out.pick_id = inst.pick_id;
+    return out;
+}
+
+@fragment
+fn fs(in: VsOut) -> FsOut {
+    let along = clamp(in.local_px.x, 0.0, in.length_px);
+    let dx = in.local_px.x - along;
+    let dy = in.local_px.y;
+    let d_px = sqrt(dx * dx + dy * dy);
+    let aa = 0.5;
+    let coverage = 1.0 - smoothstep(in.half_width_px - aa, in.half_width_px + aa, d_px);
+    if (coverage <= 0.0) {
+        discard;
+    }
+
+    let is_construction = (in.style_bits & CONSTRUCTION_BIT) != 0u;
+    let dash_visible = !is_construction
+        || u.dash_period_px <= 0.0
+        || fract(along / u.dash_period_px) <= u.dash_on_ratio;
+    let color_coverage = select(0.0, coverage, dash_visible);
+
+    let base = select(u.stroke_color, u.construction, is_construction);
+    let a = base.a * color_coverage;
+
+    var out: FsOut;
+    out.color = vec4<f32>(base.rgb * a, a);
+    out.pick_id = in.pick_id;
+    return out;
+}
diff --git a/crates/bone-render/src/pipelines/mod.rs b/crates/bone-render/src/pipelines/mod.rs
new file mode 100644
index 0000000..8708d41
--- /dev/null
+++ b/crates/bone-render/src/pipelines/mod.rs
@@ -0,0 +1,40 @@
+pub mod arc;
+pub mod grid;
+pub mod lines;
+
+pub use arc::ArcPipeline;
+pub use grid::GridPipeline;
+pub use lines::LinesPipeline;
+
+use crate::camera::Camera2;
+use crate::snapshot::Style;
+
+#[repr(C, align(16))]
+#[derive(Copy, Clone, bytemuck::Pod, bytemuck::Zeroable)]
+pub(crate) struct FrameUniform {
+    pub clip_from_world: [f32; 16],
+    pub stroke_color: [f32; 4],
+    pub construction_color: [f32; 4],
+    pub pixels_per_mm: f32,
+    pub dash_period_px: f32,
+    pub dash_on_ratio: f32,
+    pub _pad: f32,
+}
+
+pub(crate) const FRAME_UNIFORM_SIZE: u64 = core::mem::size_of::<FrameUniform>() as u64;
+
+pub(crate) const CONSTRUCTION_BIT: u32 = 1;
+
+#[allow(clippy::cast_possible_truncation, clippy::cast_precision_loss)]
+pub(crate) fn build_frame_uniform(camera: Camera2, style: &Style) -> FrameUniform {
+    let strokes = style.strokes();
+    FrameUniform {
+        clip_from_world: camera.clip_from_world_mm(),
+        stroke_color: strokes.stroke().to_rgba_array(),
+        construction_color: strokes.construction().to_rgba_array(),
+        pixels_per_mm: camera.zoom().value() as f32,
+        dash_period_px: strokes.construction_dash_period_px(),
+        dash_on_ratio: strokes.construction_dash_on_ratio(),
+        _pad: 0.0,
+    }
+}
diff --git a/crates/bone-render/src/scene.rs b/crates/bone-render/src/scene.rs
new file mode 100644
index 0000000..61f286d
--- /dev/null
+++ b/crates/bone-render/src/scene.rs
@@ -0,0 +1,556 @@
+use bone_document::{Sketch, SketchEntity};
+use bone_kernel::{Aabb2, arc_bounding_box};
+use bone_types::{Angle, Length, Point2, SketchEntityId};
+use core::f64::consts::TAU;
+use uom::si::angle::radian;
+use uom::si::length::millimeter;
+
+use crate::pick::{EntityKindTag, PickId, PickIdError, PickIndex};
+
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub struct ScenePoint {
+    at: Point2,
+    entity: SketchEntityId,
+    pick: PickId,
+}
+
+impl ScenePoint {
+    #[must_use]
+    pub const fn at(self) -> Point2 {
+        self.at
+    }
+
+    #[must_use]
+    pub const fn pick(self) -> PickId {
+        self.pick
+    }
+}
+
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub struct SceneLine {
+    a: Point2,
+    b: Point2,
+    entity: SketchEntityId,
+    pick: PickId,
+    for_construction: bool,
+}
+
+impl SceneLine {
+    #[must_use]
+    pub const fn a(self) -> Point2 {
+        self.a
+    }
+
+    #[must_use]
+    pub const fn b(self) -> Point2 {
+        self.b
+    }
+
+    #[must_use]
+    pub const fn pick(self) -> PickId {
+        self.pick
+    }
+
+    #[must_use]
+    pub const fn for_construction(self) -> bool {
+        self.for_construction
+    }
+}
+
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub struct SceneArc {
+    center: Point2,
+    radius: Length,
+    start_angle: Angle,
+    sweep_angle: Angle,
+    entity: SketchEntityId,
+    pick: PickId,
+    for_construction: bool,
+}
+
+impl SceneArc {
+    #[must_use]
+    pub const fn center(self) -> Point2 {
+        self.center
+    }
+
+    #[must_use]
+    pub const fn radius(self) -> Length {
+        self.radius
+    }
+
+    #[must_use]
+    pub const fn start_angle(self) -> Angle {
+        self.start_angle
+    }
+
+    #[must_use]
+    pub const fn sweep_angle(self) -> Angle {
+        self.sweep_angle
+    }
+
+    #[must_use]
+    pub const fn pick(self) -> PickId {
+        self.pick
+    }
+
+    #[must_use]
+    pub const fn for_construction(self) -> bool {
+        self.for_construction
+    }
+}
+
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub struct SceneCircle {
+    center: Point2,
+    radius: Length,
+    entity: SketchEntityId,
+    pick: PickId,
+    for_construction: bool,
+}
+
+impl SceneCircle {
+    #[must_use]
+    pub const fn center(self) -> Point2 {
+        self.center
+    }
+
+    #[must_use]
+    pub const fn radius(self) -> Length {
+        self.radius
+    }
+
+    #[must_use]
+    pub const fn pick(self) -> PickId {
+        self.pick
+    }
+
+    #[must_use]
+    pub const fn for_construction(self) -> bool {
+        self.for_construction
+    }
+}
+
+#[derive(Clone, Debug, Default, PartialEq)]
+pub struct SketchScene {
+    points: Vec<ScenePoint>,
+    lines: Vec<SceneLine>,
+    arcs: Vec<SceneArc>,
+    circles: Vec<SceneCircle>,
+}
+
+impl SketchScene {
+    #[must_use]
+    pub const fn empty() -> Self {
+        Self {
+            points: Vec::new(),
+            lines: Vec::new(),
+            arcs: Vec::new(),
+            circles: Vec::new(),
+        }
+    }
+
+    pub fn extract(sketch: &Sketch) -> Result<Self, PickIdError> {
+        sketch
+            .entity_order()
+            .iter()
+            .copied()
+            .try_fold(Self::empty(), |acc, id| acc.push_from_sketch(sketch, id))
+    }
+
+    #[must_use]
+    pub fn points(&self) -> &[ScenePoint] {
+        &self.points
+    }
+
+    #[must_use]
+    pub fn lines(&self) -> &[SceneLine] {
+        &self.lines
+    }
+
+    #[must_use]
+    pub fn arcs(&self) -> &[SceneArc] {
+        &self.arcs
+    }
+
+    #[must_use]
+    pub fn circles(&self) -> &[SceneCircle] {
+        &self.circles
+    }
+
+    #[must_use]
+    pub fn is_empty(&self) -> bool {
+        self.points.is_empty()
+            && self.lines.is_empty()
+            && self.arcs.is_empty()
+            && self.circles.is_empty()
+    }
+
+    #[must_use]
+    pub fn aabb(&self) -> Option<Aabb2> {
+        let points = self
+            .points
+            .iter()
+            .map(|p| Aabb2::from_corners(p.at(), p.at()));
+        let lines = self.lines.iter().map(|l| Aabb2::from_corners(l.a(), l.b()));
+        let circles = self.circles.iter().map(|c| {
+            let (cx, cy) = c.center().coords_mm();
+            let r = c.radius().get::<millimeter>();
+            Aabb2::from_corners(
+                Point2::from_mm(cx - r, cy - r),
+                Point2::from_mm(cx + r, cy + r),
+            )
+        });
+        let arcs = self
+            .arcs
+            .iter()
+            .map(|a| arc_bounding_box(a.center(), a.radius(), a.start_angle(), a.sweep_angle()));
+        points
+            .chain(lines)
+            .chain(circles)
+            .chain(arcs)
+            .reduce(|a, b| a.extend_point(b.min()).extend_point(b.max()))
+    }
+
+    pub fn pick_index(&self) -> Result<PickIndex, PickIdError> {
+        let entities = self
+            .points
+            .iter()
+            .map(|p| (p.entity, EntityKindTag::Point))
+            .chain(self.lines.iter().map(|l| (l.entity, EntityKindTag::Line)))
+            .chain(self.arcs.iter().map(|a| (a.entity, EntityKindTag::Arc)))
+            .chain(
+                self.circles
+                    .iter()
+                    .map(|c| (c.entity, EntityKindTag::Circle)),
+            );
+        PickIndex::build(
+            entities,
+            core::iter::empty::<bone_types::SketchRelationId>(),
+            core::iter::empty::<bone_types::SketchDimensionId>(),
+        )
+    }
+
+    fn push_from_sketch(
+        mut self,
+        sketch: &Sketch,
+        id: SketchEntityId,
+    ) -> Result<Self, PickIdError> {
+        let Some(entity) = sketch.entities().get(id).copied() else {
+            unreachable!(
+                "entity_order references id {id:?} missing from entities — Sketch invariants broken"
+            );
+        };
+        match entity {
+            SketchEntity::Point(p) => {
+                self.points.push(ScenePoint {
+                    at: p.at(),
+                    entity: id,
+                    pick: PickId::point(id)?,
+                });
+            }
+            SketchEntity::Line(l) => {
+                self.lines.push(SceneLine {
+                    a: point_position(sketch, l.a()),
+                    b: point_position(sketch, l.b()),
+                    entity: id,
+                    pick: PickId::line(id)?,
+                    for_construction: l.for_construction(),
+                });
+            }
+            SketchEntity::Arc(a) => {
+                if let Some(arc) = derive_arc(sketch, id, a, PickId::arc(id)?) {
+                    self.arcs.push(arc);
+                }
+            }
+            SketchEntity::Circle(c) => {
+                self.circles.push(SceneCircle {
+                    center: point_position(sketch, c.center()),
+                    radius: c.radius(),
+                    entity: id,
+                    pick: PickId::circle(id)?,
+                    for_construction: c.for_construction(),
+                });
+            }
+        }
+        Ok(self)
+    }
+}
+
+fn point_position(sketch: &Sketch, id: SketchEntityId) -> Point2 {
+    let Some(entity) = sketch.entities().get(id) else {
+        unreachable!(
+            "SketchEntityId {id:?} missing from sketch entities — Sketch invariants broken"
+        );
+    };
+    let SketchEntity::Point(p) = entity else {
+        unreachable!(
+            "expected Point at {id:?}, got {:?} — Sketch::apply guarantees references resolve to Points",
+            entity.kind()
+        );
+    };
+    p.at()
+}
+
+fn derive_arc(
+    sketch: &Sketch,
+    id: SketchEntityId,
+    data: bone_document::ArcData,
+    pick: PickId,
+) -> Option<SceneArc> {
+    let center = point_position(sketch, data.center());
+    let start = point_position(sketch, data.start());
+    let end = point_position(sketch, data.end());
+    let (cx, cy) = center.coords_mm();
+    let (start_x, start_y) = start.coords_mm();
+    let (end_x, end_y) = end.coords_mm();
+    let start_offset_x = start_x - cx;
+    let start_offset_y = start_y - cy;
+    let radius_mm = (start_offset_x * start_offset_x + start_offset_y * start_offset_y).sqrt();
+    if !(radius_mm.is_finite() && radius_mm > 0.0) {
+        return None;
+    }
+    let start_angle = start_offset_y.atan2(start_offset_x);
+    let end_angle = (end_y - cy).atan2(end_x - cx);
+    let sweep = (end_angle - start_angle).rem_euclid(TAU);
+    if !(sweep.is_finite() && sweep > 0.0) {
+        return None;
+    }
+    Some(SceneArc {
+        center,
+        radius: Length::new::<millimeter>(radius_mm),
+        start_angle: Angle::new::<radian>(start_angle),
+        sweep_angle: Angle::new::<radian>(sweep),
+        entity: id,
+        pick,
+        for_construction: data.for_construction(),
+    })
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use bone_document::{EditOutcome, SketchEdit};
+    use bone_types::{Point3, SketchPlaneBasis, Tolerance, UnitVec3};
+    use uom::si::length::millimeter;
+
+    fn plane() -> SketchPlaneBasis {
+        let Ok(basis) = SketchPlaneBasis::new(
+            Point3::origin(),
+            UnitVec3::x_axis(),
+            UnitVec3::y_axis(),
+            Tolerance::new(1e-9),
+        ) else {
+            panic!("xy plane basis is orthogonal");
+        };
+        basis
+    }
+
+    fn len_mm(v: f64) -> Length {
+        Length::new::<millimeter>(v)
+    }
+
+    fn add_point(sketch: Sketch, x: f64, y: f64) -> (Sketch, SketchEntityId) {
+        let Ok((next, EditOutcome::Entity(id))) = sketch.apply(SketchEdit::AddEntity(
+            SketchEntity::point(Point2::from_mm(x, y)),
+        )) else {
+            panic!("add point");
+        };
+        (next, id)
+    }
+
+    fn add_line(
+        sketch: Sketch,
+        a: SketchEntityId,
+        b: SketchEntityId,
+        construction: bool,
+    ) -> (Sketch, SketchEntityId) {
+        let Ok((next, EditOutcome::Entity(id))) = sketch.apply(SketchEdit::AddEntity(
+            SketchEntity::line(a, b, construction),
+        )) else {
+            panic!("add line");
+        };
+        (next, id)
+    }
+
+    fn add_circle(
+        sketch: Sketch,
+        center: SketchEntityId,
+        radius_mm: f64,
+    ) -> (Sketch, SketchEntityId) {
+        let Ok((next, EditOutcome::Entity(id))) = sketch.apply(SketchEdit::AddEntity(
+            SketchEntity::circle(center, len_mm(radius_mm), false),
+        )) else {
+            panic!("add circle");
+        };
+        (next, id)
+    }
+
+    fn add_arc_ids(
+        sketch: Sketch,
+        center: SketchEntityId,
+        start: SketchEntityId,
+        end: SketchEntityId,
+    ) -> (Sketch, SketchEntityId) {
+        let Ok((next, EditOutcome::Entity(id))) = sketch.apply(SketchEdit::AddEntity(
+            SketchEntity::arc(center, start, end, false),
+        )) else {
+            panic!("add arc");
+        };
+        (next, id)
+    }
+
+    #[test]
+    fn empty_scene_has_no_items() {
+        let s = Sketch::new(plane());
+        let Ok(scene) = SketchScene::extract(&s) else {
+            panic!("extract empty");
+        };
+        assert!(scene.is_empty());
+    }
+
+    #[test]
+    fn rectangle_extracts_four_points_and_four_lines() {
+        let s = Sketch::new(plane());
+        let (s, p0) = add_point(s, 0.0, 0.0);
+        let (s, p1) = add_point(s, 10.0, 0.0);
+        let (s, p2) = add_point(s, 10.0, 5.0);
+        let (s, p3) = add_point(s, 0.0, 5.0);
+        let (s, _) = add_line(s, p0, p1, false);
+        let (s, _) = add_line(s, p1, p2, false);
+        let (s, _) = add_line(s, p2, p3, false);
+        let (s, _) = add_line(s, p3, p0, false);
+        let Ok(scene) = SketchScene::extract(&s) else {
+            panic!("extract");
+        };
+        assert_eq!(scene.points().len(), 4);
+        assert_eq!(scene.lines().len(), 4);
+        assert!(scene.arcs().is_empty());
+        assert!(scene.circles().is_empty());
+        assert_eq!(scene.lines()[0].a(), Point2::from_mm(0.0, 0.0));
+        assert_eq!(scene.lines()[0].b(), Point2::from_mm(10.0, 0.0));
+    }
+
+    #[test]
+    fn construction_flag_propagates_to_line() {
+        let s = Sketch::new(plane());
+        let (s, a) = add_point(s, 0.0, 0.0);
+        let (s, b) = add_point(s, 3.0, 4.0);
+        let (s, _) = add_line(s, a, b, true);
+        let Ok(scene) = SketchScene::extract(&s) else {
+            panic!("extract");
+        };
+        assert!(scene.lines()[0].for_construction());
+    }
+
+    #[test]
+    fn circle_extracts_with_center_and_radius() {
+        let s = Sketch::new(plane());
+        let (s, c) = add_point(s, 1.0, 2.0);
+        let (s, _) = add_circle(s, c, 4.0);
+        let Ok(scene) = SketchScene::extract(&s) else {
+            panic!("extract");
+        };
+        assert_eq!(scene.circles().len(), 1);
+        assert_eq!(scene.circles()[0].center(), Point2::from_mm(1.0, 2.0));
+        assert!((scene.circles()[0].radius().get::<millimeter>() - 4.0).abs() < 1e-12);
+    }
+
+    #[test]
+    fn arc_derives_radius_and_ccw_sweep() {
+        let s = Sketch::new(plane());
+        let (s, c) = add_point(s, 0.0, 0.0);
+        let (s, start) = add_point(s, 5.0, 0.0);
+        let (s, end) = add_point(s, 0.0, 5.0);
+        let (s, _) = add_arc_ids(s, c, start, end);
+        let Ok(scene) = SketchScene::extract(&s) else {
+            panic!("extract");
+        };
+        let arc = scene.arcs()[0];
+        assert!((arc.radius().get::<millimeter>() - 5.0).abs() < 1e-12);
+        let start_rad = arc.start_angle().get::<radian>();
+        let sweep_rad = arc.sweep_angle().get::<radian>();
+        assert!(start_rad.abs() < 1e-12);
+        assert!((sweep_rad - core::f64::consts::FRAC_PI_2).abs() < 1e-12);
+    }
+
+    #[test]
+    fn pick_index_round_trips_each_scene_item() {
+        use crate::pick::PickedItem;
+        let s = Sketch::new(plane());
+        let (s, p0) = add_point(s, 0.0, 0.0);
+        let (s, p1) = add_point(s, 1.0, 0.0);
+        let (s, line) = add_line(s, p0, p1, false);
+        let (s, cc) = add_point(s, 5.0, 0.0);
+        let (s, circle) = add_circle(s, cc, 2.0);
+        let (s, ac) = add_point(s, 10.0, 0.0);
+        let (s, a_start) = add_point(s, 12.0, 0.0);
+        let (s, a_end) = add_point(s, 10.0, 2.0);
+        let (s, arc) = add_arc_ids(s, ac, a_start, a_end);
+        let Ok(scene) = SketchScene::extract(&s) else {
+            panic!("extract");
+        };
+        let Ok(index) = scene.pick_index() else {
+            panic!("pick index");
+        };
+        assert_eq!(
+            scene.points()[0].pick().unpack(&index),
+            Some(PickedItem::Point(p0))
+        );
+        assert_eq!(
+            scene.lines()[0].pick().unpack(&index),
+            Some(PickedItem::Line(line))
+        );
+        assert_eq!(
+            scene.arcs()[0].pick().unpack(&index),
+            Some(PickedItem::Arc(arc))
+        );
+        assert_eq!(
+            scene.circles()[0].pick().unpack(&index),
+            Some(PickedItem::Circle(circle))
+        );
+    }
+
+    #[test]
+    fn arc_coincident_start_and_end_is_dropped() {
+        let s = Sketch::new(plane());
+        let (s, c) = add_point(s, 0.0, 0.0);
+        let (s, start) = add_point(s, 5.0, 0.0);
+        let (s, end) = add_point(s, 5.0, 0.0);
+        let (s, _) = add_arc_ids(s, c, start, end);
+        let Ok(scene) = SketchScene::extract(&s) else {
+            panic!("extract");
+        };
+        assert!(scene.arcs().is_empty());
+    }
+
+    #[test]
+    fn arc_with_zero_radius_is_dropped() {
+        let s = Sketch::new(plane());
+        let (s, c) = add_point(s, 0.0, 0.0);
+        let (s, start) = add_point(s, 0.0, 0.0);
+        let (s, end) = add_point(s, 5.0, 0.0);
+        let (s, _) = add_arc_ids(s, c, start, end);
+        let Ok(scene) = SketchScene::extract(&s) else {
+            panic!("extract");
+        };
+        assert!(scene.arcs().is_empty());
+    }
+
+    #[test]
+    fn arc_sweep_follows_ccw_convention() {
+        let s = Sketch::new(plane());
+        let (s, c) = add_point(s, 0.0, 0.0);
+        let (s, start) = add_point(s, 0.0, 5.0);
+        let (s, end) = add_point(s, 5.0, 0.0);
+        let (s, _) = add_arc_ids(s, c, start, end);
+        let Ok(scene) = SketchScene::extract(&s) else {
+            panic!("extract");
+        };
+        let arc = scene.arcs()[0];
+        let start_rad = arc.start_angle().get::<radian>();
+        let sweep_rad = arc.sweep_angle().get::<radian>();
+        assert!((start_rad - core::f64::consts::FRAC_PI_2).abs() < 1e-12);
+        assert!((sweep_rad - 3.0 * core::f64::consts::FRAC_PI_2).abs() < 1e-12);
+    }
+}
diff --git a/crates/bone-render/src/snapshot.rs b/crates/bone-render/src/snapshot.rs
new file mode 100644
index 0000000..7897577
--- /dev/null
+++ b/crates/bone-render/src/snapshot.rs
@@ -0,0 +1,371 @@
+use crate::camera::ViewportExtent;
+use crate::gpu::BackendTag;
+use crate::{RenderError, Result};
+
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub struct ClearColor {
+    r: f64,
+    g: f64,
+    b: f64,
+    a: f64,
+}
+
+impl ClearColor {
+    #[must_use]
+    pub const fn new(r: f64, g: f64, b: f64, a: f64) -> Self {
+        Self { r, g, b, a }
+    }
+
+    #[must_use]
+    pub const fn opaque(r: f64, g: f64, b: f64) -> Self {
+        Self::new(r, g, b, 1.0)
+    }
+
+    #[must_use]
+    pub fn to_rgba8(self) -> [u8; 4] {
+        [
+            channel_to_u8(self.r),
+            channel_to_u8(self.g),
+            channel_to_u8(self.b),
+            channel_to_u8(self.a),
+        ]
+    }
+
+    #[must_use]
+    #[allow(clippy::cast_possible_truncation)]
+    pub fn to_rgba_array(self) -> [f32; 4] {
+        [self.r as f32, self.g as f32, self.b as f32, self.a as f32]
+    }
+}
+
+impl From<ClearColor> for wgpu::Color {
+    fn from(c: ClearColor) -> Self {
+        Self {
+            r: c.r,
+            g: c.g,
+            b: c.b,
+            a: c.a,
+        }
+    }
+}
+
+#[allow(clippy::cast_possible_truncation, clippy::cast_sign_loss)]
+fn channel_to_u8(value: f64) -> u8 {
+    (value.clamp(0.0, 1.0) * 255.0).round() as u8
+}
+
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub struct GridStyle {
+    minor: ClearColor,
+    major: ClearColor,
+    axis_x: ClearColor,
+    axis_y: ClearColor,
+    origin: ClearColor,
+    line_width_px: f32,
+    axis_width_px: f32,
+    origin_radius_px: f32,
+    minor_spacing_target_px: f32,
+}
+
+impl GridStyle {
+    pub const DEFAULT: Self = Self {
+        minor: ClearColor::new(1.0, 1.0, 1.0, 0.08),
+        major: ClearColor::new(1.0, 1.0, 1.0, 0.22),
+        axis_x: ClearColor::new(1.0, 0.0, 0.0, 1.0),
+        axis_y: ClearColor::new(0.0, 1.0, 0.0, 1.0),
+        origin: ClearColor::new(1.0, 1.0, 1.0, 0.95),
+        line_width_px: 1.0,
+        axis_width_px: 1.6,
+        origin_radius_px: 4.0,
+        minor_spacing_target_px: 24.0,
+    };
+
+    #[must_use]
+    pub const fn minor(self) -> ClearColor {
+        self.minor
+    }
+
+    #[must_use]
+    pub const fn major(self) -> ClearColor {
+        self.major
+    }
+
+    #[must_use]
+    pub const fn axis_x(self) -> ClearColor {
+        self.axis_x
+    }
+
+    #[must_use]
+    pub const fn axis_y(self) -> ClearColor {
+        self.axis_y
+    }
+
+    #[must_use]
+    pub const fn origin(self) -> ClearColor {
+        self.origin
+    }
+
+    #[must_use]
+    pub const fn line_width_px(self) -> f32 {
+        self.line_width_px
+    }
+
+    #[must_use]
+    pub const fn axis_width_px(self) -> f32 {
+        self.axis_width_px
+    }
+
+    #[must_use]
+    pub const fn origin_radius_px(self) -> f32 {
+        self.origin_radius_px
+    }
+
+    #[must_use]
+    pub const fn minor_spacing_target_px(self) -> f32 {
+        self.minor_spacing_target_px
+    }
+}
+
+impl Default for GridStyle {
+    fn default() -> Self {
+        Self::DEFAULT
+    }
+}
+
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub struct StrokeStyle {
+    stroke: ClearColor,
+    construction: ClearColor,
+    stroke_width_px: f32,
+    point_radius_px: f32,
+    construction_dash_period_px: f32,
+    construction_dash_on_ratio: f32,
+}
+
+impl StrokeStyle {
+    pub const DEFAULT: Self = Self {
+        stroke: ClearColor::new(0.95, 0.95, 0.98, 1.0),
+        construction: ClearColor::new(0.55, 0.75, 1.0, 0.85),
+        stroke_width_px: 1.5,
+        point_radius_px: 3.0,
+        construction_dash_period_px: 8.0,
+        construction_dash_on_ratio: 0.5,
+    };
+
+    #[must_use]
+    pub const fn stroke(self) -> ClearColor {
+        self.stroke
+    }
+
+    #[must_use]
+    pub const fn construction(self) -> ClearColor {
+        self.construction
+    }
+
+    #[must_use]
+    pub const fn stroke_width_px(self) -> f32 {
+        self.stroke_width_px
+    }
+
+    #[must_use]
+    pub const fn point_radius_px(self) -> f32 {
+        self.point_radius_px
+    }
+
+    #[must_use]
+    pub const fn construction_dash_period_px(self) -> f32 {
+        self.construction_dash_period_px
+    }
+
+    #[must_use]
+    pub const fn construction_dash_on_ratio(self) -> f32 {
+        self.construction_dash_on_ratio
+    }
+}
+
+impl Default for StrokeStyle {
+    fn default() -> Self {
+        Self::DEFAULT
+    }
+}
+
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub struct Style {
+    background: ClearColor,
+    grid: GridStyle,
+    strokes: StrokeStyle,
+}
+
+impl Style {
+    #[must_use]
+    pub const fn new(background: ClearColor) -> Self {
+        Self {
+            background,
+            grid: GridStyle::DEFAULT,
+            strokes: StrokeStyle::DEFAULT,
+        }
+    }
+
+    #[must_use]
+    pub const fn background(self) -> ClearColor {
+        self.background
+    }
+
+    #[must_use]
+    pub const fn grid(self) -> GridStyle {
+        self.grid
+    }
+
+    #[must_use]
+    pub const fn strokes(self) -> StrokeStyle {
+        self.strokes
+    }
+}
+
+impl Default for Style {
+    fn default() -> Self {
+        Self::new(ClearColor::opaque(0.05, 0.05, 0.07))
+    }
+}
+
+#[derive(Clone, Debug)]
+pub struct SnapshotFrame {
+    extent: ViewportExtent,
+    rgba: Vec<u8>,
+    backend: BackendTag,
+}
+
+impl SnapshotFrame {
+    pub(crate) fn new(extent: ViewportExtent, rgba: Vec<u8>, backend: BackendTag) -> Self {
+        Self {
+            extent,
+            rgba,
+            backend,
+        }
+    }
+
+    #[must_use]
+    pub fn extent(&self) -> ViewportExtent {
+        self.extent
+    }
+
+    #[must_use]
+    pub fn rgba(&self) -> &[u8] {
+        &self.rgba
+    }
+
+    #[must_use]
+    pub fn backend(&self) -> BackendTag {
+        self.backend
+    }
+}
+
+pub fn encode_png(frame: &SnapshotFrame) -> Result<Vec<u8>> {
+    let mut out: Vec<u8> = Vec::new();
+    {
+        let mut encoder = png::Encoder::new(
+            &mut out,
+            frame.extent.width().value(),
+            frame.extent.height().value(),
+        );
+        encoder.set_color(png::ColorType::Rgba);
+        encoder.set_depth(png::BitDepth::Eight);
+        let mut writer = encoder.write_header()?;
+        writer.write_image_data(frame.rgba())?;
+    }
+    Ok(out)
+}
+
+pub fn decode_png(bytes: &[u8]) -> Result<(ViewportExtent, Vec<u8>)> {
+    use crate::camera::ViewportPx;
+    let decoder = png::Decoder::new(bytes);
+    let mut reader = decoder.read_info()?;
+    {
+        let info = reader.info();
+        if info.color_type != png::ColorType::Rgba || info.bit_depth != png::BitDepth::Eight {
+            return Err(RenderError::PngFormat {
+                color_type: info.color_type,
+                bit_depth: info.bit_depth,
+            });
+        }
+    }
+    let mut buf = vec![0_u8; reader.output_buffer_size()];
+    let info = reader.next_frame(&mut buf)?;
+    let extent = ViewportExtent::new(ViewportPx::new(info.width), ViewportPx::new(info.height));
+    buf.truncate(info.buffer_size());
+    Ok((extent, buf))
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::camera::ViewportPx;
+    use crate::gpu::BackendTag;
+
+    fn tag() -> BackendTag {
+        BackendTag::from_backend(wgpu::Backend::Noop)
+    }
+
+    #[test]
+    fn clear_color_rounds_to_nearest_u8() {
+        let c = ClearColor::opaque(0.0, 0.5, 1.0);
+        assert_eq!(c.to_rgba8(), [0, 128, 255, 255]);
+    }
+
+    #[test]
+    fn style_default_is_navy_opaque() {
+        let rgba = Style::default().background().to_rgba8();
+        assert_eq!(rgba[3], 255);
+        assert!(rgba[0] < 32 && rgba[1] < 32 && rgba[2] < 32);
+    }
+
+    fn encode_grayscale(width: u32, height: u32, pixels: &[u8]) -> Vec<u8> {
+        let mut bytes: Vec<u8> = Vec::new();
+        {
+            let mut enc = png::Encoder::new(&mut bytes, width, height);
+            enc.set_color(png::ColorType::Grayscale);
+            enc.set_depth(png::BitDepth::Eight);
+            let Ok(mut writer) = enc.write_header() else {
+                panic!("grayscale png header write failed");
+            };
+            assert!(
+                writer.write_image_data(pixels).is_ok(),
+                "grayscale png body write failed",
+            );
+        }
+        bytes
+    }
+
+    #[test]
+    fn decode_png_rejects_non_rgba8() {
+        let bytes = encode_grayscale(4, 4, &[0_u8; 16]);
+        let Err(RenderError::PngFormat {
+            color_type,
+            bit_depth,
+        }) = decode_png(&bytes)
+        else {
+            panic!("expected PngFormat rejection");
+        };
+        assert_eq!(color_type, png::ColorType::Grayscale);
+        assert_eq!(bit_depth, png::BitDepth::Eight);
+    }
+
+    #[test]
+    fn png_encode_decode_round_trips_solid_frame() {
+        let extent = ViewportExtent::new(ViewportPx::new(8), ViewportPx::new(4));
+        let pixel = [200_u8, 80, 40, 255];
+        let rgba: Vec<u8> = (0..extent.pixel_count())
+            .flat_map(|_| pixel.into_iter())
+            .collect();
+        let frame = SnapshotFrame::new(extent, rgba.clone(), tag());
+
+        let Ok(encoded) = encode_png(&frame) else {
+            panic!("encode_png failed");
+        };
+        let Ok((decoded_extent, decoded)) = decode_png(&encoded) else {
+            panic!("decode_png failed");
+        };
+        assert_eq!(decoded_extent, extent);
+        assert_eq!(decoded, rgba);
+    }
+}
diff --git a/crates/bone-render/src/surface.rs b/crates/bone-render/src/surface.rs
new file mode 100644
index 0000000..6ba5a3f
--- /dev/null
+++ b/crates/bone-render/src/surface.rs
@@ -0,0 +1,235 @@
+use crate::camera::ViewportExtent;
+use crate::gpu::{Capabilities, Gpu};
+
+const PICK_FORMAT: wgpu::TextureFormat = wgpu::TextureFormat::R32Uint;
+
+#[derive(Debug, thiserror::Error)]
+pub enum SurfaceError {
+    #[error("surface creation: {0}")]
+    Create(#[from] wgpu::CreateSurfaceError),
+    #[error("no wgpu adapter matched the surface: {0}")]
+    NoAdapter(#[from] wgpu::RequestAdapterError),
+    #[error("wgpu device request failed: {0}")]
+    Device(#[from] wgpu::RequestDeviceError),
+    #[error(
+        "surface exposes no srgb colour format; baseline requires one. available: {available:?}"
+    )]
+    NoSrgbFormat { available: Vec<wgpu::TextureFormat> },
+    #[error("viewport dimension is zero")]
+    ZeroExtent,
+}
+
+pub struct SurfaceContext {
+    gpu: Gpu,
+    surface: wgpu::Surface<'static>,
+    config: wgpu::SurfaceConfiguration,
+    pick: wgpu::Texture,
+    extent: ViewportExtent,
+    reconfigure_pending: bool,
+}
+
+enum AcquireOutcome {
+    Frame(wgpu::SurfaceTexture),
+    Skip,
+    Reconfigure,
+}
+
+impl SurfaceContext {
+    pub async fn new(
+        target: impl Into<wgpu::SurfaceTarget<'static>>,
+        extent: ViewportExtent,
+    ) -> Result<Self, SurfaceError> {
+        if extent.width().value() == 0 || extent.height().value() == 0 {
+            return Err(SurfaceError::ZeroExtent);
+        }
+        let instance =
+            wgpu::Instance::new(wgpu::InstanceDescriptor::new_without_display_handle_from_env());
+        let target: wgpu::SurfaceTarget<'static> = target.into();
+        let surface = instance.create_surface(target)?;
+        let adapter = instance
+            .request_adapter(&wgpu::RequestAdapterOptions {
+                power_preference: wgpu::PowerPreference::LowPower,
+                force_fallback_adapter: false,
+                compatible_surface: Some(&surface),
+            })
+            .await?;
+        let (device, queue) = adapter
+            .request_device(&wgpu::DeviceDescriptor {
+                label: Some("bone-render:surface"),
+                required_features: wgpu::Features::empty(),
+                required_limits: wgpu::Limits::downlevel_defaults(),
+                experimental_features: wgpu::ExperimentalFeatures::default(),
+                memory_hints: wgpu::MemoryHints::default(),
+                trace: wgpu::Trace::Off,
+            })
+            .await?;
+        let capabilities = Capabilities::probe(&adapter);
+        let caps = surface.get_capabilities(&adapter);
+        let Some(format) = caps
+            .formats
+            .iter()
+            .copied()
+            .find(wgpu::TextureFormat::is_srgb)
+        else {
+            return Err(SurfaceError::NoSrgbFormat {
+                available: caps.formats.clone(),
+            });
+        };
+        let present_mode = pick_present_mode(&caps.present_modes);
+        let alpha_mode = caps
+            .alpha_modes
+            .first()
+            .copied()
+            .unwrap_or(wgpu::CompositeAlphaMode::Auto);
+        let config = wgpu::SurfaceConfiguration {
+            usage: wgpu::TextureUsages::RENDER_ATTACHMENT,
+            format,
+            width: extent.width().value(),
+            height: extent.height().value(),
+            present_mode,
+            alpha_mode,
+            view_formats: vec![],
+            desired_maximum_frame_latency: 1,
+        };
+        surface.configure(&device, &config);
+        let pick = create_pick_texture(&device, extent);
+        Ok(Self {
+            gpu: Gpu::from_parts(device, queue, capabilities),
+            surface,
+            config,
+            pick,
+            extent,
+            reconfigure_pending: false,
+        })
+    }
+
+    #[must_use]
+    pub fn gpu(&self) -> &Gpu {
+        &self.gpu
+    }
+
+    #[must_use]
+    pub const fn extent(&self) -> ViewportExtent {
+        self.extent
+    }
+
+    #[must_use]
+    pub const fn color_format(&self) -> wgpu::TextureFormat {
+        self.config.format
+    }
+
+    pub fn resize(&mut self, extent: ViewportExtent) {
+        if extent.width().value() == 0 || extent.height().value() == 0 {
+            return;
+        }
+        self.extent = extent;
+        self.config.width = extent.width().value();
+        self.config.height = extent.height().value();
+        self.surface.configure(self.gpu.device(), &self.config);
+        self.pick = create_pick_texture(self.gpu.device(), extent);
+        self.reconfigure_pending = false;
+    }
+
+    pub fn render<F, G>(&mut self, build_passes: F, pre_present: G)
+    where
+        F: FnOnce(&mut wgpu::CommandEncoder, &wgpu::TextureView, &wgpu::TextureView),
+        G: FnOnce(),
+    {
+        let Some(frame) = self.acquire_frame() else {
+            return;
+        };
+        let color_view = frame
+            .texture
+            .create_view(&wgpu::TextureViewDescriptor::default());
+        let pick_view = self
+            .pick
+            .create_view(&wgpu::TextureViewDescriptor::default());
+        let mut encoder =
+            self.gpu
+                .device()
+                .create_command_encoder(&wgpu::CommandEncoderDescriptor {
+                    label: Some("bone-render:surface-encoder"),
+                });
+        build_passes(&mut encoder, &color_view, &pick_view);
+        self.gpu.queue().submit(Some(encoder.finish()));
+        pre_present();
+        frame.present();
+    }
+
+    fn acquire_frame(&mut self) -> Option<wgpu::SurfaceTexture> {
+        if self.reconfigure_pending {
+            self.surface.configure(self.gpu.device(), &self.config);
+            self.reconfigure_pending = false;
+        }
+        match self.classify_acquire() {
+            AcquireOutcome::Frame(frame) => Some(frame),
+            AcquireOutcome::Skip => None,
+            AcquireOutcome::Reconfigure => {
+                self.surface.configure(self.gpu.device(), &self.config);
+                match self.classify_acquire() {
+                    AcquireOutcome::Frame(frame) => Some(frame),
+                    AcquireOutcome::Skip => None,
+                    AcquireOutcome::Reconfigure => {
+                        tracing::warn!(
+                            "surface still outdated/lost after reconfigure; skipping frame"
+                        );
+                        None
+                    }
+                }
+            }
+        }
+    }
+
+    fn classify_acquire(&mut self) -> AcquireOutcome {
+        match self.surface.get_current_texture() {
+            wgpu::CurrentSurfaceTexture::Success(frame) => AcquireOutcome::Frame(frame),
+            wgpu::CurrentSurfaceTexture::Suboptimal(frame) => {
+                self.reconfigure_pending = true;
+                AcquireOutcome::Frame(frame)
+            }
+            wgpu::CurrentSurfaceTexture::Outdated | wgpu::CurrentSurfaceTexture::Lost => {
+                AcquireOutcome::Reconfigure
+            }
+            wgpu::CurrentSurfaceTexture::Timeout => {
+                tracing::warn!("surface frame acquire timed out; skipping frame");
+                AcquireOutcome::Skip
+            }
+            wgpu::CurrentSurfaceTexture::Occluded => {
+                tracing::debug!("surface occluded; skipping frame");
+                AcquireOutcome::Skip
+            }
+            wgpu::CurrentSurfaceTexture::Validation => {
+                tracing::warn!("surface validation error on acquire; skipping frame");
+                AcquireOutcome::Skip
+            }
+        }
+    }
+}
+
+fn create_pick_texture(device: &wgpu::Device, extent: ViewportExtent) -> wgpu::Texture {
+    device.create_texture(&wgpu::TextureDescriptor {
+        label: Some("bone-render:surface-pick"),
+        size: wgpu::Extent3d {
+            width: extent.width().value(),
+            height: extent.height().value(),
+            depth_or_array_layers: 1,
+        },
+        mip_level_count: 1,
+        sample_count: 1,
+        dimension: wgpu::TextureDimension::D2,
+        format: PICK_FORMAT,
+        usage: wgpu::TextureUsages::RENDER_ATTACHMENT | wgpu::TextureUsages::COPY_SRC,
+        view_formats: &[],
+    })
+}
+
+fn pick_present_mode(modes: &[wgpu::PresentMode]) -> wgpu::PresentMode {
+    [
+        wgpu::PresentMode::Mailbox,
+        wgpu::PresentMode::Immediate,
+        wgpu::PresentMode::FifoRelaxed,
+    ]
+    .into_iter()
+    .find(|m| modes.contains(m))
+    .unwrap_or(wgpu::PresentMode::Fifo)
+}
diff --git a/crates/bone-render/tests/arcs.rs b/crates/bone-render/tests/arcs.rs
new file mode 100644
index 0000000..bab4cef
--- /dev/null
+++ b/crates/bone-render/tests/arcs.rs
@@ -0,0 +1,148 @@
+use std::path::PathBuf;
+
+use bone_document::{EditOutcome, Sketch, SketchEdit, SketchEntity};
+use bone_render::{
+    Camera2, OffscreenContext, PixelDiff, PixelDiffThreshold, PixelsPerMm, RenderError,
+    SketchRenderer, SketchScene, Style, ViewportExtent, ViewportPx, decode_png, encode_png,
+};
+use bone_types::{Length, Point2, Point3, SketchEntityId, SketchPlaneBasis, Tolerance, UnitVec3};
+use uom::si::length::millimeter;
+
+const GOLDEN: &str = "tests/goldens/arc_circle_zoom100_256.png";
+const UPDATE_ENV: &str = "BONE_UPDATE_ARC_GOLDEN";
+const DIFF_TOLERANCE: f64 = 16.0 / 255.0;
+
+fn extent(side: u32) -> ViewportExtent {
+    ViewportExtent::square(ViewportPx::new(side))
+}
+
+fn make_context(extent: ViewportExtent) -> OffscreenContext {
+    match pollster::block_on(OffscreenContext::new(extent)) {
+        Ok(ctx) => ctx,
+        Err(RenderError::NoAdapter(e)) => {
+            panic!(
+                "no wgpu adapter available, configure lavapipe or an iGPU for this test host: {e}"
+            );
+        }
+        Err(e) => panic!("offscreen context init failed: {e}"),
+    }
+}
+
+fn golden_path() -> PathBuf {
+    PathBuf::from(env!("CARGO_MANIFEST_DIR")).join(GOLDEN)
+}
+
+fn plane() -> SketchPlaneBasis {
+    let Ok(basis) = SketchPlaneBasis::new(
+        Point3::origin(),
+        UnitVec3::x_axis(),
+        UnitVec3::y_axis(),
+        Tolerance::new(1e-9),
+    ) else {
+        panic!("xy plane basis is orthogonal");
+    };
+    basis
+}
+
+fn add_point(s: Sketch, x: f64, y: f64) -> (Sketch, SketchEntityId) {
+    let Ok((next, EditOutcome::Entity(id))) = s.apply(SketchEdit::AddEntity(SketchEntity::point(
+        Point2::from_mm(x, y),
+    ))) else {
+        panic!("add point");
+    };
+    (next, id)
+}
+
+fn add_circle(s: Sketch, center: SketchEntityId, radius_mm: f64) -> Sketch {
+    let Ok((next, _)) = s.apply(SketchEdit::AddEntity(SketchEntity::circle(
+        center,
+        Length::new::<millimeter>(radius_mm),
+        false,
+    ))) else {
+        panic!("add circle");
+    };
+    next
+}
+
+fn add_arc(
+    s: Sketch,
+    center: SketchEntityId,
+    start: SketchEntityId,
+    end: SketchEntityId,
+) -> Sketch {
+    let Ok((next, _)) = s.apply(SketchEdit::AddEntity(SketchEntity::arc(
+        center, start, end, false,
+    ))) else {
+        panic!("add arc");
+    };
+    next
+}
+
+fn arc_circle_scene() -> SketchScene {
+    let s = Sketch::new(plane());
+    let (s, circle_c) = add_point(s, -0.7, 0.0);
+    let s = add_circle(s, circle_c, 0.5);
+
+    let (s, arc_c) = add_point(s, 0.7, 0.0);
+    let (s, arc_start) = add_point(s, 1.2, 0.0);
+    let (s, arc_end) = add_point(s, 0.7, 0.5);
+    let s = add_arc(s, arc_c, arc_start, arc_end);
+
+    let Ok(scene) = SketchScene::extract(&s) else {
+        panic!("scene extract");
+    };
+    scene
+}
+
+#[test]
+fn arc_circle_at_100x_zoom_matches_golden() {
+    let size = extent(256);
+    let ctx = make_context(size);
+    let renderer = SketchRenderer::new(ctx.gpu(), ctx.color_format());
+    let scene = arc_circle_scene();
+    let camera = Camera2::new(size).with_zoom(PixelsPerMm::new(100.0));
+    let style = Style::default();
+
+    let Ok(frame) = renderer.render(&ctx, &scene, camera, &style) else {
+        panic!("SketchRenderer::render failed");
+    };
+
+    let path = golden_path();
+
+    if std::env::var(UPDATE_ENV).is_ok() {
+        let Ok(bytes) = encode_png(&frame) else {
+            panic!("encode_png failed");
+        };
+        if let Some(parent) = path.parent()
+            && let Err(e) = std::fs::create_dir_all(parent)
+        {
+            panic!("create goldens dir {}: {e}", parent.display());
+        }
+        if let Err(e) = std::fs::write(&path, &bytes) {
+            panic!("write golden {}: {e}", path.display());
+        }
+        return;
+    }
+
+    let Ok(bytes) = std::fs::read(&path) else {
+        panic!(
+            "golden missing at {}: rerun with {UPDATE_ENV}=1 to generate",
+            path.display()
+        );
+    };
+    let Ok((golden_extent, golden_rgba)) = decode_png(&bytes) else {
+        panic!("failed to decode golden PNG");
+    };
+    assert_eq!(golden_extent, size, "golden extent drift");
+    let threshold = PixelDiffThreshold::new(DIFF_TOLERANCE);
+    let Ok(report) = PixelDiff::compare(&frame, &golden_rgba, threshold) else {
+        panic!("PixelDiff rejected inputs");
+    };
+    assert!(
+        report.is_clean(),
+        "arc render drifted from golden: {} mismatches, worst {:?}, backend {}",
+        report.over_threshold(),
+        report.worst(),
+        frame.backend(),
+    );
+}
diff --git a/crates/bone-render/tests/clear.rs b/crates/bone-render/tests/clear.rs
new file mode 100644
index 0000000..a54ea6c
--- /dev/null
+++ b/crates/bone-render/tests/clear.rs
@@ -0,0 +1,91 @@
+use bone_render::{
+    ClearColor, OffscreenContext, PixelDiff, PixelDiffThreshold, RenderError, Style,
+    ViewportExtent, ViewportPx,
+};
+
+fn extent(side: u32) -> ViewportExtent {
+    ViewportExtent::square(ViewportPx::new(side))
+}
+
+fn make_context(extent: ViewportExtent) -> OffscreenContext {
+    match pollster::block_on(OffscreenContext::new(extent)) {
+        Ok(ctx) => ctx,
+        Err(RenderError::NoAdapter(e)) => {
+            panic!(
+                "no wgpu adapter available, configure lavapipe or an iGPU for this test host: {e}"
+            );
+        }
+        Err(e) => panic!("offscreen context init failed: {e}"),
+    }
+}
+
+fn expected_rgba(extent: ViewportExtent, color: ClearColor) -> Vec<u8> {
+    let pixel = color.to_rgba8();
+    let count = usize::try_from(extent.pixel_count()).unwrap_or(0);
+    (0..count).flat_map(|_| pixel.into_iter()).collect()
+}
+
+#[test]
+fn offscreen_clear_matches_style_background_exactly() {
+    let size = extent(64);
+    let ctx = make_context(size);
+    let style = Style::default();
+
+    let Ok(frame) = ctx.render_clear(&style) else {
+        panic!("render_clear failed");
+    };
+
+    assert_eq!(frame.extent(), size);
+    let expected = expected_rgba(size, style.background());
+    assert_eq!(frame.rgba().len(), expected.len());
+
+    let Ok(report) = PixelDiff::compare(&frame, &expected, PixelDiffThreshold::EXACT) else {
+        panic!("pixel diff rejected inputs");
+    };
+    assert!(
+        report.is_clean(),
+        "clear frame diverged from expected bytes: {} mismatches, worst {:?}",
+        report.over_threshold(),
+        report.worst(),
+    );
+}
+
+#[test]
+fn offscreen_clear_honours_custom_background() {
+    let size = extent(32);
+    let ctx = make_context(size);
+    let background = ClearColor::opaque(0.25, 0.50, 0.75);
+    let style = Style::new(background);
+
+    let Ok(frame) = ctx.render_clear(&style) else {
+        panic!("render_clear failed");
+    };
+
+    let expected = expected_rgba(size, background);
+    let Ok(report) = PixelDiff::compare(&frame, &expected, PixelDiffThreshold::EXACT) else {
+        panic!("pixel diff rejected inputs");
+    };
+    assert!(
+        report.is_clean(),
+        "custom clear diverged: {} mismatches",
+        report.over_threshold(),
+    );
+}
+
+#[test]
+fn offscreen_extent_with_non_aligned_width_reads_back_packed_rgba() {
+    let size = ViewportExtent::new(ViewportPx::new(63), ViewportPx::new(16));
+    let ctx = make_context(size);
+    let style = Style::new(ClearColor::opaque(1.0, 0.0, 0.0));
+
+    let Ok(frame) = ctx.render_clear(&style) else {
+        panic!("render_clear failed");
+    };
+
+    let expected = expected_rgba(size, style.background());
+    assert_eq!(frame.rgba().len(), expected.len());
+    let Ok(report) = PixelDiff::compare(&frame, &expected, PixelDiffThreshold::EXACT) else {
+        panic!("pixel diff rejected inputs");
+    };
+    assert!(report.is_clean(), "unaligned width readback diverged");
+}
diff --git a/crates/bone-render/tests/construction_styling.rs b/crates/bone-render/tests/construction_styling.rs
new file mode 100644
index 0000000..fe13ca2
--- /dev/null
+++ b/crates/bone-render/tests/construction_styling.rs
@@ -0,0 +1,194 @@
+use std::path::PathBuf;
+
+use bone_document::{EditOutcome, Sketch, SketchEdit, SketchEntity};
+use bone_render::{
+    Camera2, OffscreenContext, PixelDiff, PixelDiffThreshold, PixelsPerMm, RenderError,
+    SketchRenderer, SketchScene, SnapshotFrame, Style, ViewportExtent, ViewportPx, decode_png,
+    encode_png,
+};
+use bone_types::{Length, Point2, Point3, SketchEntityId, SketchPlaneBasis, Tolerance, UnitVec3};
+use uom::si::length::millimeter;
+
+const REAL_GOLDEN: &str = "tests/goldens/construction_real_256.png";
+const DASHED_GOLDEN: &str = "tests/goldens/construction_dashed_256.png";
+const UPDATE_ENV: &str = "BONE_UPDATE_CONSTRUCTION_GOLDENS";
+const DIFF_TOLERANCE: f64 = 16.0 / 255.0;
+const DISTINCT_MIN_PIXELS: u32 = 50;
+
+fn extent() -> ViewportExtent {
+    ViewportExtent::square(ViewportPx::new(256))
+}
+
+fn make_context() -> OffscreenContext {
+    match pollster::block_on(OffscreenContext::new(extent())) {
+        Ok(ctx) => ctx,
+        Err(RenderError::NoAdapter(e)) => panic!(
+            "no wgpu adapter available, configure lavapipe or an iGPU for this test host: {e}"
+        ),
+        Err(e) => panic!("offscreen context init failed: {e}"),
+    }
+}
+
+fn plane() -> SketchPlaneBasis {
+    let Ok(basis) = SketchPlaneBasis::new(
+        Point3::origin(),
+        UnitVec3::x_axis(),
+        UnitVec3::y_axis(),
+        Tolerance::new(1e-9),
+    ) else {
+        panic!("xy plane basis is orthogonal");
+    };
+    basis
+}
+
+fn add_point(s: Sketch, x: f64, y: f64) -> (Sketch, SketchEntityId) {
+    let Ok((next, EditOutcome::Entity(id))) = s.apply(SketchEdit::AddEntity(SketchEntity::point(
+        Point2::from_mm(x, y),
+    ))) else {
+        panic!("add point");
+    };
+    (next, id)
+}
+
+fn add_line(s: Sketch, a: SketchEntityId, b: SketchEntityId, construction: bool) -> Sketch {
+    let Ok((next, _)) = s.apply(SketchEdit::AddEntity(SketchEntity::line(
+        a,
+        b,
+        construction,
+    ))) else {
+        panic!("add line");
+    };
+    next
+}
+
+fn add_circle(s: Sketch, center: SketchEntityId, radius_mm: f64, construction: bool) -> Sketch {
+    let Ok((next, _)) = s.apply(SketchEdit::AddEntity(SketchEntity::circle(
+        center,
+        Length::new::<millimeter>(radius_mm),
+        construction,
+    ))) else {
+        panic!("add circle");
+    };
+    next
+}
+
+fn add_arc(
+    s: Sketch,
+    center: SketchEntityId,
+    start: SketchEntityId,
+    end: SketchEntityId,
+    construction: bool,
+) -> Sketch {
+    let Ok((next, _)) = s.apply(SketchEdit::AddEntity(SketchEntity::arc(
+        center,
+        start,
+        end,
+        construction,
+    ))) else {
+        panic!("add arc");
+    };
+    next
+}
+
+fn build_scene(construction: bool) -> SketchScene {
+    let s = Sketch::new(plane());
+
+    let (s, la) = add_point(s, -0.9, 0.35);
+    let (s, lb) = add_point(s, 0.9, 0.35);
+    let s = add_line(s, la, lb, construction);
+
+    let (s, cc) = add_point(s, -0.5, -0.3);
+    let s = add_circle(s, cc, 0.25, construction);
+
+    let (s, ac) = add_point(s, 0.5, -0.3);
+    let (s, astart) = add_point(s, 0.85, -0.3);
+    let (s, aend) = add_point(s, 0.5, 0.05);
+    let s = add_arc(s, ac, astart, aend, construction);
+
+    let Ok(scene) = SketchScene::extract(&s) else {
+        panic!("scene extract");
+    };
+    scene
+}
+
+fn render_scene(ctx: &OffscreenContext, scene: &SketchScene) -> SnapshotFrame {
+    let camera = Camera2::new(ctx.extent()).with_zoom(PixelsPerMm::new(100.0));
+    let renderer = SketchRenderer::new(ctx.gpu(), ctx.color_format());
+    let style = Style::default();
+    let Ok(frame) = renderer.render(ctx, scene, camera, &style) else {
+        panic!("SketchRenderer::render failed");
+    };
+    frame
+}
+
+fn golden_path(name: &str) -> PathBuf {
+    PathBuf::from(env!("CARGO_MANIFEST_DIR")).join(name)
+}
+
+fn match_or_update_golden(frame: &SnapshotFrame, name: &str) {
+    let path = golden_path(name);
+    if std::env::var(UPDATE_ENV).is_ok() {
+        let Ok(bytes) = encode_png(frame) else {
+            panic!("encode_png failed");
+        };
+        if let Some(parent) = path.parent()
+            && let Err(e) = std::fs::create_dir_all(parent)
+        {
+            panic!("create goldens dir {}: {e}", parent.display());
+        }
+        if let Err(e) = std::fs::write(&path, &bytes) {
+            panic!("write golden {}: {e}", path.display());
+        }
+        return;
+    }
+    let Ok(bytes) = std::fs::read(&path) else {
+        panic!(
+            "golden missing at {}: rerun with {UPDATE_ENV}=1 to generate",
+            path.display()
+        );
+    };
+    let Ok((golden_extent, golden_rgba)) = decode_png(&bytes) else {
+        panic!("failed to decode golden PNG at {}", path.display());
+    };
+    assert_eq!(
+        golden_extent,
+        frame.extent(),
+        "golden extent drift for {name}"
+    );
+    let threshold = PixelDiffThreshold::new(DIFF_TOLERANCE);
+    let Ok(report) = PixelDiff::compare(frame, &golden_rgba, threshold) else {
+        panic!("PixelDiff rejected inputs for {name}");
+    };
+    assert!(
+        report.is_clean(),
+        "{name} drifted: {} mismatches, worst {:?}, backend {}",
+        report.over_threshold(),
+        report.worst(),
+        frame.backend(),
+    );
+}
+
+#[test]
+fn real_and_dashed_goldens_match() {
+    let ctx = make_context();
+    let real = render_scene(&ctx, &build_scene(false));
+    match_or_update_golden(&real, REAL_GOLDEN);
+    let dashed = render_scene(&ctx, &build_scene(true));
+    match_or_update_golden(&dashed, DASHED_GOLDEN);
+}
+
+#[test]
+fn construction_render_differs_from_real_render() {
+    let ctx = make_context();
+    let real = render_scene(&ctx, &build_scene(false));
+    let dashed = render_scene(&ctx, &build_scene(true));
+    assert_eq!(real.extent(), dashed.extent());
+    let Ok(report) = PixelDiff::compare(&real, dashed.rgba(), PixelDiffThreshold::EXACT) else {
+        panic!("PixelDiff rejected inputs");
+    };
+    assert!(
+        report.over_threshold() >= DISTINCT_MIN_PIXELS,
+        "construction styling not distinct from real: only {} pixels differ",
+        report.over_threshold(),
+    );
+}
diff --git a/crates/bone-render/tests/goldens/arc_circle_zoom100_256.png b/crates/bone-render/tests/goldens/arc_circle_zoom100_256.png
new file mode 100644
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diff --git a/crates/bone-render/tests/grid.rs b/crates/bone-render/tests/grid.rs
new file mode 100644
index 0000000..d56277e
--- /dev/null
+++ b/crates/bone-render/tests/grid.rs
@@ -0,0 +1,86 @@
+use std::path::PathBuf;
+
+use bone_render::{
+    Camera2, OffscreenContext, PixelDiff, PixelDiffThreshold, RenderError, SketchRenderer,
+    SketchScene, Style, ViewportExtent, ViewportPx, decode_png, encode_png,
+};
+
+const GOLDEN: &str = "tests/goldens/grid_empty_256.png";
+const UPDATE_ENV: &str = "BONE_UPDATE_GRID_GOLDEN";
+const DIFF_TOLERANCE: f64 = 16.0 / 255.0;
+
+fn extent(side: u32) -> ViewportExtent {
+    ViewportExtent::square(ViewportPx::new(side))
+}
+
+fn make_context(extent: ViewportExtent) -> OffscreenContext {
+    match pollster::block_on(OffscreenContext::new(extent)) {
+        Ok(ctx) => ctx,
+        Err(RenderError::NoAdapter(e)) => {
+            panic!(
+                "no wgpu adapter available, configure lavapipe or an iGPU for this test host: {e}"
+            );
+        }
+        Err(e) => panic!("offscreen context init failed: {e}"),
+    }
+}
+
+fn golden_path() -> PathBuf {
+    PathBuf::from(env!("CARGO_MANIFEST_DIR")).join(GOLDEN)
+}
+
+#[test]
+fn grid_empty_sketch_matches_golden() {
+    let size = extent(256);
+    let ctx = make_context(size);
+    let renderer = SketchRenderer::new(ctx.gpu(), ctx.color_format());
+    let scene = SketchScene::empty();
+    let camera = Camera2::new(size);
+    let style = Style::default();
+
+    let Ok(frame) = renderer.render(&ctx, &scene, camera, &style) else {
+        panic!("SketchRenderer::render failed");
+    };
+
+    let path = golden_path();
+
+    if std::env::var(UPDATE_ENV).is_ok() {
+        let Ok(bytes) = encode_png(&frame) else {
+            panic!("encode_png failed");
+        };
+        if let Some(parent) = path.parent() {
+            assert!(
+                std::fs::create_dir_all(parent).is_ok(),
+                "failed to create goldens dir"
+            );
+        }
+        assert!(
+            std::fs::write(&path, &bytes).is_ok(),
+            "failed to write golden {}",
+            path.display()
+        );
+        return;
+    }
+
+    let Ok(bytes) = std::fs::read(&path) else {
+        panic!(
+            "golden missing at {}: rerun with {UPDATE_ENV}=1 to generate",
+            path.display()
+        );
+    };
+    let Ok((golden_extent, golden_rgba)) = decode_png(&bytes) else {
+        panic!("failed to decode golden PNG");
+    };
+    assert_eq!(golden_extent, size, "golden extent drift");
+    let threshold = PixelDiffThreshold::new(DIFF_TOLERANCE);
+    let Ok(report) = PixelDiff::compare(&frame, &golden_rgba, threshold) else {
+        panic!("PixelDiff rejected inputs");
+    };
+    assert!(
+        report.is_clean(),
+        "grid render drifted from golden: {} mismatches, worst {:?}, backend {}",
+        report.over_threshold(),
+        report.worst(),
+        frame.backend(),
+    );
+}
diff --git a/crates/bone-render/tests/picker.rs b/crates/bone-render/tests/picker.rs
new file mode 100644
index 0000000..a5a2962
--- /dev/null
+++ b/crates/bone-render/tests/picker.rs
@@ -0,0 +1,266 @@
+use bone_document::{EditOutcome, Sketch, SketchEdit, SketchEntity};
+use bone_render::{
+    Camera2, OffscreenContext, PickQuery, PickedItem, RenderError, SketchRenderer, SketchScene,
+    Style, ViewportExtent, ViewportPx,
+};
+use bone_types::{Length, Point2, Point3, SketchEntityId, SketchPlaneBasis, Tolerance, UnitVec3};
+use uom::si::length::millimeter;
+
+const SIDE: u32 = 256;
+const CENTER_PX: f32 = 128.0;
+const PX_PER_MM: f32 = 10.0;
+
+fn extent() -> ViewportExtent {
+    ViewportExtent::square(ViewportPx::new(SIDE))
+}
+
+fn make_context() -> OffscreenContext {
+    match pollster::block_on(OffscreenContext::new(extent())) {
+        Ok(ctx) => ctx,
+        Err(RenderError::NoAdapter(e)) => panic!(
+            "no wgpu adapter available, configure lavapipe or an iGPU for this test host: {e}"
+        ),
+        Err(e) => panic!("offscreen context init failed: {e}"),
+    }
+}
+
+fn plane() -> SketchPlaneBasis {
+    let Ok(basis) = SketchPlaneBasis::new(
+        Point3::origin(),
+        UnitVec3::x_axis(),
+        UnitVec3::y_axis(),
+        Tolerance::new(1e-9),
+    ) else {
+        panic!("xy plane basis is orthogonal");
+    };
+    basis
+}
+
+fn add_point(s: Sketch, x: f64, y: f64) -> (Sketch, SketchEntityId) {
+    let Ok((next, EditOutcome::Entity(id))) = s.apply(SketchEdit::AddEntity(SketchEntity::point(
+        Point2::from_mm(x, y),
+    ))) else {
+        panic!("add point");
+    };
+    (next, id)
+}
+
+fn add_line(s: Sketch, a: SketchEntityId, b: SketchEntityId) -> (Sketch, SketchEntityId) {
+    let Ok((next, EditOutcome::Entity(id))) =
+        s.apply(SketchEdit::AddEntity(SketchEntity::line(a, b, false)))
+    else {
+        panic!("add line");
+    };
+    (next, id)
+}
+
+fn add_circle(s: Sketch, center: SketchEntityId, radius_mm: f64) -> (Sketch, SketchEntityId) {
+    let Ok((next, EditOutcome::Entity(id))) = s.apply(SketchEdit::AddEntity(SketchEntity::circle(
+        center,
+        Length::new::<millimeter>(radius_mm),
+        false,
+    ))) else {
+        panic!("add circle");
+    };
+    (next, id)
+}
+
+fn add_arc(
+    s: Sketch,
+    center: SketchEntityId,
+    start: SketchEntityId,
+    end: SketchEntityId,
+) -> (Sketch, SketchEntityId) {
+    let Ok((next, EditOutcome::Entity(id))) = s.apply(SketchEdit::AddEntity(SketchEntity::arc(
+        center, start, end, false,
+    ))) else {
+        panic!("add arc");
+    };
+    (next, id)
+}
+
+#[allow(clippy::cast_possible_truncation, clippy::cast_sign_loss)]
+fn world_to_px(x_mm: f32, y_mm: f32) -> PickQuery {
+    let x_px = (CENTER_PX + x_mm * PX_PER_MM).round() as u32;
+    let y_px = (CENTER_PX - y_mm * PX_PER_MM).round() as u32;
+    PickQuery::new(ViewportPx::new(x_px), ViewportPx::new(y_px))
+}
+
+struct Fixture {
+    scene: SketchScene,
+    probe_point: (PickQuery, SketchEntityId),
+    probe_line: (PickQuery, SketchEntityId),
+    probe_circle: (PickQuery, SketchEntityId),
+    probe_arc: (PickQuery, SketchEntityId),
+}
+
+fn build_fixture() -> Fixture {
+    let s = Sketch::new(plane());
+
+    let (s, line_a) = add_point(s, -6.0, 2.0);
+    let (s, line_b) = add_point(s, -6.0, -2.0);
+    let (s, line) = add_line(s, line_a, line_b);
+
+    let (s, standalone) = add_point(s, 6.0, 0.0);
+
+    let (s, circle_center) = add_point(s, 0.0, 4.0);
+    let (s, circle) = add_circle(s, circle_center, 2.0);
+
+    let (s, arc_center) = add_point(s, 0.0, -4.0);
+    let (s, arc_start) = add_point(s, 3.0, -4.0);
+    let (s, arc_end) = add_point(s, 0.0, -1.0);
+    let (s, arc) = add_arc(s, arc_center, arc_start, arc_end);
+
+    let Ok(scene) = SketchScene::extract(&s) else {
+        panic!("scene extract");
+    };
+
+    Fixture {
+        scene,
+        probe_point: (world_to_px(6.0, 0.0), standalone),
+        probe_line: (world_to_px(-6.0, 0.0), line),
+        probe_circle: (world_to_px(2.0, 4.0), circle),
+        probe_arc: (
+            world_to_px(
+                3.0 * core::f32::consts::FRAC_1_SQRT_2,
+                -4.0 + 3.0 * core::f32::consts::FRAC_1_SQRT_2,
+            ),
+            arc,
+        ),
+    }
+}
+
+#[test]
+fn each_entity_centroid_round_trips_to_pick_id() {
+    let ctx = make_context();
+    let renderer = SketchRenderer::new(ctx.gpu(), ctx.color_format());
+    let camera = Camera2::new(extent());
+    let style = Style::default();
+
+    let fx = build_fixture();
+    let Ok(_) = renderer.render(&ctx, &fx.scene, camera, &style) else {
+        panic!("SketchRenderer::render failed");
+    };
+    let Ok(index) = fx.scene.pick_index() else {
+        panic!("pick index build");
+    };
+    let picker = ctx.picker(index);
+
+    let cases = [
+        (
+            "point",
+            fx.probe_point.0,
+            PickedItem::Point(fx.probe_point.1),
+        ),
+        ("line", fx.probe_line.0, PickedItem::Line(fx.probe_line.1)),
+        (
+            "circle",
+            fx.probe_circle.0,
+            PickedItem::Circle(fx.probe_circle.1),
+        ),
+        ("arc", fx.probe_arc.0, PickedItem::Arc(fx.probe_arc.1)),
+    ];
+
+    let mismatches: Vec<String> = cases
+        .into_iter()
+        .filter_map(|(label, query, expected)| match picker.at(query) {
+            Ok(Some(got)) if got == expected => None,
+            Ok(got) => Some(format!(
+                "{label} pick at {query}: expected {expected:?}, got {got:?}"
+            )),
+            Err(e) => Some(format!("{label} pick at {query}: error {e}")),
+        })
+        .collect();
+    assert!(mismatches.is_empty(), "pick mismatches: {mismatches:#?}");
+}
+
+#[test]
+fn empty_space_decodes_to_none() {
+    let ctx = make_context();
+    let renderer = SketchRenderer::new(ctx.gpu(), ctx.color_format());
+    let camera = Camera2::new(extent());
+    let style = Style::default();
+
+    let fx = build_fixture();
+    let Ok(_) = renderer.render(&ctx, &fx.scene, camera, &style) else {
+        panic!("SketchRenderer::render failed");
+    };
+    let Ok(index) = fx.scene.pick_index() else {
+        panic!("pick index");
+    };
+    let picker = ctx.picker(index);
+
+    let empty = world_to_px(-8.0, 8.0);
+    let Ok(hit) = picker.at(empty) else {
+        panic!("picker at empty failed");
+    };
+    assert_eq!(hit, None, "empty pixel expected to decode to None");
+}
+
+#[test]
+fn repeated_render_picks_deterministic() {
+    let ctx = make_context();
+    let renderer = SketchRenderer::new(ctx.gpu(), ctx.color_format());
+    let camera = Camera2::new(extent());
+    let style = Style::default();
+
+    let fx = build_fixture();
+    let Ok(index) = fx.scene.pick_index() else {
+        panic!("pick index");
+    };
+
+    let collect_picks = || {
+        let Ok(_) = renderer.render(&ctx, &fx.scene, camera, &style) else {
+            panic!("render");
+        };
+        let pp = ctx.picker(index.clone());
+        [
+            fx.probe_point.0,
+            fx.probe_line.0,
+            fx.probe_circle.0,
+            fx.probe_arc.0,
+        ]
+        .into_iter()
+        .map(|q| {
+            let Ok(raw) = pp.raw_at(q) else {
+                panic!("raw_at failed");
+            };
+            raw.raw()
+        })
+        .collect::<Vec<_>>()
+    };
+
+    let first = collect_picks();
+    let second = collect_picks();
+    assert_eq!(
+        first, second,
+        "repeated render produced non-deterministic ids"
+    );
+}
+
+#[test]
+fn out_of_bounds_query_is_rejected() {
+    let ctx = make_context();
+    let Ok(index) = SketchScene::empty().pick_index() else {
+        panic!("pick index");
+    };
+    let picker = ctx.picker(index);
+    let cases = [
+        (
+            "x-axis edge",
+            PickQuery::new(ViewportPx::new(SIDE), ViewportPx::new(0)),
+        ),
+        (
+            "y-axis edge",
+            PickQuery::new(ViewportPx::new(0), ViewportPx::new(SIDE)),
+        ),
+    ];
+    let failures: Vec<String> = cases
+        .into_iter()
+        .filter_map(|(label, query)| match picker.at(query) {
+            Err(RenderError::PickOutOfBounds { .. }) => None,
+            other => Some(format!("{label}: expected OOB, got {other:?}")),
+        })
+        .collect();
+    assert!(failures.is_empty(), "OOB rejection failures: {failures:#?}");
+}
diff --git a/crates/bone-render/tests/rectangle.rs b/crates/bone-render/tests/rectangle.rs
new file mode 100644
index 0000000..10175ba
--- /dev/null
+++ b/crates/bone-render/tests/rectangle.rs
@@ -0,0 +1,129 @@
+use std::path::PathBuf;
+
+use bone_document::{EditOutcome, Sketch, SketchEdit, SketchEntity};
+use bone_render::{
+    Camera2, OffscreenContext, PixelDiff, PixelDiffThreshold, RenderError, SketchRenderer,
+    SketchScene, Style, ViewportExtent, ViewportPx, decode_png, encode_png,
+};
+use bone_types::{Point2, Point3, SketchEntityId, SketchPlaneBasis, Tolerance, UnitVec3};
+
+const GOLDEN: &str = "tests/goldens/rectangle_256.png";
+const UPDATE_ENV: &str = "BONE_UPDATE_RECTANGLE_GOLDEN";
+const DIFF_TOLERANCE: f64 = 16.0 / 255.0;
+
+fn extent(side: u32) -> ViewportExtent {
+    ViewportExtent::square(ViewportPx::new(side))
+}
+
+fn make_context(extent: ViewportExtent) -> OffscreenContext {
+    match pollster::block_on(OffscreenContext::new(extent)) {
+        Ok(ctx) => ctx,
+        Err(RenderError::NoAdapter(e)) => {
+            panic!(
+                "no wgpu adapter available, configure lavapipe or an iGPU for this test host: {e}"
+            );
+        }
+        Err(e) => panic!("offscreen context init failed: {e}"),
+    }
+}
+
+fn golden_path() -> PathBuf {
+    PathBuf::from(env!("CARGO_MANIFEST_DIR")).join(GOLDEN)
+}
+
+fn plane() -> SketchPlaneBasis {
+    let Ok(basis) = SketchPlaneBasis::new(
+        Point3::origin(),
+        UnitVec3::x_axis(),
+        UnitVec3::y_axis(),
+        Tolerance::new(1e-9),
+    ) else {
+        panic!("xy plane basis is orthogonal");
+    };
+    basis
+}
+
+fn add_point(s: Sketch, x: f64, y: f64) -> (Sketch, SketchEntityId) {
+    let Ok((next, EditOutcome::Entity(id))) = s.apply(SketchEdit::AddEntity(SketchEntity::point(
+        Point2::from_mm(x, y),
+    ))) else {
+        panic!("add point");
+    };
+    (next, id)
+}
+
+fn add_line(s: Sketch, a: SketchEntityId, b: SketchEntityId) -> Sketch {
+    let Ok((next, _)) = s.apply(SketchEdit::AddEntity(SketchEntity::line(a, b, false))) else {
+        panic!("add line");
+    };
+    next
+}
+
+fn rectangle_scene() -> SketchScene {
+    let s = Sketch::new(plane());
+    let (s, p0) = add_point(s, -8.0, -5.0);
+    let (s, p1) = add_point(s, 8.0, -5.0);
+    let (s, p2) = add_point(s, 8.0, 5.0);
+    let (s, p3) = add_point(s, -8.0, 5.0);
+    let s = add_line(s, p0, p1);
+    let s = add_line(s, p1, p2);
+    let s = add_line(s, p2, p3);
+    let s = add_line(s, p3, p0);
+    let Ok(scene) = SketchScene::extract(&s) else {
+        panic!("scene extract");
+    };
+    scene
+}
+
+#[test]
+fn rectangle_sketch_matches_golden() {
+    let size = extent(256);
+    let ctx = make_context(size);
+    let renderer = SketchRenderer::new(ctx.gpu(), ctx.color_format());
+    let scene = rectangle_scene();
+    let camera = Camera2::new(size);
+    let style = Style::default();
+
+    let Ok(frame) = renderer.render(&ctx, &scene, camera, &style) else {
+        panic!("SketchRenderer::render failed");
+    };
+
+    let path = golden_path();
+
+    if std::env::var(UPDATE_ENV).is_ok() {
+        let Ok(bytes) = encode_png(&frame) else {
+            panic!("encode_png failed");
+        };
+        if let Some(parent) = path.parent()
+            && let Err(e) = std::fs::create_dir_all(parent)
+        {
+            panic!("create goldens dir {}: {e}", parent.display());
+        }
+        if let Err(e) = std::fs::write(&path, &bytes) {
+            panic!("write golden {}: {e}", path.display());
+        }
+        return;
+    }
+
+    let Ok(bytes) = std::fs::read(&path) else {
+        panic!(
+            "golden missing at {}: rerun with {UPDATE_ENV}=1 to generate",
+            path.display()
+        );
+    };
+    let Ok((golden_extent, golden_rgba)) = decode_png(&bytes) else {
+        panic!("failed to decode golden PNG");
+    };
+    assert_eq!(golden_extent, size, "golden extent drift");
+    let threshold = PixelDiffThreshold::new(DIFF_TOLERANCE);
+    let Ok(report) = PixelDiff::compare(&frame, &golden_rgba, threshold) else {
+        panic!("PixelDiff rejected inputs");
+    };
+    assert!(
+        report.is_clean(),
+        "rectangle render drifted from golden: {} mismatches, worst {:?}, backend {}",
+        report.over_threshold(),
+        report.worst(),
+        frame.backend(),
+    );
+}
diff --git a/crates/bone-solver/Cargo.toml b/crates/bone-solver/Cargo.toml
new file mode 100644
index 0000000..4129225
--- /dev/null
+++ b/crates/bone-solver/Cargo.toml
@@ -0,0 +1,19 @@
+[package]
+name = "bone-solver"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+rust-version.workspace = true
+
+[dependencies]
+bone-types = { workspace = true }
+bone-kernel = { workspace = true }
+faer = { workspace = true }
+nalgebra = { workspace = true }
+thiserror = { workspace = true }
+
+[dev-dependencies]
+insta = { workspace = true }
+
+[lints]
+workspace = true
diff --git a/crates/bone-solver/src/dof.rs b/crates/bone-solver/src/dof.rs
new file mode 100644
index 0000000..11b0c0c
--- /dev/null
+++ b/crates/bone-solver/src/dof.rs
@@ -0,0 +1,329 @@
+use bone_types::{
+    DegreesOfFreedom, Parameter, ParameterIndex, ResidualIndex, SolverResidual, SolverSeed,
+};
+use nalgebra::{DMatrix, DVector};
+
+use crate::jacobian::{assemble_jacobian, evaluate_residuals};
+use crate::system::ConstraintSystem;
+
+#[derive(Clone, Debug, PartialEq)]
+pub struct DofReport {
+    dof: DegreesOfFreedom,
+    under_constrained: Vec<ParameterIndex>,
+    over_constrained: Vec<ResidualIndex>,
+    redundant_consistent: Vec<ResidualIndex>,
+}
+
+impl DofReport {
+    #[must_use]
+    pub fn dof(&self) -> DegreesOfFreedom {
+        self.dof
+    }
+
+    #[must_use]
+    pub fn under_constrained(&self) -> &[ParameterIndex] {
+        &self.under_constrained
+    }
+
+    #[must_use]
+    pub fn over_constrained(&self) -> &[ResidualIndex] {
+        &self.over_constrained
+    }
+
+    #[must_use]
+    pub fn redundant_consistent(&self) -> &[ResidualIndex] {
+        &self.redundant_consistent
+    }
+
+    #[must_use]
+    pub fn display(&self) -> String {
+        format!(
+            "dof={} under=[{}] over=[{}] redundant=[{}]",
+            self.dof.value(),
+            self.under_constrained
+                .iter()
+                .map(ToString::to_string)
+                .collect::<Vec<_>>()
+                .join(","),
+            self.over_constrained
+                .iter()
+                .map(ToString::to_string)
+                .collect::<Vec<_>>()
+                .join(","),
+            self.redundant_consistent
+                .iter()
+                .map(ToString::to_string)
+                .collect::<Vec<_>>()
+                .join(","),
+        )
+    }
+}
+
+#[derive(Copy, Clone, Debug)]
+pub struct DofConfig {
+    pub seed: SolverSeed,
+    pub witness_scale: f64,
+    pub rank_tolerance: f64,
+    pub residual_tolerance: SolverResidual,
+}
+
+impl DofConfig {
+    pub const DEFAULT: Self = Self {
+        seed: SolverSeed::DEFAULT,
+        witness_scale: 0.5,
+        rank_tolerance: 1e-9,
+        residual_tolerance: SolverResidual::new(1e-7),
+    };
+}
+
+#[must_use]
+pub fn analyze_dof(system: &ConstraintSystem, cfg: DofConfig) -> DofReport {
+    analyze_dof_at(system, system.parameters(), cfg)
+}
+
+#[must_use]
+pub fn analyze_dof_at(
+    system: &ConstraintSystem,
+    params: &[Parameter],
+    cfg: DofConfig,
+) -> DofReport {
+    let current_params: Vec<f64> = params.iter().map(|p| p.value()).collect();
+    let n = system.parameter_count();
+    let m = system.row_count();
+    if m == 0 || n == 0 {
+        let Ok(dof_n) = u32::try_from(n) else {
+            unreachable!("parameter count fits in u32 via ParameterIndex")
+        };
+        return DofReport {
+            dof: DegreesOfFreedom::new(dof_n),
+            under_constrained: (0..dof_n).map(ParameterIndex::new).collect(),
+            over_constrained: Vec::new(),
+            redundant_consistent: Vec::new(),
+        };
+    }
+    let witness_params = perturb(&current_params, cfg.seed, cfg.witness_scale);
+    let jacobian = assemble_jacobian(system, &witness_params);
+    let qr_cols = col_piv_qr(&jacobian, cfg.rank_tolerance);
+    let transpose = jacobian.transpose();
+    let qr_rows = col_piv_qr(&transpose, cfg.rank_tolerance);
+    let rank = qr_cols.rank;
+    let Ok(dof_n) = u32::try_from(n.saturating_sub(rank)) else {
+        unreachable!("rank <= n <= ParameterIndex::MAX")
+    };
+    let under_constrained: Vec<ParameterIndex> = qr_cols.col_perm[rank..]
+        .iter()
+        .map(|&c| {
+            let Ok(cv) = u32::try_from(c) else {
+                unreachable!("pivoted column index fits in u32 via ParameterIndex")
+            };
+            ParameterIndex::new(cv)
+        })
+        .collect();
+    let mut under_sorted = under_constrained;
+    under_sorted.sort_by_key(|p| p.value());
+
+    let residuals_at_current = evaluate_residuals(system, &current_params);
+    let tol = cfg.residual_tolerance.value();
+    let (over_constrained, redundant_consistent): (Vec<ResidualIndex>, Vec<ResidualIndex>) =
+        qr_rows.col_perm[rank..]
+            .iter()
+            .map(|&row| {
+                let Ok(rv) = u32::try_from(row) else {
+                    unreachable!("dependent row index fits in u32 via ResidualIndex")
+                };
+                let value = residuals_at_current[row];
+                (ResidualIndex::new(rv), value.abs() <= tol)
+            })
+            .fold(
+                (Vec::new(), Vec::new()),
+                |(mut over, mut red), (idx, consistent)| {
+                    if consistent {
+                        red.push(idx);
+                    } else {
+                        over.push(idx);
+                    }
+                    (over, red)
+                },
+            );
+    let mut over_sorted = over_constrained;
+    let mut red_sorted = redundant_consistent;
+    over_sorted.sort_by_key(|r| r.value());
+    red_sorted.sort_by_key(|r| r.value());
+    DofReport {
+        dof: DegreesOfFreedom::new(dof_n),
+        under_constrained: under_sorted,
+        over_constrained: over_sorted,
+        redundant_consistent: red_sorted,
+    }
+}
+
+fn perturb(params: &[f64], seed: SolverSeed, scale: f64) -> Vec<f64> {
+    params
+        .iter()
+        .scan(WitnessRng::new(seed), |rng, &v| {
+            let (u, next) = rng.step();
+            *rng = next;
+            Some(v + (u - 0.5) * scale)
+        })
+        .collect()
+}
+
+#[derive(Copy, Clone, Debug)]
+struct WitnessRng {
+    state: u64,
+}
+
+impl WitnessRng {
+    fn new(seed: SolverSeed) -> Self {
+        Self {
+            state: seed.value(),
+        }
+    }
+
+    fn step(self) -> (f64, Self) {
+        let state = self.state.wrapping_add(0x9E37_79B9_7F4A_7C15);
+        let mixed = mix64(state);
+        let mantissa = mixed & 0x000F_FFFF_FFFF_FFFF;
+        let uniform = f64::from_bits(0x3FF0_0000_0000_0000 | mantissa) - 1.0;
+        (uniform, Self { state })
+    }
+}
+
+fn mix64(x: u64) -> u64 {
+    let a = (x ^ (x >> 30)).wrapping_mul(0xBF58_476D_1CE4_E5B9);
+    let b = (a ^ (a >> 27)).wrapping_mul(0x94D0_49BB_1331_11EB);
+    b ^ (b >> 31)
+}
+
+#[derive(Clone, Debug)]
+struct ColPivQr {
+    rank: usize,
+    col_perm: Vec<usize>,
+}
+
+fn col_piv_qr(input: &DMatrix<f64>, tol: f64) -> ColPivQr {
+    let m = input.nrows();
+    let n = input.ncols();
+    let initial_norms: Vec<f64> = (0..n).map(|j| input.column(j).norm_squared()).collect();
+    let initial = QrState {
+        a: input.clone(),
+        col_perm: (0..n).collect(),
+        col_norms: initial_norms,
+        rank: 0,
+    };
+    let max_steps = m.min(n);
+    let final_state = (0..max_steps).try_fold(initial, |state, k| state.step(k, tol));
+    let state = match final_state {
+        Ok(s) | Err(s) => s,
+    };
+    ColPivQr {
+        rank: state.rank,
+        col_perm: state.col_perm,
+    }
+}
+
+#[derive(Clone, Debug)]
+struct QrState {
+    a: DMatrix<f64>,
+    col_perm: Vec<usize>,
+    col_norms: Vec<f64>,
+    rank: usize,
+}
+
+impl QrState {
+    fn step(self, pivot_row: usize, tol: f64) -> Result<Self, Self> {
+        let QrState {
+            a: matrix,
+            mut col_perm,
+            mut col_norms,
+            rank,
+        } = self;
+        let rows = matrix.nrows();
+        let cols = matrix.ncols();
+        let (best_col, best_norm2) = col_norms.iter().enumerate().skip(pivot_row).fold(
+            (pivot_row, f64::NEG_INFINITY),
+            |(best, best_v), (j, &value)| {
+                if value > best_v {
+                    (j, value)
+                } else {
+                    (best, best_v)
+                }
+            },
+        );
+        if best_norm2 <= tol * tol {
+            return Err(QrState {
+                a: matrix,
+                col_perm,
+                col_norms,
+                rank,
+            });
+        }
+        let mut matrix = matrix;
+        if best_col != pivot_row {
+            matrix.swap_columns(pivot_row, best_col);
+            col_perm.swap(pivot_row, best_col);
+            col_norms.swap(pivot_row, best_col);
+        }
+        let tail = rows - pivot_row;
+        let column: DVector<f64> = (0..tail)
+            .map(|i| matrix[(pivot_row + i, pivot_row)])
+            .collect::<Vec<_>>()
+            .into();
+        let column_norm = column.norm();
+        if column_norm <= tol {
+            return Err(QrState {
+                a: matrix,
+                col_perm,
+                col_norms,
+                rank,
+            });
+        }
+        let alpha = if column[0] >= 0.0 {
+            -column_norm
+        } else {
+            column_norm
+        };
+        let reflector: DVector<f64> = column
+            .iter()
+            .enumerate()
+            .map(|(idx, &value)| if idx == 0 { value - alpha } else { value })
+            .collect::<Vec<_>>()
+            .into();
+        let reflector_dot = reflector.dot(&reflector);
+        let applied = if reflector_dot > 0.0 {
+            let beta = 2.0 / reflector_dot;
+            (pivot_row..cols).fold(matrix, |mut mat, col| {
+                let projection: f64 = (0..tail)
+                    .map(|i| reflector[i] * mat[(pivot_row + i, col)])
+                    .sum();
+                let coef = beta * projection;
+                (0..tail).for_each(|i| mat[(pivot_row + i, col)] -= coef * reflector[i]);
+                mat
+            })
+        } else {
+            matrix
+        };
+        let new_norms: Vec<f64> = col_norms
+            .iter()
+            .enumerate()
+            .map(|(col, &prev)| {
+                if col <= pivot_row || pivot_row + 1 >= rows {
+                    prev
+                } else {
+                    (0..(tail - 1))
+                        .map(|i| {
+                            let entry = applied[(pivot_row + 1 + i, col)];
+                            entry * entry
+                        })
+                        .sum::<f64>()
+                }
+            })
+            .collect();
+        Ok(QrState {
+            a: applied,
+            col_perm,
+            col_norms: new_norms,
+            rank: rank + 1,
+        })
+    }
+}
diff --git a/crates/bone-solver/src/error.rs b/crates/bone-solver/src/error.rs
new file mode 100644
index 0000000..86ee91c
--- /dev/null
+++ b/crates/bone-solver/src/error.rs
@@ -0,0 +1,17 @@
+use core::time::Duration;
+
+use bone_types::{ParameterIndex, SketchItemId, SolverResidual};
+
+#[derive(Debug, Clone, PartialEq, thiserror::Error)]
+pub enum SolverError {
+    #[error("no solution found (last residual = {last})")]
+    NoSolutionFound { last: SolverResidual },
+    #[error("sketch is over-defined ({} conflicting items)", conflicts.len())]
+    OverDefined { conflicts: Vec<SketchItemId> },
+    #[error("invalid solution found: jacobian singular at parameter {at}")]
+    InvalidSolutionFound { at: ParameterIndex },
+    #[error("solver budget exhausted after {elapsed:?}")]
+    Budget { elapsed: Duration },
+}
+
+pub type Result<T, E = SolverError> = core::result::Result<T, E>;
diff --git a/crates/bone-solver/src/graph.rs b/crates/bone-solver/src/graph.rs
new file mode 100644
index 0000000..ccbaff8
--- /dev/null
+++ b/crates/bone-solver/src/graph.rs
@@ -0,0 +1,146 @@
+use std::collections::BTreeMap;
+
+use bone_types::{ParameterIndex, ParentIndex, ResidualIndex};
+
+use crate::system::ConstraintSystem;
+
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub struct Component {
+    parameters: Vec<ParameterIndex>,
+    residual_parents: Vec<ParentIndex>,
+}
+
+impl Component {
+    #[must_use]
+    pub fn parameters(&self) -> &[ParameterIndex] {
+        &self.parameters
+    }
+
+    #[must_use]
+    pub fn residual_parents(&self) -> &[ParentIndex] {
+        &self.residual_parents
+    }
+
+    #[must_use]
+    pub fn residual_rows(&self, system: &ConstraintSystem) -> Vec<ResidualIndex> {
+        let offsets = system.row_offsets();
+        self.residual_parents
+            .iter()
+            .flat_map(|&parent| {
+                let start = offsets[parent.as_usize()].value();
+                (0..system.residual_at_parent(parent).rows()).map(move |o| {
+                    let Ok(ov) = u32::try_from(o) else {
+                        unreachable!("residual row count is a small constant")
+                    };
+                    ResidualIndex::new(start + ov)
+                })
+            })
+            .collect()
+    }
+}
+
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub struct Decomposition {
+    components: Vec<Component>,
+}
+
+impl Decomposition {
+    #[must_use]
+    pub fn components(&self) -> &[Component] {
+        &self.components
+    }
+}
+
+#[must_use]
+pub fn decompose(system: &ConstraintSystem) -> Decomposition {
+    let n = system.parameter_count();
+    let dsu_final = system.residuals().iter().fold(Dsu::new(n), |dsu, r| {
+        let ps = r.parameters();
+        match ps.split_first() {
+            None => dsu,
+            Some((&first, rest)) => rest
+                .iter()
+                .fold(dsu, |acc, &p| acc.union(first.as_usize(), p.as_usize())),
+        }
+    });
+    let roots: Vec<usize> = (0..n).map(|p| dsu_final.find(p)).collect();
+    let param_groups: BTreeMap<usize, Vec<ParameterIndex>> =
+        (0..n).fold(BTreeMap::new(), |mut acc, p| {
+            let Ok(pv) = u32::try_from(p) else {
+                unreachable!("parameter count stays within ParameterIndex range")
+            };
+            acc.entry(roots[p])
+                .or_default()
+                .push(ParameterIndex::new(pv));
+            acc
+        });
+    let parent_groups: BTreeMap<usize, Vec<ParentIndex>> = system
+        .residuals()
+        .iter()
+        .enumerate()
+        .fold(BTreeMap::new(), |mut acc, (pi, r)| {
+            if let Some(&first) = r.parameters().first() {
+                let Ok(piv) = u32::try_from(pi) else {
+                    unreachable!("parent count fits in u32 via ResidualIndex")
+                };
+                acc.entry(roots[first.as_usize()])
+                    .or_default()
+                    .push(ParentIndex::new(piv));
+            }
+            acc
+        });
+    let mut keyed: Vec<(usize, Component)> = param_groups
+        .into_iter()
+        .map(|(root, parameters)| {
+            let residual_parents = parent_groups.get(&root).cloned().unwrap_or_default();
+            let min_param = parameters
+                .first()
+                .copied()
+                .map_or(usize::MAX, ParameterIndex::as_usize);
+            (
+                min_param,
+                Component {
+                    parameters,
+                    residual_parents,
+                },
+            )
+        })
+        .collect();
+    keyed.sort_by_key(|(min_param, _)| *min_param);
+    Decomposition {
+        components: keyed.into_iter().map(|(_, c)| c).collect(),
+    }
+}
+
+#[derive(Clone, Debug)]
+struct Dsu {
+    parent: Vec<usize>,
+}
+
+impl Dsu {
+    fn new(n: usize) -> Self {
+        Self {
+            parent: (0..n).collect(),
+        }
+    }
+
+    fn find(&self, x: usize) -> usize {
+        std::iter::successors(Some(x), |&i| {
+            let p = self.parent[i];
+            (p != i).then_some(p)
+        })
+        .fold(x, |_, i| i)
+    }
+
+    fn union(self, a: usize, b: usize) -> Self {
+        let ra = self.find(a);
+        let rb = self.find(b);
+        if ra == rb {
+            return self;
+        }
+        let (lo, hi) = if ra < rb { (ra, rb) } else { (rb, ra) };
+        let Self { mut parent } = self;
+        parent[hi] = lo;
+        Self { parent }
+    }
+}
diff --git a/crates/bone-solver/src/jacobian.rs b/crates/bone-solver/src/jacobian.rs
new file mode 100644
index 0000000..c9c331f
--- /dev/null
+++ b/crates/bone-solver/src/jacobian.rs
@@ -0,0 +1,111 @@
+use bone_types::ParameterIndex;
+use nalgebra::DMatrix;
+
+use crate::residual::Triplet;
+use crate::system::ConstraintSystem;
+
+#[derive(Clone, Debug, PartialEq)]
+pub struct SparsityPattern {
+    rows: usize,
+    cols: usize,
+    entries: Vec<(usize, ParameterIndex)>,
+}
+
+impl SparsityPattern {
+    #[must_use]
+    pub fn rows(&self) -> usize {
+        self.rows
+    }
+
+    #[must_use]
+    pub fn cols(&self) -> usize {
+        self.cols
+    }
+
+    #[must_use]
+    pub fn entries(&self) -> &[(usize, ParameterIndex)] {
+        &self.entries
+    }
+
+    #[must_use]
+    pub fn display(&self) -> String {
+        let mut grid = vec![vec![false; self.cols]; self.rows];
+        self.entries.iter().for_each(|(r, c)| {
+            grid[*r][c.as_usize()] = true;
+        });
+        grid.into_iter()
+            .map(|row| {
+                row.into_iter()
+                    .map(|filled| if filled { 'X' } else { '.' })
+                    .collect::<String>()
+            })
+            .collect::<Vec<_>>()
+            .join("\n")
+    }
+}
+
+#[must_use]
+pub fn evaluate_residuals(system: &ConstraintSystem, params: &[f64]) -> Vec<f64> {
+    let mut out = vec![0.0; system.row_count()];
+    let offsets = system.row_offsets();
+    system
+        .residuals()
+        .iter()
+        .zip(offsets.iter().copied())
+        .for_each(|(r, start)| {
+            let len = r.rows();
+            let begin = start.as_usize();
+            r.evaluate(params, &mut out[begin..begin + len]);
+        });
+    out
+}
+
+#[must_use]
+pub fn assemble_jacobian(system: &ConstraintSystem, params: &[f64]) -> DMatrix<f64> {
+    let rows = system.row_count();
+    let cols = system.parameter_count();
+    let triplets = collect_triplets(system, params);
+    triplets.into_iter().fold(
+        DMatrix::<f64>::zeros(rows, cols),
+        |mut mat, (row, col, val)| {
+            mat[(row.as_usize(), col.as_usize())] += val;
+            mat
+        },
+    )
+}
+
+#[must_use]
+pub fn sparsity_pattern(system: &ConstraintSystem) -> SparsityPattern {
+    let dummy: Vec<f64> = std::iter::successors(Some(1.0_f64), |v| Some(v + 0.25))
+        .take(system.parameter_count())
+        .collect();
+    let triplets = collect_triplets(system, &dummy);
+    let mut entries: Vec<(usize, ParameterIndex)> = triplets
+        .into_iter()
+        .map(|(r, c, _)| (r.as_usize(), c))
+        .collect();
+    entries.sort_unstable_by(|a, b| a.0.cmp(&b.0).then(a.1.value().cmp(&b.1.value())));
+    entries.dedup();
+    SparsityPattern {
+        rows: system.row_count(),
+        cols: system.parameter_count(),
+        entries,
+    }
+}
+
+#[must_use]
+pub fn jacobian_triplets(system: &ConstraintSystem, params: &[f64]) -> Vec<Triplet> {
+    collect_triplets(system, params)
+}
+
+fn collect_triplets(system: &ConstraintSystem, params: &[f64]) -> Vec<Triplet> {
+    let offsets = system.row_offsets();
+    system
+        .residuals()
+        .iter()
+        .zip(offsets)
+        .fold(Vec::new(), |mut triplets, (r, start)| {
+            r.jacobian(params, start, &mut triplets);
+            triplets
+        })
+}
diff --git a/crates/bone-solver/src/lib.rs b/crates/bone-solver/src/lib.rs
new file mode 100644
index 0000000..8ddbe72
--- /dev/null
+++ b/crates/bone-solver/src/lib.rs
@@ -0,0 +1,21 @@
+pub mod dof;
+pub mod error;
+pub mod graph;
+pub mod jacobian;
+pub mod mus;
+pub mod newton;
+pub mod residual;
+pub mod system;
+pub mod util;
+
+pub use dof::{DofConfig, DofReport, analyze_dof, analyze_dof_at};
+pub use error::{Result, SolverError};
+pub use graph::{Component, Decomposition, decompose};
+pub use jacobian::{
+    SparsityPattern, assemble_jacobian, evaluate_residuals, jacobian_triplets, sparsity_pattern,
+};
+pub use mus::minimal_unsatisfiable_subset;
+pub use newton::{NewtonConfig, residual_norm, solve_newton, solve_newton_decomposed};
+pub use residual::{CurveRadius, LineHandle, PointHandle, Residual, Triplet};
+pub use system::{ConstraintSystem, Subsystem};
+pub use util::dedup_preserving_order;
diff --git a/crates/bone-solver/src/mus.rs b/crates/bone-solver/src/mus.rs
new file mode 100644
index 0000000..3ae740a
--- /dev/null
+++ b/crates/bone-solver/src/mus.rs
@@ -0,0 +1,125 @@
+use std::collections::BTreeSet;
+
+use bone_types::{ParentIndex, ResidualIndex};
+
+use crate::dof::{DofConfig, analyze_dof_at};
+use crate::graph::decompose;
+use crate::newton::{NewtonConfig, solve_newton_decomposed};
+use crate::system::ConstraintSystem;
+use crate::util::dedup_preserving_order;
+
+#[must_use]
+pub fn minimal_unsatisfiable_subset(
+    system: &ConstraintSystem,
+    over_flagged: &[ResidualIndex],
+    cfg: DofConfig,
+) -> Vec<ResidualIndex> {
+    let candidate_parents: Vec<ParentIndex> = dedup_preserving_order(
+        over_flagged
+            .iter()
+            .copied()
+            .map(|row| system.parent_of_row(row)),
+    );
+    if candidate_parents.is_empty() {
+        return Vec::new();
+    }
+    let solver_cfg = NewtonConfig::DEFAULT;
+    let kept_parents = candidate_parents
+        .iter()
+        .copied()
+        .fold(
+            (BTreeSet::<ParentIndex>::new(), Vec::<ParentIndex>::new()),
+            |(mut excluded, mut kept), parent| {
+                let mut trial = excluded.clone();
+                trial.insert(parent);
+                if reduced_is_over(system, &trial, solver_cfg, cfg) {
+                    excluded.insert(parent);
+                } else {
+                    kept.push(parent);
+                }
+                (excluded, kept)
+            },
+        )
+        .1;
+    kept_parents
+        .into_iter()
+        .flat_map(|p| system.rows_of_parent(p))
+        .filter(|row| over_flagged.contains(row))
+        .collect()
+}
+
+fn reduced_is_over(
+    system: &ConstraintSystem,
+    exclude: &BTreeSet<ParentIndex>,
+    newton_cfg: NewtonConfig,
+    dof_cfg: DofConfig,
+) -> bool {
+    let sub = system.without_parents(exclude);
+    if sub.row_count() == 0 {
+        return false;
+    }
+    let decomposition = decompose(&sub);
+    match solve_newton_decomposed(&sub, &decomposition, newton_cfg) {
+        Ok(solved) => {
+            let report = analyze_dof_at(&sub, &solved, dof_cfg);
+            !report.over_constrained().is_empty()
+        }
+        Err(_) => true,
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::residual::Residual;
+    use bone_types::{Parameter, ParameterIndex};
+
+    fn p(i: u32) -> ParameterIndex {
+        ParameterIndex::new(i)
+    }
+
+    fn pin(at: u32, target: f64) -> Residual {
+        Residual::Pin {
+            param: p(at),
+            target,
+        }
+    }
+
+    fn over_flagged_after_solve(system: &ConstraintSystem) -> Vec<ResidualIndex> {
+        let decomposition = decompose(system);
+        let solved = solve_newton_decomposed(system, &decomposition, NewtonConfig::DEFAULT)
+            .unwrap_or_else(|_| system.parameters().to_vec());
+        analyze_dof_at(system, &solved, DofConfig::DEFAULT)
+            .over_constrained()
+            .to_vec()
+    }
+
+    #[test]
+    fn three_conflicting_pins_isolate_the_odd_one_out() {
+        let system = ConstraintSystem::new(
+            vec![Parameter::new(0.0)],
+            vec![pin(0, 1.0), pin(0, 2.0), pin(0, 1.0)],
+        );
+        let over = over_flagged_after_solve(&system);
+        assert!(
+            !over.is_empty(),
+            "fixture must produce at least one over row"
+        );
+        let mus = minimal_unsatisfiable_subset(&system, &over, DofConfig::DEFAULT);
+        assert!(
+            !mus.is_empty(),
+            "MUS must keep at least one essential row, got empty",
+        );
+        assert!(
+            mus.len() <= over.len(),
+            "MUS must be a subset of over-flagged rows: mus={mus:?} over={over:?}",
+        );
+    }
+
+    #[test]
+    fn empty_over_flagged_yields_empty_mus() {
+        let system = ConstraintSystem::new(vec![Parameter::new(0.0)], vec![pin(0, 0.0)]);
+        let mus = minimal_unsatisfiable_subset(&system, &[], DofConfig::DEFAULT);
+        assert!(mus.is_empty());
+    }
+}
diff --git a/crates/bone-solver/src/newton.rs b/crates/bone-solver/src/newton.rs
new file mode 100644
index 0000000..aba6227
--- /dev/null
+++ b/crates/bone-solver/src/newton.rs
@@ -0,0 +1,378 @@
+use std::collections::BTreeMap;
+use std::time::{Duration, Instant};
+
+use bone_types::{
+    BudgetCeiling, NewtonDamping, NewtonStepTolerance, Parameter, ParameterIndex, ResidualIndex,
+    SolverResidual,
+};
+use faer::Mat;
+use faer::linalg::solvers::Solve;
+use faer::sparse::{SparseColMat, Triplet as FaerTriplet};
+use nalgebra::DVector;
+
+use crate::error::{Result, SolverError};
+use crate::graph::Decomposition;
+use crate::jacobian::{evaluate_residuals, jacobian_triplets};
+use crate::residual::Triplet;
+use crate::system::ConstraintSystem;
+
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub struct NewtonConfig {
+    pub max_iterations: u32,
+    pub residual_tolerance: SolverResidual,
+    pub step_tolerance: NewtonStepTolerance,
+    pub line_search_shrinks: u32,
+    pub damping: NewtonDamping,
+    pub budget: Option<BudgetCeiling>,
+}
+
+impl NewtonConfig {
+    pub const DEFAULT: Self = Self {
+        max_iterations: 64,
+        residual_tolerance: SolverResidual::new(1e-10),
+        step_tolerance: NewtonStepTolerance::DEFAULT,
+        line_search_shrinks: 20,
+        damping: NewtonDamping::DEFAULT,
+        budget: None,
+    };
+}
+
+#[must_use]
+pub fn residual_norm(residuals: &[f64]) -> SolverResidual {
+    let sum: f64 = residuals.iter().map(|r| r * r).sum();
+    SolverResidual::new(sum.sqrt())
+}
+
+#[derive(Copy, Clone, Debug)]
+struct Stopwatch {
+    started_at: Instant,
+    budget: Option<BudgetCeiling>,
+}
+
+impl Stopwatch {
+    fn start(budget: Option<BudgetCeiling>) -> Self {
+        Self {
+            started_at: Instant::now(),
+            budget,
+        }
+    }
+
+    fn check(self) -> Result<(), SolverError> {
+        let Some(budget) = self.budget else {
+            return Ok(());
+        };
+        let elapsed = self.started_at.elapsed();
+        if elapsed >= budget.duration() {
+            Err(SolverError::Budget { elapsed })
+        } else {
+            Ok(())
+        }
+    }
+}
+
+#[must_use]
+fn remaining_budget(outer: BudgetCeiling, elapsed: Duration) -> BudgetCeiling {
+    BudgetCeiling::new(outer.duration().saturating_sub(elapsed))
+}
+
+struct Iterate {
+    params: Vec<f64>,
+    residuals: Vec<f64>,
+    norm: SolverResidual,
+}
+
+impl Iterate {
+    fn evaluate(system: &ConstraintSystem, params: Vec<f64>) -> Self {
+        let residuals = evaluate_residuals(system, &params);
+        let norm = residual_norm(&residuals);
+        Self {
+            params,
+            residuals,
+            norm,
+        }
+    }
+}
+
+pub fn solve_newton(system: &ConstraintSystem, cfg: NewtonConfig) -> Result<Vec<Parameter>> {
+    let stopwatch = Stopwatch::start(cfg.budget);
+    let seed: Vec<f64> = system.parameters().iter().map(|p| p.value()).collect();
+    converge(
+        system,
+        cfg,
+        &Iterate::evaluate(system, seed),
+        cfg.max_iterations,
+        stopwatch,
+    )
+}
+
+pub fn solve_newton_decomposed(
+    system: &ConstraintSystem,
+    decomposition: &Decomposition,
+    cfg: NewtonConfig,
+) -> Result<Vec<Parameter>> {
+    let stopwatch = Stopwatch::start(cfg.budget);
+    let seed: Vec<Parameter> = system.parameters().to_vec();
+    decomposition
+        .components()
+        .iter()
+        .try_fold(seed, |acc, component| {
+            stopwatch.check()?;
+            let sub = system.subsystem(component);
+            if sub.system().row_count() == 0 {
+                return Ok(acc);
+            }
+            let sub_cfg = NewtonConfig {
+                budget: cfg
+                    .budget
+                    .map(|b| remaining_budget(b, stopwatch.started_at.elapsed())),
+                ..cfg
+            };
+            let solved = solve_newton(sub.system(), sub_cfg)
+                .map_err(|err| lift_subsystem_error(err, sub.param_map()))?;
+            Ok(splice(acc, sub.param_map(), &solved))
+        })
+}
+
+fn lift_subsystem_error(err: SolverError, param_map: &[ParameterIndex]) -> SolverError {
+    match err {
+        SolverError::InvalidSolutionFound { at } => SolverError::InvalidSolutionFound {
+            at: param_map[at.as_usize()],
+        },
+        other => other,
+    }
+}
+
+fn splice(
+    mut whole: Vec<Parameter>,
+    param_map: &[bone_types::ParameterIndex],
+    sub: &[Parameter],
+) -> Vec<Parameter> {
+    param_map
+        .iter()
+        .zip(sub.iter().copied())
+        .for_each(|(orig, value)| {
+            whole[orig.as_usize()] = value;
+        });
+    whole
+}
+
+fn converge(
+    system: &ConstraintSystem,
+    cfg: NewtonConfig,
+    state: &Iterate,
+    remaining: u32,
+    stopwatch: Stopwatch,
+) -> Result<Vec<Parameter>> {
+    if state.norm.value() < cfg.residual_tolerance.value() {
+        return Ok(to_parameters(&state.params));
+    }
+    if remaining == 0 {
+        return Err(SolverError::NoSolutionFound { last: state.norm });
+    }
+    stopwatch.check()?;
+    let triplets = jacobian_triplets(system, &state.params);
+    let step = least_squares_step(
+        system.parameter_count(),
+        &triplets,
+        &state.residuals,
+        cfg.damping,
+    )?;
+    if step.norm() < cfg.step_tolerance.value() {
+        return Ok(to_parameters(&state.params));
+    }
+    match line_search(
+        system,
+        &state.params,
+        &step,
+        state.norm,
+        cfg.line_search_shrinks,
+    ) {
+        Some(next) => converge(system, cfg, &next, remaining - 1, stopwatch),
+        None => Err(SolverError::NoSolutionFound { last: state.norm }),
+    }
+}
+
+fn to_parameters(values: &[f64]) -> Vec<Parameter> {
+    values.iter().copied().map(Parameter::new).collect()
+}
+
+fn least_squares_step(
+    params_len: usize,
+    triplets: &[Triplet],
+    residuals: &[f64],
+    damping: NewtonDamping,
+) -> Result<DVector<f64>> {
+    let (jtj_triplets, jtr) =
+        assemble_normal_equations(params_len, triplets, residuals, damping.value());
+    let Ok(jtj) =
+        SparseColMat::<usize, f64>::try_new_from_triplets(params_len, params_len, &jtj_triplets)
+    else {
+        unreachable!("normal-equations triplets are built with in-bounds indices")
+    };
+    let lu = jtj.sp_lu().map_err(|err| match err {
+        faer::sparse::linalg::LuError::SymbolicSingular { index } => {
+            let Ok(at) = u32::try_from(index) else {
+                unreachable!("J^T J column count fits in u32 via ParameterIndex")
+            };
+            SolverError::InvalidSolutionFound {
+                at: ParameterIndex::new(at),
+            }
+        }
+        faer::sparse::linalg::LuError::Generic(_) => {
+            unreachable!("faer sp_lu returns Generic only on malformed input")
+        }
+    })?;
+    let mut rhs = Mat::<f64>::from_fn(params_len, 1, |i, _| -jtr[i]);
+    lu.solve_in_place(rhs.as_mut());
+    Ok(DVector::from_iterator(
+        params_len,
+        (0..params_len).map(|i| rhs[(i, 0)]),
+    ))
+}
+
+fn assemble_normal_equations(
+    params_len: usize,
+    triplets: &[Triplet],
+    residuals: &[f64],
+    damping: f64,
+) -> (Vec<FaerTriplet<usize, usize, f64>>, Vec<f64>) {
+    let rows: BTreeMap<ResidualIndex, Vec<(usize, f64)>> =
+        triplets
+            .iter()
+            .fold(BTreeMap::new(), |mut acc, (row, col, val)| {
+                acc.entry(*row).or_default().push((col.as_usize(), *val));
+                acc
+            });
+    let jtj: BTreeMap<(usize, usize), f64> = rows
+        .values()
+        .flat_map(|entries| {
+            entries
+                .iter()
+                .flat_map(move |(i, vi)| entries.iter().map(move |(j, vj)| ((*i, *j), vi * vj)))
+        })
+        .fold(BTreeMap::new(), |mut acc, (key, val)| {
+            *acc.entry(key).or_insert(0.0) += val;
+            acc
+        });
+    let damped = (0..params_len).fold(jtj, |mut acc, i| {
+        *acc.entry((i, i)).or_insert(0.0) += damping;
+        acc
+    });
+    let jtj_triplets: Vec<FaerTriplet<usize, usize, f64>> = damped
+        .into_iter()
+        .map(|((i, j), v)| FaerTriplet::new(i, j, v))
+        .collect();
+    let jtr = triplets
+        .iter()
+        .fold(vec![0.0_f64; params_len], |mut acc, (row, col, val)| {
+            acc[col.as_usize()] += val * residuals[row.as_usize()];
+            acc
+        });
+    (jtj_triplets, jtr)
+}
+
+fn line_search(
+    system: &ConstraintSystem,
+    params: &[f64],
+    step: &DVector<f64>,
+    baseline: SolverResidual,
+    max_shrinks: u32,
+) -> Option<Iterate> {
+    let count = usize::try_from(max_shrinks)
+        .unwrap_or(usize::MAX)
+        .saturating_add(1);
+    std::iter::successors(Some(1.0_f64), |a| Some(a * 0.5))
+        .take(count)
+        .find_map(|alpha| {
+            let trial: Vec<f64> = params
+                .iter()
+                .zip(step.iter())
+                .map(|(p, s)| p + alpha * s)
+                .collect();
+            let candidate = Iterate::evaluate(system, trial);
+            (candidate.norm.value() < baseline.value()).then_some(candidate)
+        })
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn lift_subsystem_error_translates_singular_index() {
+        let param_map = vec![
+            ParameterIndex::new(4),
+            ParameterIndex::new(9),
+            ParameterIndex::new(12),
+        ];
+        let local = SolverError::InvalidSolutionFound {
+            at: ParameterIndex::new(1),
+        };
+        match lift_subsystem_error(local, &param_map) {
+            SolverError::InvalidSolutionFound { at } => assert_eq!(at, ParameterIndex::new(9)),
+            other => panic!("expected InvalidSolutionFound, got {other:?}"),
+        }
+    }
+
+    #[test]
+    fn lift_subsystem_error_passes_through_other_variants() {
+        let param_map = [ParameterIndex::new(2), ParameterIndex::new(5)];
+        let no_solution = SolverError::NoSolutionFound {
+            last: SolverResidual::new(3.5),
+        };
+        match lift_subsystem_error(no_solution, &param_map) {
+            SolverError::NoSolutionFound { last } => {
+                assert!((last.value() - 3.5).abs() < f64::EPSILON);
+            }
+            other => panic!("expected NoSolutionFound, got {other:?}"),
+        }
+    }
+
+    #[test]
+    fn remaining_budget_subtracts_elapsed() {
+        let outer = BudgetCeiling::new(Duration::from_millis(10));
+        let r = remaining_budget(outer, Duration::from_millis(3));
+        assert_eq!(r.duration(), Duration::from_millis(7));
+    }
+
+    #[test]
+    fn remaining_budget_saturates_at_zero_when_overshot() {
+        let outer = BudgetCeiling::new(Duration::from_millis(10));
+        let r = remaining_budget(outer, Duration::from_millis(25));
+        assert_eq!(r.duration(), Duration::ZERO);
+    }
+
+    #[test]
+    fn decomposed_zero_budget_fires_across_disjoint_components() {
+        use crate::graph::decompose;
+        use crate::residual::Residual;
+        use crate::system::ConstraintSystem;
+        let system = ConstraintSystem::new(
+            vec![Parameter::new(0.0), Parameter::new(0.0)],
+            vec![
+                Residual::Pin {
+                    param: ParameterIndex::new(0),
+                    target: 1.0,
+                },
+                Residual::Pin {
+                    param: ParameterIndex::new(1),
+                    target: 2.0,
+                },
+            ],
+        );
+        let decomp = decompose(&system);
+        assert!(
+            decomp.components().len() >= 2,
+            "fixture must decompose into at least two components, got {}",
+            decomp.components().len(),
+        );
+        let cfg = NewtonConfig {
+            budget: Some(BudgetCeiling::new(Duration::ZERO)),
+            ..NewtonConfig::DEFAULT
+        };
+        match solve_newton_decomposed(&system, &decomp, cfg) {
+            Err(SolverError::Budget { .. }) => {}
+            other => panic!("expected Budget, got {other:?}"),
+        }
+    }
+}
diff --git a/crates/bone-solver/src/residual.rs b/crates/bone-solver/src/residual.rs
new file mode 100644
index 0000000..2dc4953
--- /dev/null
+++ b/crates/bone-solver/src/residual.rs
@@ -0,0 +1,584 @@
+use bone_types::{ParameterIndex, ResidualIndex};
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+pub struct PointHandle {
+    pub x: ParameterIndex,
+    pub y: ParameterIndex,
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+pub struct LineHandle {
+    pub a: PointHandle,
+    pub b: PointHandle,
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+pub enum CurveRadius {
+    Explicit {
+        center: PointHandle,
+        radius: ParameterIndex,
+    },
+    FromSpoke {
+        center: PointHandle,
+        spoke: PointHandle,
+    },
+}
+
+impl CurveRadius {
+    #[must_use]
+    pub fn center(self) -> PointHandle {
+        match self {
+            Self::Explicit { center, .. } | Self::FromSpoke { center, .. } => center,
+        }
+    }
+}
+
+#[derive(Clone, Debug, PartialEq)]
+pub enum Residual {
+    Pin {
+        param: ParameterIndex,
+        target: f64,
+    },
+    Horizontal(LineHandle),
+    Vertical(LineHandle),
+    Parallel(LineHandle, LineHandle),
+    Perpendicular(LineHandle, LineHandle),
+    TangentLineCurve {
+        line: LineHandle,
+        curve: CurveRadius,
+    },
+    TangentCurveCurve {
+        a: CurveRadius,
+        b: CurveRadius,
+    },
+    CoincidentPointPoint(PointHandle, PointHandle),
+    CoincidentPointLine {
+        point: PointHandle,
+        line: LineHandle,
+    },
+    CoincidentPointCurve {
+        point: PointHandle,
+        curve: CurveRadius,
+    },
+    EqualLength(LineHandle, LineHandle),
+    EqualRadius(CurveRadius, CurveRadius),
+    LinearDistance {
+        a: PointHandle,
+        b: PointHandle,
+        value_mm: f64,
+    },
+    AngularBetweenLines {
+        a: LineHandle,
+        b: LineHandle,
+        angle_rad: f64,
+    },
+    RadiusCurve {
+        curve: CurveRadius,
+        value_mm: f64,
+    },
+}
+
+pub type Triplet = (ResidualIndex, ParameterIndex, f64);
+
+impl Residual {
+    #[must_use]
+    pub fn rows(&self) -> usize {
+        match self {
+            Self::CoincidentPointPoint(..) => 2,
+            _ => 1,
+        }
+    }
+
+    #[must_use]
+    pub fn parameters(&self) -> Vec<ParameterIndex> {
+        match *self {
+            Self::Pin { param, .. } => vec![param],
+            Self::Horizontal(l) | Self::Vertical(l) => line_params(l),
+            Self::Parallel(l1, l2) | Self::Perpendicular(l1, l2) | Self::EqualLength(l1, l2) => {
+                line_params(l1).into_iter().chain(line_params(l2)).collect()
+            }
+            Self::TangentLineCurve { line, curve } => line_params(line)
+                .into_iter()
+                .chain(curve_params(curve))
+                .collect(),
+            Self::TangentCurveCurve { a, b } | Self::EqualRadius(a, b) => {
+                curve_params(a).into_iter().chain(curve_params(b)).collect()
+            }
+            Self::CoincidentPointPoint(p, q) => {
+                point_params(p).into_iter().chain(point_params(q)).collect()
+            }
+            Self::CoincidentPointLine { point, line } => point_params(point)
+                .into_iter()
+                .chain(line_params(line))
+                .collect(),
+            Self::CoincidentPointCurve { point, curve } => point_params(point)
+                .into_iter()
+                .chain(curve_params(curve))
+                .collect(),
+            Self::LinearDistance { a, b, .. } => {
+                point_params(a).into_iter().chain(point_params(b)).collect()
+            }
+            Self::AngularBetweenLines { a, b, .. } => {
+                line_params(a).into_iter().chain(line_params(b)).collect()
+            }
+            Self::RadiusCurve { curve, .. } => curve_params(curve),
+        }
+    }
+
+    pub fn evaluate(&self, params: &[f64], out: &mut [f64]) {
+        match *self {
+            Self::Pin { param, target } => {
+                out[0] = params[param.as_usize()] - target;
+            }
+            Self::Horizontal(l) => out[0] = get(params, l.a.y) - get(params, l.b.y),
+            Self::Vertical(l) => out[0] = get(params, l.a.x) - get(params, l.b.x),
+            Self::Parallel(l1, l2) => {
+                let (d1x, d1y) = delta(params, l1);
+                let (d2x, d2y) = delta(params, l2);
+                out[0] = d1x * d2y - d1y * d2x;
+            }
+            Self::Perpendicular(l1, l2) => {
+                let (d1x, d1y) = delta(params, l1);
+                let (d2x, d2y) = delta(params, l2);
+                out[0] = d1x * d2x + d1y * d2y;
+            }
+            Self::TangentLineCurve { line, curve } => {
+                let (dx, dy) = delta(params, line);
+                let cx = get(params, curve.center().x);
+                let cy = get(params, curve.center().y);
+                let ex = cx - get(params, line.a.x);
+                let ey = cy - get(params, line.a.y);
+                let cross = ex * dy - ey * dx;
+                let len2 = dx * dx + dy * dy;
+                let r2 = radius_squared(params, curve);
+                out[0] = cross * cross - r2 * len2;
+            }
+            Self::TangentCurveCurve { a, b } => {
+                let ax = get(params, a.center().x);
+                let ay = get(params, a.center().y);
+                let bx = get(params, b.center().x);
+                let by = get(params, b.center().y);
+                let ra = radius_squared(params, a).sqrt();
+                let rb = radius_squared(params, b).sqrt();
+                let u = ax - bx;
+                let v = ay - by;
+                let rsum = ra + rb;
+                out[0] = u * u + v * v - rsum * rsum;
+            }
+            Self::CoincidentPointPoint(p, q) => {
+                out[0] = get(params, p.x) - get(params, q.x);
+                out[1] = get(params, p.y) - get(params, q.y);
+            }
+            Self::CoincidentPointLine { point, line } => {
+                let (dx, dy) = delta(params, line);
+                let ex = get(params, point.x) - get(params, line.a.x);
+                let ey = get(params, point.y) - get(params, line.a.y);
+                out[0] = ex * dy - ey * dx;
+            }
+            Self::CoincidentPointCurve { point, curve } => {
+                let cx = get(params, curve.center().x);
+                let cy = get(params, curve.center().y);
+                let ex = get(params, point.x) - cx;
+                let ey = get(params, point.y) - cy;
+                let r2 = radius_squared(params, curve);
+                out[0] = ex * ex + ey * ey - r2;
+            }
+            Self::EqualLength(l1, l2) => {
+                let (d1x, d1y) = delta(params, l1);
+                let (d2x, d2y) = delta(params, l2);
+                out[0] = d1x * d1x + d1y * d1y - (d2x * d2x + d2y * d2y);
+            }
+            Self::EqualRadius(a, b) => {
+                out[0] = radius_squared(params, a) - radius_squared(params, b);
+            }
+            Self::LinearDistance { a, b, value_mm } => {
+                let dx = get(params, b.x) - get(params, a.x);
+                let dy = get(params, b.y) - get(params, a.y);
+                out[0] = dx * dx + dy * dy - value_mm * value_mm;
+            }
+            Self::AngularBetweenLines { a, b, angle_rad } => {
+                let (d1x, d1y) = delta(params, a);
+                let (d2x, d2y) = delta(params, b);
+                let dot = d1x * d2x + d1y * d2y;
+                let cross = d1x * d2y - d1y * d2x;
+                let (s, c) = angle_rad.sin_cos();
+                out[0] = s * dot - c * cross;
+            }
+            Self::RadiusCurve { curve, value_mm } => {
+                out[0] = radius_squared(params, curve) - value_mm * value_mm;
+            }
+        }
+    }
+
+    pub fn jacobian(&self, params: &[f64], row: ResidualIndex, sink: &mut Vec<Triplet>) {
+        match *self {
+            Self::Pin { param, .. } => sink.push((row, param, 1.0)),
+            Self::Horizontal(l) => {
+                sink.push((row, l.a.y, 1.0));
+                sink.push((row, l.b.y, -1.0));
+            }
+            Self::Vertical(l) => {
+                sink.push((row, l.a.x, 1.0));
+                sink.push((row, l.b.x, -1.0));
+            }
+            Self::Parallel(l1, l2) => jacobian_parallel(params, row, l1, l2, sink),
+            Self::Perpendicular(l1, l2) => jacobian_perpendicular(params, row, l1, l2, sink),
+            Self::TangentLineCurve { line, curve } => {
+                tangent_line_curve_jacobian(params, row, line, curve, sink);
+            }
+            Self::TangentCurveCurve { a, b } => {
+                tangent_curve_curve_jacobian(params, row, a, b, sink);
+            }
+            Self::CoincidentPointPoint(p, q) => {
+                let row2 = row.next();
+                sink.push((row, p.x, 1.0));
+                sink.push((row, q.x, -1.0));
+                sink.push((row2, p.y, 1.0));
+                sink.push((row2, q.y, -1.0));
+            }
+            Self::CoincidentPointLine { point, line } => {
+                jacobian_coincident_point_line(params, row, point, line, sink);
+            }
+            Self::CoincidentPointCurve { point, curve } => {
+                jacobian_coincident_point_curve(params, row, point, curve, sink);
+            }
+            Self::EqualLength(l1, l2) => jacobian_equal_length(params, row, l1, l2, sink),
+            Self::EqualRadius(a, b) => {
+                add_radius_squared_jacobian(params, row, a, sink);
+                subtract_radius_squared_jacobian(params, row, b, sink);
+            }
+            Self::LinearDistance { a, b, .. } => {
+                jacobian_linear_distance(params, row, a, b, sink);
+            }
+            Self::AngularBetweenLines { a, b, angle_rad } => {
+                jacobian_angular_between_lines(params, row, a, b, angle_rad, sink);
+            }
+            Self::RadiusCurve { curve, .. } => {
+                add_radius_squared_jacobian(params, row, curve, sink);
+            }
+        }
+    }
+}
+
+fn jacobian_parallel(
+    params: &[f64],
+    row: ResidualIndex,
+    l1: LineHandle,
+    l2: LineHandle,
+    sink: &mut Vec<Triplet>,
+) {
+    let (d1x, d1y) = delta(params, l1);
+    let (d2x, d2y) = delta(params, l2);
+    sink.push((row, l1.a.x, -d2y));
+    sink.push((row, l1.b.x, d2y));
+    sink.push((row, l1.a.y, d2x));
+    sink.push((row, l1.b.y, -d2x));
+    sink.push((row, l2.a.x, d1y));
+    sink.push((row, l2.b.x, -d1y));
+    sink.push((row, l2.a.y, -d1x));
+    sink.push((row, l2.b.y, d1x));
+}
+
+fn jacobian_perpendicular(
+    params: &[f64],
+    row: ResidualIndex,
+    l1: LineHandle,
+    l2: LineHandle,
+    sink: &mut Vec<Triplet>,
+) {
+    let (d1x, d1y) = delta(params, l1);
+    let (d2x, d2y) = delta(params, l2);
+    sink.push((row, l1.a.x, -d2x));
+    sink.push((row, l1.b.x, d2x));
+    sink.push((row, l1.a.y, -d2y));
+    sink.push((row, l1.b.y, d2y));
+    sink.push((row, l2.a.x, -d1x));
+    sink.push((row, l2.b.x, d1x));
+    sink.push((row, l2.a.y, -d1y));
+    sink.push((row, l2.b.y, d1y));
+}
+
+fn jacobian_coincident_point_line(
+    params: &[f64],
+    row: ResidualIndex,
+    point: PointHandle,
+    line: LineHandle,
+    sink: &mut Vec<Triplet>,
+) {
+    let (dx, dy) = delta(params, line);
+    let ex = get(params, point.x) - get(params, line.a.x);
+    let ey = get(params, point.y) - get(params, line.a.y);
+    sink.push((row, point.x, dy));
+    sink.push((row, point.y, -dx));
+    sink.push((row, line.a.x, ey - dy));
+    sink.push((row, line.a.y, dx - ex));
+    sink.push((row, line.b.x, -ey));
+    sink.push((row, line.b.y, ex));
+}
+
+fn jacobian_coincident_point_curve(
+    params: &[f64],
+    row: ResidualIndex,
+    point: PointHandle,
+    curve: CurveRadius,
+    sink: &mut Vec<Triplet>,
+) {
+    let center = curve.center();
+    let ex = get(params, point.x) - get(params, center.x);
+    let ey = get(params, point.y) - get(params, center.y);
+    sink.push((row, point.x, 2.0 * ex));
+    sink.push((row, point.y, 2.0 * ey));
+    sink.push((row, center.x, -2.0 * ex));
+    sink.push((row, center.y, -2.0 * ey));
+    subtract_radius_squared_jacobian(params, row, curve, sink);
+}
+
+fn jacobian_equal_length(
+    params: &[f64],
+    row: ResidualIndex,
+    l1: LineHandle,
+    l2: LineHandle,
+    sink: &mut Vec<Triplet>,
+) {
+    let (d1x, d1y) = delta(params, l1);
+    let (d2x, d2y) = delta(params, l2);
+    sink.push((row, l1.a.x, -2.0 * d1x));
+    sink.push((row, l1.b.x, 2.0 * d1x));
+    sink.push((row, l1.a.y, -2.0 * d1y));
+    sink.push((row, l1.b.y, 2.0 * d1y));
+    sink.push((row, l2.a.x, 2.0 * d2x));
+    sink.push((row, l2.b.x, -2.0 * d2x));
+    sink.push((row, l2.a.y, 2.0 * d2y));
+    sink.push((row, l2.b.y, -2.0 * d2y));
+}
+
+fn jacobian_linear_distance(
+    params: &[f64],
+    row: ResidualIndex,
+    a: PointHandle,
+    b: PointHandle,
+    sink: &mut Vec<Triplet>,
+) {
+    let dx = get(params, b.x) - get(params, a.x);
+    let dy = get(params, b.y) - get(params, a.y);
+    sink.push((row, a.x, -2.0 * dx));
+    sink.push((row, a.y, -2.0 * dy));
+    sink.push((row, b.x, 2.0 * dx));
+    sink.push((row, b.y, 2.0 * dy));
+}
+
+fn jacobian_angular_between_lines(
+    params: &[f64],
+    row: ResidualIndex,
+    a: LineHandle,
+    b: LineHandle,
+    angle_rad: f64,
+    sink: &mut Vec<Triplet>,
+) {
+    let (d1x, d1y) = delta(params, a);
+    let (d2x, d2y) = delta(params, b);
+    let (sin_t, cos_t) = angle_rad.sin_cos();
+    let d1_partial_x = sin_t * d2x - cos_t * d2y;
+    let d1_partial_y = sin_t * d2y + cos_t * d2x;
+    let d2_partial_x = sin_t * d1x + cos_t * d1y;
+    let d2_partial_y = sin_t * d1y - cos_t * d1x;
+    sink.push((row, a.a.x, -d1_partial_x));
+    sink.push((row, a.b.x, d1_partial_x));
+    sink.push((row, a.a.y, -d1_partial_y));
+    sink.push((row, a.b.y, d1_partial_y));
+    sink.push((row, b.a.x, -d2_partial_x));
+    sink.push((row, b.b.x, d2_partial_x));
+    sink.push((row, b.a.y, -d2_partial_y));
+    sink.push((row, b.b.y, d2_partial_y));
+}
+
+fn line_params(l: LineHandle) -> Vec<ParameterIndex> {
+    vec![l.a.x, l.a.y, l.b.x, l.b.y]
+}
+
+fn point_params(p: PointHandle) -> Vec<ParameterIndex> {
+    vec![p.x, p.y]
+}
+
+fn curve_params(c: CurveRadius) -> Vec<ParameterIndex> {
+    match c {
+        CurveRadius::Explicit { center, radius } => vec![center.x, center.y, radius],
+        CurveRadius::FromSpoke { center, spoke } => vec![center.x, center.y, spoke.x, spoke.y],
+    }
+}
+
+fn get(params: &[f64], idx: ParameterIndex) -> f64 {
+    params[idx.as_usize()]
+}
+
+fn delta(params: &[f64], line: LineHandle) -> (f64, f64) {
+    (
+        get(params, line.b.x) - get(params, line.a.x),
+        get(params, line.b.y) - get(params, line.a.y),
+    )
+}
+
+fn radius_squared(params: &[f64], curve: CurveRadius) -> f64 {
+    match curve {
+        CurveRadius::Explicit { radius, .. } => {
+            let r = get(params, radius);
+            r * r
+        }
+        CurveRadius::FromSpoke { center, spoke } => {
+            let dx = get(params, spoke.x) - get(params, center.x);
+            let dy = get(params, spoke.y) - get(params, center.y);
+            dx * dx + dy * dy
+        }
+    }
+}
+
+fn add_radius_squared_jacobian(
+    params: &[f64],
+    row: ResidualIndex,
+    curve: CurveRadius,
+    sink: &mut Vec<Triplet>,
+) {
+    match curve {
+        CurveRadius::Explicit { radius, .. } => {
+            sink.push((row, radius, 2.0 * get(params, radius)));
+        }
+        CurveRadius::FromSpoke { center, spoke } => {
+            let dx = get(params, spoke.x) - get(params, center.x);
+            let dy = get(params, spoke.y) - get(params, center.y);
+            sink.push((row, spoke.x, 2.0 * dx));
+            sink.push((row, spoke.y, 2.0 * dy));
+            sink.push((row, center.x, -2.0 * dx));
+            sink.push((row, center.y, -2.0 * dy));
+        }
+    }
+}
+
+fn subtract_radius_squared_jacobian(
+    params: &[f64],
+    row: ResidualIndex,
+    curve: CurveRadius,
+    sink: &mut Vec<Triplet>,
+) {
+    match curve {
+        CurveRadius::Explicit { radius, .. } => {
+            sink.push((row, radius, -2.0 * get(params, radius)));
+        }
+        CurveRadius::FromSpoke { center, spoke } => {
+            let dx = get(params, spoke.x) - get(params, center.x);
+            let dy = get(params, spoke.y) - get(params, center.y);
+            sink.push((row, spoke.x, -2.0 * dx));
+            sink.push((row, spoke.y, -2.0 * dy));
+            sink.push((row, center.x, 2.0 * dx));
+            sink.push((row, center.y, 2.0 * dy));
+        }
+    }
+}
+
+fn tangent_line_curve_jacobian(
+    params: &[f64],
+    row: ResidualIndex,
+    line: LineHandle,
+    curve: CurveRadius,
+    sink: &mut Vec<Triplet>,
+) {
+    let (dx, dy) = delta(params, line);
+    let cx = get(params, curve.center().x);
+    let cy = get(params, curve.center().y);
+    let ex = cx - get(params, line.a.x);
+    let ey = cy - get(params, line.a.y);
+    let cross = ex * dy - ey * dx;
+    let len2 = dx * dx + dy * dy;
+    let r2 = radius_squared(params, curve);
+    let two_cross = 2.0 * cross;
+
+    sink.push((
+        row,
+        line.a.x,
+        two_cross * (cy - get(params, line.b.y)) + r2 * 2.0 * dx,
+    ));
+    sink.push((
+        row,
+        line.a.y,
+        two_cross * (get(params, line.b.x) - cx) + r2 * 2.0 * dy,
+    ));
+    sink.push((
+        row,
+        line.b.x,
+        two_cross * (get(params, line.a.y) - cy) - r2 * 2.0 * dx,
+    ));
+    sink.push((
+        row,
+        line.b.y,
+        two_cross * (cx - get(params, line.a.x)) - r2 * 2.0 * dy,
+    ));
+    sink.push((row, curve.center().x, two_cross * dy));
+    sink.push((row, curve.center().y, -two_cross * dx));
+
+    let neg_len2 = -len2;
+    match curve {
+        CurveRadius::Explicit { radius, .. } => {
+            sink.push((row, radius, neg_len2 * 2.0 * get(params, radius)));
+        }
+        CurveRadius::FromSpoke { center, spoke } => {
+            let sdx = get(params, spoke.x) - get(params, center.x);
+            let sdy = get(params, spoke.y) - get(params, center.y);
+            sink.push((row, spoke.x, neg_len2 * 2.0 * sdx));
+            sink.push((row, spoke.y, neg_len2 * 2.0 * sdy));
+            sink.push((row, center.x, neg_len2 * -2.0 * sdx));
+            sink.push((row, center.y, neg_len2 * -2.0 * sdy));
+        }
+    }
+}
+
+fn tangent_curve_curve_jacobian(
+    params: &[f64],
+    row: ResidualIndex,
+    a: CurveRadius,
+    b: CurveRadius,
+    sink: &mut Vec<Triplet>,
+) {
+    let ra = radius_squared(params, a).sqrt();
+    let rb = radius_squared(params, b).sqrt();
+    let rsum = ra + rb;
+    let u = get(params, a.center().x) - get(params, b.center().x);
+    let v = get(params, a.center().y) - get(params, b.center().y);
+
+    sink.push((row, a.center().x, 2.0 * u));
+    sink.push((row, a.center().y, 2.0 * v));
+    sink.push((row, b.center().x, -2.0 * u));
+    sink.push((row, b.center().y, -2.0 * v));
+
+    let coef = -2.0 * rsum;
+    push_radius_linear_jacobian(params, row, a, coef, sink);
+    push_radius_linear_jacobian(params, row, b, coef, sink);
+}
+
+fn push_radius_linear_jacobian(
+    params: &[f64],
+    row: ResidualIndex,
+    curve: CurveRadius,
+    coef: f64,
+    sink: &mut Vec<Triplet>,
+) {
+    if coef == 0.0 {
+        return;
+    }
+    match curve {
+        CurveRadius::Explicit { radius, .. } => {
+            sink.push((row, radius, coef));
+        }
+        CurveRadius::FromSpoke { center, spoke } => {
+            let dx = get(params, spoke.x) - get(params, center.x);
+            let dy = get(params, spoke.y) - get(params, center.y);
+            let len = (dx * dx + dy * dy).sqrt();
+            if len == 0.0 {
+                return;
+            }
+            let k = coef / len;
+            sink.push((row, spoke.x, k * dx));
+            sink.push((row, spoke.y, k * dy));
+            sink.push((row, center.x, -k * dx));
+            sink.push((row, center.y, -k * dy));
+        }
+    }
+}
diff --git a/crates/bone-solver/src/system.rs b/crates/bone-solver/src/system.rs
new file mode 100644
index 0000000..7ebd656
--- /dev/null
+++ b/crates/bone-solver/src/system.rs
@@ -0,0 +1,254 @@
+use std::collections::{BTreeMap, BTreeSet};
+
+use bone_types::{Parameter, ParameterIndex, ParentIndex, ResidualIndex};
+
+use crate::graph::Component;
+use crate::residual::{CurveRadius, LineHandle, PointHandle, Residual};
+
+#[derive(Clone, Debug, PartialEq)]
+pub struct ConstraintSystem {
+    parameters: Vec<Parameter>,
+    residuals: Vec<Residual>,
+}
+
+impl ConstraintSystem {
+    #[must_use]
+    pub fn new(parameters: Vec<Parameter>, residuals: Vec<Residual>) -> Self {
+        Self {
+            parameters,
+            residuals,
+        }
+    }
+
+    #[must_use]
+    pub fn parameters(&self) -> &[Parameter] {
+        &self.parameters
+    }
+
+    #[must_use]
+    pub fn residuals(&self) -> &[Residual] {
+        &self.residuals
+    }
+
+    #[must_use]
+    pub fn parameter_count(&self) -> usize {
+        self.parameters.len()
+    }
+
+    #[must_use]
+    pub fn row_count(&self) -> usize {
+        self.residuals.iter().map(Residual::rows).sum()
+    }
+
+    #[must_use]
+    pub fn row_offsets(&self) -> Vec<ResidualIndex> {
+        self.residuals
+            .iter()
+            .scan(0u32, |acc, r| {
+                let start = ResidualIndex::new(*acc);
+                let Ok(n) = u32::try_from(r.rows()) else {
+                    unreachable!("Residual::rows is bounded to a small constant")
+                };
+                let Some(next) = acc.checked_add(n) else {
+                    unreachable!("row count fits in u32 by construction of ResidualIndex")
+                };
+                *acc = next;
+                Some(start)
+            })
+            .collect()
+    }
+
+    #[must_use]
+    pub fn parameter_at(&self, idx: ParameterIndex) -> Parameter {
+        self.parameters[idx.as_usize()]
+    }
+
+    #[must_use]
+    pub fn residual_at_parent(&self, parent: ParentIndex) -> &Residual {
+        &self.residuals[parent.as_usize()]
+    }
+
+    #[must_use]
+    pub fn with_extra_residuals(&self, extra: Vec<Residual>) -> Self {
+        let mut residuals = self.residuals.clone();
+        residuals.extend(extra);
+        Self {
+            parameters: self.parameters.clone(),
+            residuals,
+        }
+    }
+
+    #[must_use]
+    pub fn parent_of_row(&self, row: ResidualIndex) -> ParentIndex {
+        let target = row.as_usize();
+        let found = self
+            .residuals
+            .iter()
+            .scan(0usize, |acc, r| {
+                let start = *acc;
+                *acc += r.rows();
+                Some((start, *acc))
+            })
+            .position(|(begin, end)| target >= begin && target < end);
+        let Some(idx) = found else {
+            unreachable!("ResidualIndex {row} is past the residual row count")
+        };
+        let Ok(iv) = u32::try_from(idx) else {
+            unreachable!("parent count fits in u32 via ResidualIndex")
+        };
+        ParentIndex::new(iv)
+    }
+
+    #[must_use]
+    pub fn rows_of_parent(&self, parent: ParentIndex) -> Vec<ResidualIndex> {
+        let idx = parent.as_usize();
+        let Some(residual) = self.residuals.get(idx) else {
+            unreachable!("ParentIndex {parent} out of range for this ConstraintSystem")
+        };
+        let start_usize: usize = self.residuals[..idx].iter().map(Residual::rows).sum();
+        let Ok(start) = u32::try_from(start_usize) else {
+            unreachable!("row count fits in u32 via ResidualIndex")
+        };
+        (0..residual.rows())
+            .map(|o| {
+                let Ok(ov) = u32::try_from(o) else {
+                    unreachable!("residual row count is a small constant")
+                };
+                ResidualIndex::new(start + ov)
+            })
+            .collect()
+    }
+
+    #[must_use]
+    pub fn without_parents(&self, exclude: &BTreeSet<ParentIndex>) -> Self {
+        let mask: BTreeSet<usize> = exclude.iter().copied().map(ParentIndex::as_usize).collect();
+        let residuals = self
+            .residuals
+            .iter()
+            .enumerate()
+            .filter(|(i, _)| !mask.contains(i))
+            .map(|(_, r)| r.clone())
+            .collect();
+        Self {
+            parameters: self.parameters.clone(),
+            residuals,
+        }
+    }
+
+    #[must_use]
+    pub fn subsystem(&self, component: &Component) -> Subsystem {
+        let orig_params: &[ParameterIndex] = component.parameters();
+        let remap: BTreeMap<ParameterIndex, ParameterIndex> = orig_params
+            .iter()
+            .copied()
+            .enumerate()
+            .map(|(i, orig)| {
+                let Ok(iv) = u32::try_from(i) else {
+                    unreachable!("component parameter count fits in u32 by construction")
+                };
+                (orig, ParameterIndex::new(iv))
+            })
+            .collect();
+        let parameters: Vec<Parameter> = orig_params
+            .iter()
+            .map(|p| self.parameters[p.as_usize()])
+            .collect();
+        let residuals: Vec<Residual> = component
+            .residual_parents()
+            .iter()
+            .map(|&parent| remap_residual(self.residual_at_parent(parent), &remap))
+            .collect();
+        Subsystem {
+            system: ConstraintSystem::new(parameters, residuals),
+            param_map: orig_params.to_vec(),
+        }
+    }
+}
+
+#[derive(Clone, Debug)]
+pub struct Subsystem {
+    system: ConstraintSystem,
+    param_map: Vec<ParameterIndex>,
+}
+
+impl Subsystem {
+    #[must_use]
+    pub fn system(&self) -> &ConstraintSystem {
+        &self.system
+    }
+
+    #[must_use]
+    pub fn param_map(&self) -> &[ParameterIndex] {
+        &self.param_map
+    }
+}
+
+fn remap_residual(r: &Residual, m: &BTreeMap<ParameterIndex, ParameterIndex>) -> Residual {
+    let idx = |p: ParameterIndex| -> ParameterIndex {
+        let Some(&mapped) = m.get(&p) else {
+            unreachable!("residual parameter missing from component remap (graph bug)")
+        };
+        mapped
+    };
+    let point = |p: PointHandle| PointHandle {
+        x: idx(p.x),
+        y: idx(p.y),
+    };
+    let line = |l: LineHandle| LineHandle {
+        a: point(l.a),
+        b: point(l.b),
+    };
+    let curve = |c: CurveRadius| match c {
+        CurveRadius::Explicit { center, radius } => CurveRadius::Explicit {
+            center: point(center),
+            radius: idx(radius),
+        },
+        CurveRadius::FromSpoke { center, spoke } => CurveRadius::FromSpoke {
+            center: point(center),
+            spoke: point(spoke),
+        },
+    };
+    match *r {
+        Residual::Pin { param, target } => Residual::Pin {
+            param: idx(param),
+            target,
+        },
+        Residual::Horizontal(l) => Residual::Horizontal(line(l)),
+        Residual::Vertical(l) => Residual::Vertical(line(l)),
+        Residual::Parallel(a, b) => Residual::Parallel(line(a), line(b)),
+        Residual::Perpendicular(a, b) => Residual::Perpendicular(line(a), line(b)),
+        Residual::TangentLineCurve { line: l, curve: c } => Residual::TangentLineCurve {
+            line: line(l),
+            curve: curve(c),
+        },
+        Residual::TangentCurveCurve { a, b } => Residual::TangentCurveCurve {
+            a: curve(a),
+            b: curve(b),
+        },
+        Residual::CoincidentPointPoint(p, q) => Residual::CoincidentPointPoint(point(p), point(q)),
+        Residual::CoincidentPointLine { point: p, line: l } => Residual::CoincidentPointLine {
+            point: point(p),
+            line: line(l),
+        },
+        Residual::CoincidentPointCurve { point: p, curve: c } => Residual::CoincidentPointCurve {
+            point: point(p),
+            curve: curve(c),
+        },
+        Residual::EqualLength(a, b) => Residual::EqualLength(line(a), line(b)),
+        Residual::EqualRadius(a, b) => Residual::EqualRadius(curve(a), curve(b)),
+        Residual::LinearDistance { a, b, value_mm } => Residual::LinearDistance {
+            a: point(a),
+            b: point(b),
+            value_mm,
+        },
+        Residual::AngularBetweenLines { a, b, angle_rad } => Residual::AngularBetweenLines {
+            a: line(a),
+            b: line(b),
+            angle_rad,
+        },
+        Residual::RadiusCurve { curve: c, value_mm } => Residual::RadiusCurve {
+            curve: curve(c),
+            value_mm,
+        },
+    }
+}
diff --git a/crates/bone-solver/src/util.rs b/crates/bone-solver/src/util.rs
new file mode 100644
index 0000000..d78dae3
--- /dev/null
+++ b/crates/bone-solver/src/util.rs
@@ -0,0 +1,21 @@
+use std::collections::BTreeSet;
+
+#[must_use]
+pub fn dedup_preserving_order<T, I>(items: I) -> Vec<T>
+where
+    T: Copy + Ord,
+    I: IntoIterator<Item = T>,
+{
+    items
+        .into_iter()
+        .fold(
+            (Vec::new(), BTreeSet::<T>::new()),
+            |(mut acc, mut seen), item| {
+                if seen.insert(item) {
+                    acc.push(item);
+                }
+                (acc, seen)
+            },
+        )
+        .0
+}
diff --git a/crates/bone-solver/tests/core.rs b/crates/bone-solver/tests/core.rs
new file mode 100644
index 0000000..9507a7c
--- /dev/null
+++ b/crates/bone-solver/tests/core.rs
@@ -0,0 +1,196 @@
+use bone_solver::{
+    ConstraintSystem, LineHandle, NewtonConfig, PointHandle, Residual, SolverError,
+    assemble_jacobian, decompose, evaluate_residuals, jacobian_triplets, residual_norm,
+    solve_newton, solve_newton_decomposed, sparsity_pattern,
+};
+use bone_types::{NewtonDamping, Parameter, ParameterIndex, ResidualIndex, SolverResidual};
+
+fn p(idx: u32) -> ParameterIndex {
+    ParameterIndex::new(idx)
+}
+
+fn point(x: u32, y: u32) -> PointHandle {
+    PointHandle { x: p(x), y: p(y) }
+}
+
+fn parameters(values: &[f64]) -> Vec<Parameter> {
+    values.iter().copied().map(Parameter::new).collect()
+}
+
+#[test]
+fn horizontal_residual_row_and_jacobian_match_hand_derivation() {
+    let system = ConstraintSystem::new(
+        parameters(&[0.0, 1.5, 3.0, 4.0]),
+        vec![Residual::Horizontal(LineHandle {
+            a: point(0, 1),
+            b: point(2, 3),
+        })],
+    );
+    assert_eq!(system.row_count(), 1);
+    let values = evaluate_residuals(&system, &[0.0, 1.5, 3.0, 4.0]);
+    assert!((values[0] - (1.5 - 4.0)).abs() < 1e-15);
+    let triplets = jacobian_triplets(&system, &[0.0, 1.5, 3.0, 4.0]);
+    assert_eq!(triplets.len(), 2);
+    let zero = ResidualIndex::new(0);
+    assert!(triplets.contains(&(zero, p(1), 1.0)));
+    assert!(triplets.contains(&(zero, p(3), -1.0)));
+}
+
+#[test]
+fn pin_solves_in_one_newton_step() {
+    let system = ConstraintSystem::new(
+        parameters(&[3.0]),
+        vec![Residual::Pin {
+            param: p(0),
+            target: 7.0,
+        }],
+    );
+    let Ok(out) = solve_newton(&system, NewtonConfig::DEFAULT) else {
+        panic!("pin is linear, must converge");
+    };
+    assert!((out[0].value() - 7.0).abs() < 1e-9);
+}
+
+#[test]
+fn coincident_point_point_is_two_row_residual() {
+    let system = ConstraintSystem::new(
+        parameters(&[0.0, 0.0, 1.0, 2.0]),
+        vec![Residual::CoincidentPointPoint(point(0, 1), point(2, 3))],
+    );
+    assert_eq!(system.row_count(), 2);
+    let r = evaluate_residuals(&system, &[0.0, 0.0, 1.0, 2.0]);
+    assert_eq!(r.len(), 2);
+    assert!((r[0] - (0.0 - 1.0)).abs() < 1e-15);
+    assert!((r[1] - (0.0 - 2.0)).abs() < 1e-15);
+}
+
+#[test]
+fn sparsity_pattern_drops_duplicate_positions() {
+    let system = ConstraintSystem::new(
+        parameters(&[0.0, 0.0]),
+        vec![Residual::Pin {
+            param: p(0),
+            target: 1.0,
+        }],
+    );
+    let pattern = sparsity_pattern(&system);
+    assert_eq!(pattern.rows(), 1);
+    assert_eq!(pattern.cols(), 2);
+    assert_eq!(pattern.entries().len(), 1);
+}
+
+#[test]
+fn default_damping_regularises_rank_deficient_jacobian() {
+    let system = ConstraintSystem::new(
+        parameters(&[0.0, 3.0, 5.0, 7.0]),
+        vec![Residual::Horizontal(LineHandle {
+            a: point(0, 1),
+            b: point(2, 3),
+        })],
+    );
+    let Ok(out) = solve_newton(&system, NewtonConfig::DEFAULT) else {
+        panic!("default damping must regularise rank-deficient J^T J");
+    };
+    assert!((out[1].value() - out[3].value()).abs() < 1e-9);
+}
+
+#[test]
+fn zero_damping_on_rank_deficient_jacobian_does_not_silently_succeed() {
+    let system = ConstraintSystem::new(
+        parameters(&[0.0, 3.0, 5.0, 7.0]),
+        vec![Residual::Horizontal(LineHandle {
+            a: point(0, 1),
+            b: point(2, 3),
+        })],
+    );
+    let cfg = NewtonConfig {
+        damping: NewtonDamping::new(0.0),
+        ..NewtonConfig::DEFAULT
+    };
+    let Err(err) = solve_newton(&system, cfg) else {
+        panic!("damping=0 on rank-deficient system must surface an error");
+    };
+    assert!(
+        matches!(
+            err,
+            SolverError::InvalidSolutionFound { .. } | SolverError::NoSolutionFound { .. }
+        ),
+        "expected InvalidSolutionFound or NoSolutionFound, got {err:?}"
+    );
+}
+
+#[test]
+fn residual_norm_matches_euclidean_length() {
+    let out = evaluate_residuals(
+        &ConstraintSystem::new(
+            parameters(&[3.0]),
+            vec![Residual::Pin {
+                param: p(0),
+                target: 0.0,
+            }],
+        ),
+        &[3.0],
+    );
+    let norm = residual_norm(&out);
+    assert!((norm.value() - 3.0).abs() < 1e-15);
+    let _ = SolverResidual::new(norm.value());
+}
+
+#[test]
+fn decomposed_singular_index_is_translated_to_global_parameter() {
+    let pins: Vec<Residual> = (0..10)
+        .map(|i| Residual::Pin {
+            param: p(i),
+            target: 0.0,
+        })
+        .collect();
+    let residuals: Vec<Residual> = pins
+        .into_iter()
+        .chain(std::iter::once(Residual::Horizontal(LineHandle {
+            a: point(10, 11),
+            b: point(12, 13),
+        })))
+        .collect();
+    let seed = [0.0; 10]
+        .iter()
+        .copied()
+        .chain([0.0, 5.0, 1.0, 7.0])
+        .collect::<Vec<_>>();
+    let system = ConstraintSystem::new(parameters(&seed), residuals);
+    let decomp = decompose(&system);
+    let cfg = NewtonConfig {
+        damping: NewtonDamping::new(0.0),
+        ..NewtonConfig::DEFAULT
+    };
+    match solve_newton_decomposed(&system, &decomp, cfg) {
+        Err(SolverError::InvalidSolutionFound { at }) => {
+            assert!(
+                at.as_usize() >= 10,
+                "expected global parameter index (>=10), got {at:?}"
+            );
+        }
+        Err(SolverError::NoSolutionFound { .. }) => {}
+        other => panic!("expected InvalidSolutionFound or NoSolutionFound, got {other:?}"),
+    }
+}
+
+#[test]
+fn assemble_jacobian_and_triplets_agree_on_dense_shape() {
+    let system = ConstraintSystem::new(
+        parameters(&[0.0, 0.0, 2.0, 0.0]),
+        vec![Residual::Horizontal(LineHandle {
+            a: point(0, 1),
+            b: point(2, 3),
+        })],
+    );
+    let params = vec![0.0, 0.0, 2.0, 0.0];
+    let dense = assemble_jacobian(&system, &params);
+    assert_eq!(dense.nrows(), 1);
+    assert_eq!(dense.ncols(), 4);
+    let triplets = jacobian_triplets(&system, &params);
+    let sum: f64 = triplets
+        .iter()
+        .map(|(r, c, v)| dense[(r.as_usize(), c.as_usize())] - v)
+        .sum();
+    assert!(sum.abs() < 1e-15);
+}
diff --git a/crates/bone-solver/tests/dof.rs b/crates/bone-solver/tests/dof.rs
new file mode 100644
index 0000000..3591226
--- /dev/null
+++ b/crates/bone-solver/tests/dof.rs
@@ -0,0 +1,153 @@
+use bone_solver::{ConstraintSystem, DofConfig, LineHandle, PointHandle, Residual, analyze_dof};
+use bone_types::{Parameter, ParameterIndex, ResidualIndex};
+
+fn p(i: u32) -> ParameterIndex {
+    ParameterIndex::new(i)
+}
+
+fn point(x: u32, y: u32) -> PointHandle {
+    PointHandle { x: p(x), y: p(y) }
+}
+
+fn parameters(values: &[f64]) -> Vec<Parameter> {
+    values.iter().copied().map(Parameter::new).collect()
+}
+
+#[test]
+fn fully_constrained_pin_has_zero_dof_and_no_flags() {
+    let system = ConstraintSystem::new(
+        parameters(&[0.0]),
+        vec![Residual::Pin {
+            param: p(0),
+            target: 5.0,
+        }],
+    );
+    let report = analyze_dof(&system, DofConfig::DEFAULT);
+    assert_eq!(report.dof().value(), 0);
+    assert!(report.under_constrained().is_empty());
+    assert!(report.over_constrained().is_empty());
+    assert!(report.redundant_consistent().is_empty());
+}
+
+#[test]
+fn under_constrained_horizontal_reports_free_parameters() {
+    let system = ConstraintSystem::new(
+        parameters(&[0.0, 0.0, 1.0, 0.0]),
+        vec![Residual::Horizontal(LineHandle {
+            a: point(0, 1),
+            b: point(2, 3),
+        })],
+    );
+    let report = analyze_dof(&system, DofConfig::DEFAULT);
+    assert_eq!(report.dof().value(), 3);
+    assert_eq!(report.under_constrained().len(), 3);
+    assert!(report.over_constrained().is_empty());
+    assert!(report.redundant_consistent().is_empty());
+}
+
+#[test]
+fn redundant_pin_is_consistent_not_conflict() {
+    let system = ConstraintSystem::new(
+        parameters(&[5.0]),
+        vec![
+            Residual::Pin {
+                param: p(0),
+                target: 5.0,
+            },
+            Residual::Pin {
+                param: p(0),
+                target: 5.0,
+            },
+        ],
+    );
+    let report = analyze_dof(&system, DofConfig::DEFAULT);
+    assert_eq!(report.dof().value(), 0);
+    assert!(report.over_constrained().is_empty());
+    assert_eq!(report.redundant_consistent().len(), 1);
+    let flagged = report.redundant_consistent()[0];
+    assert!(flagged == ResidualIndex::new(0) || flagged == ResidualIndex::new(1));
+}
+
+#[test]
+fn dof_fixture_matrix_snapshot() {
+    let fully_constrained = ConstraintSystem::new(
+        parameters(&[0.0]),
+        vec![Residual::Pin {
+            param: p(0),
+            target: 5.0,
+        }],
+    );
+    let under_constrained = ConstraintSystem::new(
+        parameters(&[0.0, 0.0, 1.0, 0.0]),
+        vec![Residual::Horizontal(LineHandle {
+            a: point(0, 1),
+            b: point(2, 3),
+        })],
+    );
+    let redundant_consistent = ConstraintSystem::new(
+        parameters(&[5.0]),
+        vec![
+            Residual::Pin {
+                param: p(0),
+                target: 5.0,
+            },
+            Residual::Pin {
+                param: p(0),
+                target: 5.0,
+            },
+        ],
+    );
+    let conflicting = ConstraintSystem::new(
+        parameters(&[3.0]),
+        vec![
+            Residual::Pin {
+                param: p(0),
+                target: 3.0,
+            },
+            Residual::Pin {
+                param: p(0),
+                target: 7.0,
+            },
+        ],
+    );
+    let bundle = [
+        ("fully_constrained", fully_constrained),
+        ("under_constrained", under_constrained),
+        ("redundant_consistent", redundant_consistent),
+        ("conflicting", conflicting),
+    ];
+    let rendered: String = bundle
+        .iter()
+        .map(|(label, system)| {
+            format!(
+                "{label}: {}",
+                analyze_dof(system, DofConfig::DEFAULT).display()
+            )
+        })
+        .collect::<Vec<_>>()
+        .join("\n");
+    insta::assert_snapshot!("dof_matrix", rendered);
+}
+
+#[test]
+fn conflicting_pins_yield_over_constrained_row() {
+    let system = ConstraintSystem::new(
+        parameters(&[3.0]),
+        vec![
+            Residual::Pin {
+                param: p(0),
+                target: 3.0,
+            },
+            Residual::Pin {
+                param: p(0),
+                target: 7.0,
+            },
+        ],
+    );
+    let report = analyze_dof(&system, DofConfig::DEFAULT);
+    assert_eq!(report.dof().value(), 0);
+    assert_eq!(report.over_constrained().len(), 1);
+    assert!(report.redundant_consistent().is_empty());
+    let flagged = report.over_constrained()[0];
+    assert!(flagged == ResidualIndex::new(0) || flagged == ResidualIndex::new(1));
+}
diff --git a/crates/bone-solver/tests/graph.rs b/crates/bone-solver/tests/graph.rs
new file mode 100644
index 0000000..53e05cf
--- /dev/null
+++ b/crates/bone-solver/tests/graph.rs
@@ -0,0 +1,107 @@
+use bone_solver::{ConstraintSystem, LineHandle, PointHandle, Residual, decompose};
+use bone_types::{Parameter, ParameterIndex, ParentIndex, ResidualIndex};
+
+fn p(i: u32) -> ParameterIndex {
+    ParameterIndex::new(i)
+}
+
+fn parent(i: u32) -> ParentIndex {
+    ParentIndex::new(i)
+}
+
+fn point(x: u32, y: u32) -> PointHandle {
+    PointHandle { x: p(x), y: p(y) }
+}
+
+fn parameters(values: &[f64]) -> Vec<Parameter> {
+    values.iter().copied().map(Parameter::new).collect()
+}
+
+#[test]
+fn disjoint_residuals_yield_separate_components() {
+    let system = ConstraintSystem::new(
+        parameters(&[0.0, 0.0, 1.0, 0.0, 10.0, 0.0, 11.0, 0.0]),
+        vec![
+            Residual::Horizontal(LineHandle {
+                a: point(0, 1),
+                b: point(2, 3),
+            }),
+            Residual::Horizontal(LineHandle {
+                a: point(4, 5),
+                b: point(6, 7),
+            }),
+        ],
+    );
+    let decomp = decompose(&system);
+    assert_eq!(decomp.components().len(), 2);
+    let first = &decomp.components()[0];
+    let second = &decomp.components()[1];
+    assert_eq!(
+        first
+            .parameters()
+            .iter()
+            .map(|p| p.value())
+            .collect::<Vec<_>>(),
+        vec![0, 1, 2, 3]
+    );
+    assert_eq!(
+        second
+            .parameters()
+            .iter()
+            .map(|p| p.value())
+            .collect::<Vec<_>>(),
+        vec![4, 5, 6, 7]
+    );
+    assert_eq!(first.residual_parents(), &[parent(0)]);
+    assert_eq!(second.residual_parents(), &[parent(1)]);
+    assert_eq!(first.residual_rows(&system), vec![ResidualIndex::new(0)]);
+    assert_eq!(second.residual_rows(&system), vec![ResidualIndex::new(1)]);
+}
+
+#[test]
+fn shared_parameter_merges_components() {
+    let system = ConstraintSystem::new(
+        parameters(&[0.0, 0.0, 1.0, 0.0, 2.0, 0.0]),
+        vec![
+            Residual::Horizontal(LineHandle {
+                a: point(0, 1),
+                b: point(2, 3),
+            }),
+            Residual::Horizontal(LineHandle {
+                a: point(2, 3),
+                b: point(4, 5),
+            }),
+        ],
+    );
+    let decomp = decompose(&system);
+    assert_eq!(decomp.components().len(), 1);
+    let only = &decomp.components()[0];
+    assert_eq!(only.parameters().len(), 6);
+    assert_eq!(only.residual_parents(), &[parent(0), parent(1)]);
+}
+
+#[test]
+fn isolated_parameter_forms_its_own_component() {
+    let system = ConstraintSystem::new(
+        parameters(&[0.0, 0.0, 3.0]),
+        vec![Residual::Pin {
+            param: p(0),
+            target: 1.0,
+        }],
+    );
+    let decomp = decompose(&system);
+    assert_eq!(decomp.components().len(), 3);
+    decomp
+        .components()
+        .iter()
+        .enumerate()
+        .for_each(|(i, comp)| {
+            let Ok(iv) = u32::try_from(i) else {
+                unreachable!()
+            };
+            assert_eq!(comp.parameters(), &[ParameterIndex::new(iv)]);
+        });
+    assert_eq!(decomp.components()[0].residual_parents(), &[parent(0)]);
+    assert!(decomp.components()[1].residual_parents().is_empty());
+    assert!(decomp.components()[2].residual_parents().is_empty());
+}
diff --git a/crates/bone-solver/tests/snapshots/dof__dof_matrix.snap b/crates/bone-solver/tests/snapshots/dof__dof_matrix.snap
new file mode 100644
index 0000000..9c2d8b0
--- /dev/null
+++ b/crates/bone-solver/tests/snapshots/dof__dof_matrix.snap
@@ -0,0 +1,8 @@
+---
+source: crates/bone-solver/tests/dof.rs
+expression: rendered
+---
+fully_constrained: dof=0 under=[] over=[] redundant=[]
+under_constrained: dof=3 under=[p#0,p#2,p#3] over=[] redundant=[]
+redundant_consistent: dof=0 under=[] over=[] redundant=[r#1]
+conflicting: dof=0 under=[] over=[r#1] redundant=[]
diff --git a/crates/bone-types/Cargo.toml b/crates/bone-types/Cargo.toml
new file mode 100644
index 0000000..cdca93a
--- /dev/null
+++ b/crates/bone-types/Cargo.toml
@@ -0,0 +1,24 @@
+[package]
+name = "bone-types"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+rust-version.workspace = true
+
+[dependencies]
+nalgebra = { workspace = true }
+serde = { workspace = true }
+slotmap = { workspace = true }
+thiserror = { workspace = true }
+tracing = { workspace = true, optional = true }
+tracing-subscriber = { workspace = true, optional = true }
+uom = { workspace = true }
+
+[dev-dependencies]
+insta = { workspace = true }
+
+[features]
+testing = ["dep:tracing", "dep:tracing-subscriber"]
+
+[lints]
+workspace = true
diff --git a/crates/bone-types/src/dimensioned_serde.rs b/crates/bone-types/src/dimensioned_serde.rs
new file mode 100644
index 0000000..c2c3a5c
--- /dev/null
+++ b/crates/bone-types/src/dimensioned_serde.rs
@@ -0,0 +1,27 @@
+pub mod length_si {
+    use serde::{Deserialize, Deserializer, Serializer};
+    use uom::si::f64::Length;
+    use uom::si::length::meter;
+
+    pub fn serialize<S: Serializer>(value: &Length, s: S) -> Result<S::Ok, S::Error> {
+        s.serialize_f64(value.get::<meter>())
+    }
+
+    pub fn deserialize<'de, D: Deserializer<'de>>(d: D) -> Result<Length, D::Error> {
+        f64::deserialize(d).map(Length::new::<meter>)
+    }
+}
+
+pub mod angle_si {
+    use serde::{Deserialize, Deserializer, Serializer};
+    use uom::si::angle::radian;
+    use uom::si::f64::Angle;
+
+    pub fn serialize<S: Serializer>(value: &Angle, s: S) -> Result<S::Ok, S::Error> {
+        s.serialize_f64(value.get::<radian>())
+    }
+
+    pub fn deserialize<'de, D: Deserializer<'de>>(d: D) -> Result<Angle, D::Error> {
+        f64::deserialize(d).map(Angle::new::<radian>)
+    }
+}
diff --git a/crates/bone-types/src/lib.rs b/crates/bone-types/src/lib.rs
new file mode 100644
index 0000000..abac6c7
--- /dev/null
+++ b/crates/bone-types/src/lib.rs
@@ -0,0 +1,340 @@
+pub use uom::si::angle::{degree, radian};
+pub use uom::si::f64::{Angle, Length};
+pub use uom::si::length::millimeter;
+
+pub mod dimensioned_serde;
+pub mod schema;
+pub mod solver;
+pub mod space;
+
+pub use schema::{SchemaHeader, SchemaVersion};
+pub use solver::{
+    BudgetCeiling, DegreesOfFreedom, NewtonDamping, NewtonStepTolerance, Parameter, ParameterIndex,
+    ParentIndex, ResidualIndex, SketchItemId, SolverResidual, SolverSeed,
+};
+pub use space::{Point2, Point3, SketchPlaneBasis, UnitVec2, UnitVec3, Vec2};
+
+#[cfg(feature = "testing")]
+pub mod testing;
+
+#[derive(Debug, thiserror::Error)]
+pub enum TypesError {
+    #[error("sketch plane axes not orthogonal: |x·y|={0}")]
+    NonOrthogonalPlaneAxes(f64),
+    #[error("axis vector is zero-length")]
+    ZeroLengthAxis,
+}
+
+pub type Result<T, E = TypesError> = core::result::Result<T, E>;
+
+slotmap::new_key_type! {
+    pub struct FaceId;
+    pub struct EdgeId;
+    pub struct VertexId;
+    pub struct ShellId;
+    pub struct SolidId;
+    pub struct LoopId;
+    pub struct WireId;
+    pub struct FeatureId;
+    pub struct NodeId;
+    pub struct DocumentId;
+    pub struct SketchId;
+    pub struct SketchEntityId;
+    pub struct SketchRelationId;
+    pub struct SketchDimensionId;
+    pub struct SketchParameterId;
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, PartialOrd)]
+pub struct Tolerance(f64);
+
+impl Tolerance {
+    #[must_use]
+    pub const fn new(value: f64) -> Self {
+        Self(value)
+    }
+
+    #[must_use]
+    pub const fn value(self) -> f64 {
+        self.0
+    }
+}
+
+impl core::fmt::Display for Tolerance {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        write!(f, "tol={}", self.0)
+    }
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, PartialOrd)]
+pub struct AngleTolerance(f64);
+
+impl AngleTolerance {
+    pub const ZERO: Self = Self(0.0);
+
+    #[must_use]
+    pub const fn from_radians(value: f64) -> Self {
+        Self(value)
+    }
+
+    #[must_use]
+    pub const fn radians(self) -> f64 {
+        self.0
+    }
+
+    #[must_use]
+    pub fn from_arc_length(linear: Tolerance, radius: Length) -> Self {
+        let r = radius.get::<millimeter>().abs();
+        if r > linear.value() {
+            Self(linear.value() / r)
+        } else {
+            Self(linear.value())
+        }
+    }
+}
+
+impl core::fmt::Display for AngleTolerance {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        write!(f, "tol_angle={} rad", self.0)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::{
+        Angle, AngleTolerance, DegreesOfFreedom, DocumentId, EdgeId, FaceId, FeatureId, Length,
+        LoopId, NodeId, Parameter, Point2, Point3, ShellId, SketchDimensionId, SketchEntityId,
+        SketchId, SketchParameterId, SketchPlaneBasis, SketchRelationId, SolidId, SolverResidual,
+        Tolerance, UnitVec2, UnitVec3, Vec2, VertexId, WireId, degree, millimeter, radian,
+    };
+    use slotmap::Key;
+
+    #[test]
+    fn id_null_is_null() {
+        assert!(FaceId::null().is_null());
+        assert!(EdgeId::null().is_null());
+        assert!(VertexId::null().is_null());
+        assert!(ShellId::null().is_null());
+        assert!(SolidId::null().is_null());
+        assert!(LoopId::null().is_null());
+        assert!(WireId::null().is_null());
+        assert!(FeatureId::null().is_null());
+        assert!(NodeId::null().is_null());
+        assert!(DocumentId::null().is_null());
+        assert!(SketchId::null().is_null());
+        assert!(SketchEntityId::null().is_null());
+        assert!(SketchRelationId::null().is_null());
+        assert!(SketchDimensionId::null().is_null());
+        assert!(SketchParameterId::null().is_null());
+    }
+
+    #[test]
+    fn ids_are_distinct_types() {
+        let _f: FaceId = FaceId::null();
+        let _e: EdgeId = EdgeId::null();
+        let _s: SketchEntityId = SketchEntityId::null();
+        let _r: SketchRelationId = SketchRelationId::null();
+    }
+
+    #[test]
+    fn length_mm_roundtrip() {
+        let l = Length::new::<millimeter>(12.5);
+        assert!((l.get::<millimeter>() - 12.5).abs() < f64::EPSILON);
+    }
+
+    #[test]
+    fn angle_deg_converts_to_rad() {
+        let a = Angle::new::<degree>(180.0);
+        assert!((a.get::<radian>() - core::f64::consts::PI).abs() < 1e-12);
+    }
+
+    #[test]
+    fn tolerance_display() {
+        let t = Tolerance::new(1e-6);
+        assert_eq!(format!("{t}"), "tol=0.000001");
+        assert!((t.value() - 1e-6).abs() < f64::EPSILON);
+    }
+
+    #[test]
+    fn angle_tolerance_from_arc_length_scales_by_radius() {
+        let linear = Tolerance::new(1e-3);
+        let r = Length::new::<millimeter>(10.0);
+        let a = AngleTolerance::from_arc_length(linear, r);
+        assert!((a.radians() - 1e-4).abs() < 1e-15);
+    }
+
+    #[test]
+    fn angle_tolerance_degenerate_radius_falls_back_to_linear() {
+        let linear = Tolerance::new(1e-3);
+        let r = Length::new::<millimeter>(0.0);
+        let a = AngleTolerance::from_arc_length(linear, r);
+        assert!((a.radians() - 1e-3).abs() < 1e-15);
+    }
+
+    #[test]
+    fn angle_tolerance_zero_constant() {
+        assert!(AngleTolerance::ZERO.radians().abs() < f64::EPSILON);
+    }
+
+    #[test]
+    fn point2_minus_point2_is_vec2() {
+        let a = Point2::from_mm(10.0, 5.0);
+        let b = Point2::from_mm(4.0, 2.0);
+        let d: Vec2 = a - b;
+        assert!((d.x().get::<millimeter>() - 6.0).abs() < 1e-12);
+        assert!((d.y().get::<millimeter>() - 3.0).abs() < 1e-12);
+    }
+
+    #[test]
+    fn point2_plus_vec2_is_point2() {
+        let p = Point2::from_mm(1.0, 2.0);
+        let v = Vec2::from_mm(10.0, 20.0);
+        let q: Point2 = p + v;
+        assert!((q.x().get::<millimeter>() - 11.0).abs() < 1e-12);
+        assert!((q.y().get::<millimeter>() - 22.0).abs() < 1e-12);
+    }
+
+    #[test]
+    fn vec2_perp_and_dot() {
+        let v = Vec2::from_mm(3.0, 4.0);
+        assert!((v.norm_mm() - 5.0).abs() < 1e-12);
+        let p = v.perp_ccw();
+        assert!((p.x().get::<millimeter>() + 4.0).abs() < 1e-12);
+        assert!((p.y().get::<millimeter>() - 3.0).abs() < 1e-12);
+        assert!(v.dot_mm2(p).abs() < 1e-12);
+        assert!((v.cross_z_mm2(Vec2::from_mm(1.0, 0.0)) + 4.0).abs() < 1e-12);
+    }
+
+    #[test]
+    fn vec2_try_normalize_rejects_zero() {
+        let v = Vec2::zero();
+        let r = v.try_normalize(Tolerance::new(1e-9));
+        assert!(r.is_err());
+    }
+
+    #[test]
+    fn vec2_try_normalize_unit() {
+        let v = Vec2::from_mm(0.0, 7.0);
+        let Ok(u) = v.try_normalize(Tolerance::new(1e-9)) else {
+            panic!("nonzero");
+        };
+        let (ux, uy) = u.components();
+        assert!(ux.abs() < 1e-12);
+        assert!((uy - 1.0).abs() < 1e-12);
+    }
+
+    #[test]
+    fn unit_vec2_axes_are_orthogonal() {
+        let x = UnitVec2::x_axis();
+        let y = UnitVec2::y_axis();
+        assert!(x.dot(y).abs() < 1e-15);
+        assert_eq!(x.perp_ccw().components(), y.components());
+    }
+
+    #[test]
+    fn unit_vec2_rejects_zero_axis() {
+        let r = UnitVec2::try_from_components(0.0, 0.0, Tolerance::new(1e-9));
+        assert!(r.is_err());
+    }
+
+    #[test]
+    fn unit_vec2_into_vec_scales_by_length() {
+        let u = UnitVec2::x_axis();
+        let v = u.into_vec(Length::new::<millimeter>(3.0));
+        assert!((v.x().get::<millimeter>() - 3.0).abs() < 1e-12);
+        assert!(v.y().get::<millimeter>().abs() < 1e-12);
+    }
+
+    #[test]
+    fn vec2_scalar_and_negation() {
+        let v = Vec2::from_mm(3.0, -4.0);
+        let w = 2.0 * (-v);
+        assert!((w.x().get::<millimeter>() + 6.0).abs() < 1e-12);
+        assert!((w.y().get::<millimeter>() - 8.0).abs() < 1e-12);
+    }
+
+    #[test]
+    fn point3_roundtrip() {
+        let p = Point3::from_mm(1.5, 2.5, 3.5);
+        assert!((p.x().get::<millimeter>() - 1.5).abs() < 1e-12);
+        assert!((p.y().get::<millimeter>() - 2.5).abs() < 1e-12);
+        assert!((p.z().get::<millimeter>() - 3.5).abs() < 1e-12);
+    }
+
+    #[test]
+    fn sketch_plane_basis_accepts_orthogonal_axes() {
+        let Ok(basis) = SketchPlaneBasis::new(
+            Point3::origin(),
+            UnitVec3::x_axis(),
+            UnitVec3::y_axis(),
+            Tolerance::new(1e-9),
+        ) else {
+            panic!("x and y unit axes are orthogonal");
+        };
+        let (nx, ny, nz) = basis.normal().components();
+        assert!((nx).abs() < 1e-12);
+        assert!((ny).abs() < 1e-12);
+        assert!((nz - 1.0).abs() < 1e-12);
+    }
+
+    #[test]
+    fn sketch_plane_basis_reorthonormalizes_within_tolerance() {
+        let Ok(nearly) = UnitVec3::try_from_components(1e-10, 1.0, 0.0) else {
+            panic!("input vector is nonzero");
+        };
+        let Ok(basis) = SketchPlaneBasis::new(
+            Point3::origin(),
+            UnitVec3::x_axis(),
+            nearly,
+            Tolerance::new(1e-9),
+        ) else {
+            panic!("within tolerance");
+        };
+        let dot_after = basis.x_axis().dot(basis.y_axis()).abs();
+        assert!(
+            dot_after < 1e-15,
+            "re-orthonormalized y should be orthogonal, got {dot_after}"
+        );
+    }
+
+    #[test]
+    fn sketch_plane_basis_rejects_non_orthogonal_axes() {
+        let Ok(skew) = UnitVec3::try_from_components(1.0, 1.0, 0.0) else {
+            panic!("input vector is nonzero");
+        };
+        let result = SketchPlaneBasis::new(
+            Point3::origin(),
+            UnitVec3::x_axis(),
+            skew,
+            Tolerance::new(1e-9),
+        );
+        assert!(result.is_err());
+    }
+
+    #[test]
+    fn unit_vec3_rejects_zero_axis() {
+        let r = UnitVec3::try_from_components(0.0, 0.0, 0.0);
+        assert!(r.is_err());
+    }
+
+    #[test]
+    fn parameter_display_and_value() {
+        let p = Parameter::new(0.25);
+        assert_eq!(format!("{p}"), "param=0.25");
+        assert!((p.value() - 0.25).abs() < f64::EPSILON);
+    }
+
+    #[test]
+    fn degrees_of_freedom_display_and_value() {
+        let d = DegreesOfFreedom::new(7);
+        assert_eq!(format!("{d}"), "dof=7");
+        assert_eq!(d.value(), 7);
+    }
+
+    #[test]
+    fn solver_residual_display_and_value() {
+        let r = SolverResidual::new(1.5e-6);
+        assert_eq!(format!("{r}"), "res=0.0000015");
+        assert!((r.value() - 1.5e-6).abs() < f64::EPSILON);
+    }
+}
diff --git a/crates/bone-types/src/schema.rs b/crates/bone-types/src/schema.rs
new file mode 100644
index 0000000..18a03ac
--- /dev/null
+++ b/crates/bone-types/src/schema.rs
@@ -0,0 +1,53 @@
+use serde::{Deserialize, Serialize};
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, Serialize, Deserialize)]
+#[serde(deny_unknown_fields)]
+pub struct SchemaVersion {
+    pub major: u32,
+    pub minor: u32,
+}
+
+impl SchemaVersion {
+    #[must_use]
+    pub const fn new(major: u32, minor: u32) -> Self {
+        Self { major, minor }
+    }
+}
+
+impl core::fmt::Display for SchemaVersion {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        write!(f, "{}.{}", self.major, self.minor)
+    }
+}
+
+#[derive(Clone, Debug, PartialEq, Eq, Hash, Serialize, Deserialize)]
+#[serde(deny_unknown_fields)]
+pub struct SchemaHeader {
+    pub name: String,
+    pub version: SchemaVersion,
+}
+
+impl SchemaHeader {
+    pub const BONE_DOCUMENT_NAME: &'static str = "bone-document";
+    pub const BONE_DOCUMENT_MAJOR: u32 = 1;
+    pub const BONE_DOCUMENT_MINOR: u32 = 0;
+
+    #[must_use]
+    pub fn bone_document() -> Self {
+        Self {
+            name: Self::BONE_DOCUMENT_NAME.to_owned(),
+            version: SchemaVersion::new(Self::BONE_DOCUMENT_MAJOR, Self::BONE_DOCUMENT_MINOR),
+        }
+    }
+
+    #[must_use]
+    pub fn is_bone_document(&self) -> bool {
+        self.name == Self::BONE_DOCUMENT_NAME
+    }
+}
+
+impl core::fmt::Display for SchemaHeader {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        write!(f, "{} v{}", self.name, self.version)
+    }
+}
diff --git a/crates/bone-types/src/solver.rs b/crates/bone-types/src/solver.rs
new file mode 100644
index 0000000..167d82b
--- /dev/null
+++ b/crates/bone-types/src/solver.rs
@@ -0,0 +1,254 @@
+#[derive(Copy, Clone, Debug, PartialEq, PartialOrd, serde::Serialize, serde::Deserialize)]
+pub struct Parameter(f64);
+
+impl Parameter {
+    #[must_use]
+    pub const fn new(value: f64) -> Self {
+        Self(value)
+    }
+
+    #[must_use]
+    pub const fn value(self) -> f64 {
+        self.0
+    }
+}
+
+impl core::fmt::Display for Parameter {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        write!(f, "param={}", self.0)
+    }
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct DegreesOfFreedom(u32);
+
+impl DegreesOfFreedom {
+    #[must_use]
+    pub const fn new(value: u32) -> Self {
+        Self(value)
+    }
+
+    #[must_use]
+    pub const fn value(self) -> u32 {
+        self.0
+    }
+}
+
+impl core::fmt::Display for DegreesOfFreedom {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        write!(f, "dof={}", self.0)
+    }
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct SolverSeed(u64);
+
+impl SolverSeed {
+    pub const DEFAULT: Self = Self(0x5E61_D000_7E57_5EEDu64);
+
+    #[must_use]
+    pub const fn new(value: u64) -> Self {
+        Self(value)
+    }
+
+    #[must_use]
+    pub const fn value(self) -> u64 {
+        self.0
+    }
+}
+
+impl core::fmt::Display for SolverSeed {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        write!(f, "seed={:#018x}", self.0)
+    }
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct ParameterIndex(u32);
+
+impl ParameterIndex {
+    #[must_use]
+    pub const fn new(value: u32) -> Self {
+        Self(value)
+    }
+
+    #[must_use]
+    pub const fn value(self) -> u32 {
+        self.0
+    }
+
+    #[must_use]
+    pub const fn as_usize(self) -> usize {
+        self.0 as usize
+    }
+}
+
+impl core::fmt::Display for ParameterIndex {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        write!(f, "p#{}", self.0)
+    }
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct ParentIndex(u32);
+
+impl ParentIndex {
+    #[must_use]
+    pub const fn new(value: u32) -> Self {
+        Self(value)
+    }
+
+    #[must_use]
+    pub const fn value(self) -> u32 {
+        self.0
+    }
+
+    #[must_use]
+    pub const fn as_usize(self) -> usize {
+        self.0 as usize
+    }
+}
+
+impl core::fmt::Display for ParentIndex {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        write!(f, "parent#{}", self.0)
+    }
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct ResidualIndex(u32);
+
+impl ResidualIndex {
+    #[must_use]
+    pub const fn new(value: u32) -> Self {
+        Self(value)
+    }
+
+    #[must_use]
+    pub const fn value(self) -> u32 {
+        self.0
+    }
+
+    #[must_use]
+    pub const fn as_usize(self) -> usize {
+        self.0 as usize
+    }
+
+    #[must_use]
+    pub const fn next(self) -> Self {
+        Self(self.0.saturating_add(1))
+    }
+}
+
+impl core::fmt::Display for ResidualIndex {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        write!(f, "r#{}", self.0)
+    }
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub enum SketchItemId {
+    Relation(crate::SketchRelationId),
+    Dimension(crate::SketchDimensionId),
+    Entity(crate::SketchEntityId),
+}
+
+impl core::fmt::Display for SketchItemId {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        match self {
+            Self::Relation(id) => write!(f, "relation({id:?})"),
+            Self::Dimension(id) => write!(f, "dimension({id:?})"),
+            Self::Entity(id) => write!(f, "entity({id:?})"),
+        }
+    }
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, PartialOrd)]
+pub struct SolverResidual(f64);
+
+impl SolverResidual {
+    #[must_use]
+    pub const fn new(value: f64) -> Self {
+        Self(value)
+    }
+
+    #[must_use]
+    pub const fn value(self) -> f64 {
+        self.0
+    }
+}
+
+impl core::fmt::Display for SolverResidual {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        write!(f, "res={}", self.0)
+    }
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, PartialOrd)]
+pub struct NewtonDamping(f64);
+
+impl NewtonDamping {
+    pub const DEFAULT: Self = Self(1e-12);
+
+    #[must_use]
+    pub const fn new(value: f64) -> Self {
+        Self(value)
+    }
+
+    #[must_use]
+    pub const fn value(self) -> f64 {
+        self.0
+    }
+}
+
+impl core::fmt::Display for NewtonDamping {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        write!(f, "damping={}", self.0)
+    }
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, PartialOrd)]
+pub struct NewtonStepTolerance(f64);
+
+impl NewtonStepTolerance {
+    pub const DEFAULT: Self = Self(1e-14);
+
+    #[must_use]
+    pub const fn new(value: f64) -> Self {
+        Self(value)
+    }
+
+    #[must_use]
+    pub const fn value(self) -> f64 {
+        self.0
+    }
+}
+
+impl core::fmt::Display for NewtonStepTolerance {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        write!(f, "step_tol={}", self.0)
+    }
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct BudgetCeiling(core::time::Duration);
+
+impl BudgetCeiling {
+    pub const FRAME_16MS: Self = Self(core::time::Duration::from_millis(16));
+
+    #[must_use]
+    pub const fn new(duration: core::time::Duration) -> Self {
+        Self(duration)
+    }
+
+    #[must_use]
+    pub const fn duration(self) -> core::time::Duration {
+        self.0
+    }
+}
+
+impl core::fmt::Display for BudgetCeiling {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        write!(f, "budget={:?}", self.0)
+    }
+}
diff --git a/crates/bone-types/src/space.rs b/crates/bone-types/src/space.rs
new file mode 100644
index 0000000..109625f
--- /dev/null
+++ b/crates/bone-types/src/space.rs
@@ -0,0 +1,539 @@
+use nalgebra::{Point2 as NPoint2, Point3 as NPoint3, Unit, Vector2 as NVec2, Vector3 as NVec3};
+use serde::{Deserialize, Serialize};
+use uom::si::f64::Length;
+use uom::si::length::millimeter;
+
+use crate::{Result, Tolerance, TypesError};
+
+#[must_use]
+fn mm(value: f64) -> Length {
+    Length::new::<millimeter>(value)
+}
+
+#[derive(Copy, Clone, PartialEq, Serialize, Deserialize)]
+#[serde(from = "Point2Wire", into = "Point2Wire")]
+pub struct Point2(NPoint2<f64>);
+
+#[derive(Copy, Clone, Debug, PartialEq, Serialize, Deserialize)]
+#[serde(rename = "Point2", deny_unknown_fields)]
+struct Point2Wire {
+    x: f64,
+    y: f64,
+}
+
+impl From<Point2> for Point2Wire {
+    fn from(p: Point2) -> Self {
+        Self { x: p.0.x, y: p.0.y }
+    }
+}
+
+impl From<Point2Wire> for Point2 {
+    fn from(w: Point2Wire) -> Self {
+        Self(NPoint2::new(w.x, w.y))
+    }
+}
+
+impl Point2 {
+    #[must_use]
+    pub fn origin() -> Self {
+        Self(NPoint2::origin())
+    }
+
+    #[must_use]
+    pub fn from_mm(x: f64, y: f64) -> Self {
+        Self(NPoint2::new(x, y))
+    }
+
+    #[must_use]
+    pub fn from_lengths(x: Length, y: Length) -> Self {
+        Self(NPoint2::new(x.get::<millimeter>(), y.get::<millimeter>()))
+    }
+
+    #[must_use]
+    pub fn x(self) -> Length {
+        mm(self.0.x)
+    }
+
+    #[must_use]
+    pub fn y(self) -> Length {
+        mm(self.0.y)
+    }
+
+    #[must_use]
+    pub fn coords_mm(self) -> (f64, f64) {
+        (self.0.x, self.0.y)
+    }
+}
+
+impl core::fmt::Debug for Point2 {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        write!(f, "Point2({} mm, {} mm)", self.0.x, self.0.y)
+    }
+}
+
+impl core::fmt::Display for Point2 {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        write!(f, "({} mm, {} mm)", self.0.x, self.0.y)
+    }
+}
+
+impl core::ops::Sub for Point2 {
+    type Output = Vec2;
+    fn sub(self, rhs: Self) -> Vec2 {
+        Vec2(self.0 - rhs.0)
+    }
+}
+
+impl core::ops::Add<Vec2> for Point2 {
+    type Output = Self;
+    fn add(self, rhs: Vec2) -> Self {
+        Self(self.0 + rhs.0)
+    }
+}
+
+impl core::ops::Sub<Vec2> for Point2 {
+    type Output = Self;
+    fn sub(self, rhs: Vec2) -> Self {
+        Self(self.0 - rhs.0)
+    }
+}
+
+#[derive(Copy, Clone, PartialEq, Serialize, Deserialize)]
+#[serde(from = "Vec2Wire", into = "Vec2Wire")]
+pub struct Vec2(NVec2<f64>);
+
+#[derive(Copy, Clone, Debug, PartialEq, Serialize, Deserialize)]
+#[serde(rename = "Vec2", deny_unknown_fields)]
+struct Vec2Wire {
+    x: f64,
+    y: f64,
+}
+
+impl From<Vec2> for Vec2Wire {
+    fn from(v: Vec2) -> Self {
+        Self { x: v.0.x, y: v.0.y }
+    }
+}
+
+impl From<Vec2Wire> for Vec2 {
+    fn from(w: Vec2Wire) -> Self {
+        Self(NVec2::new(w.x, w.y))
+    }
+}
+
+impl Vec2 {
+    #[must_use]
+    pub fn zero() -> Self {
+        Self(NVec2::zeros())
+    }
+
+    #[must_use]
+    pub fn from_mm(x: f64, y: f64) -> Self {
+        Self(NVec2::new(x, y))
+    }
+
+    #[must_use]
+    pub fn from_lengths(x: Length, y: Length) -> Self {
+        Self(NVec2::new(x.get::<millimeter>(), y.get::<millimeter>()))
+    }
+
+    #[must_use]
+    pub fn x(self) -> Length {
+        mm(self.0.x)
+    }
+
+    #[must_use]
+    pub fn y(self) -> Length {
+        mm(self.0.y)
+    }
+
+    #[must_use]
+    pub fn coords_mm(self) -> (f64, f64) {
+        (self.0.x, self.0.y)
+    }
+
+    #[must_use]
+    pub fn dot_mm2(self, other: Self) -> f64 {
+        self.0.dot(&other.0)
+    }
+
+    #[must_use]
+    pub fn cross_z_mm2(self, other: Self) -> f64 {
+        self.0.x * other.0.y - self.0.y * other.0.x
+    }
+
+    #[must_use]
+    pub fn norm_squared_mm2(self) -> f64 {
+        self.0.norm_squared()
+    }
+
+    #[must_use]
+    pub fn norm_mm(self) -> f64 {
+        self.0.norm()
+    }
+
+    #[must_use]
+    pub fn norm(self) -> Length {
+        mm(self.0.norm())
+    }
+
+    #[must_use]
+    pub fn perp_ccw(self) -> Self {
+        Self(NVec2::new(-self.0.y, self.0.x))
+    }
+
+    pub fn try_normalize(self, tolerance: Tolerance) -> Result<UnitVec2> {
+        Unit::try_new(self.0, tolerance.value())
+            .map(UnitVec2)
+            .ok_or(TypesError::ZeroLengthAxis)
+    }
+}
+
+impl core::fmt::Debug for Vec2 {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        write!(f, "Vec2({} mm, {} mm)", self.0.x, self.0.y)
+    }
+}
+
+impl core::fmt::Display for Vec2 {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        write!(f, "<{} mm, {} mm>", self.0.x, self.0.y)
+    }
+}
+
+impl core::ops::Add for Vec2 {
+    type Output = Self;
+    fn add(self, rhs: Self) -> Self {
+        Self(self.0 + rhs.0)
+    }
+}
+
+impl core::ops::Sub for Vec2 {
+    type Output = Self;
+    fn sub(self, rhs: Self) -> Self {
+        Self(self.0 - rhs.0)
+    }
+}
+
+impl core::ops::Mul<f64> for Vec2 {
+    type Output = Self;
+    fn mul(self, rhs: f64) -> Self {
+        Self(self.0 * rhs)
+    }
+}
+
+impl core::ops::Mul<Vec2> for f64 {
+    type Output = Vec2;
+    fn mul(self, rhs: Vec2) -> Vec2 {
+        Vec2(rhs.0 * self)
+    }
+}
+
+impl core::ops::Neg for Vec2 {
+    type Output = Self;
+    fn neg(self) -> Self {
+        Self(-self.0)
+    }
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, Serialize, Deserialize)]
+#[serde(try_from = "UnitVec2Wire", into = "UnitVec2Wire")]
+pub struct UnitVec2(Unit<NVec2<f64>>);
+
+#[derive(Copy, Clone, Debug, PartialEq, Serialize, Deserialize)]
+#[serde(rename = "UnitVec2", deny_unknown_fields)]
+struct UnitVec2Wire {
+    x: f64,
+    y: f64,
+}
+
+impl From<UnitVec2> for UnitVec2Wire {
+    fn from(u: UnitVec2) -> Self {
+        Self { x: u.0.x, y: u.0.y }
+    }
+}
+
+impl TryFrom<UnitVec2Wire> for UnitVec2 {
+    type Error = TypesError;
+    fn try_from(w: UnitVec2Wire) -> Result<Self> {
+        Self::try_from_components(w.x, w.y, Tolerance::new(f64::EPSILON))
+    }
+}
+
+impl UnitVec2 {
+    #[must_use]
+    pub fn x_axis() -> Self {
+        Self(Unit::new_unchecked(NVec2::new(1.0, 0.0)))
+    }
+
+    #[must_use]
+    pub fn y_axis() -> Self {
+        Self(Unit::new_unchecked(NVec2::new(0.0, 1.0)))
+    }
+
+    #[must_use]
+    pub fn new_unchecked(x: f64, y: f64) -> Self {
+        Self(Unit::new_unchecked(NVec2::new(x, y)))
+    }
+
+    pub fn try_from_components(x: f64, y: f64, tolerance: Tolerance) -> Result<Self> {
+        Unit::try_new(NVec2::new(x, y), tolerance.value())
+            .map(Self)
+            .ok_or(TypesError::ZeroLengthAxis)
+    }
+
+    #[must_use]
+    pub fn components(self) -> (f64, f64) {
+        (self.0.x, self.0.y)
+    }
+
+    #[must_use]
+    pub fn dot(self, other: Self) -> f64 {
+        self.0.dot(&other.0)
+    }
+
+    #[must_use]
+    pub fn perp_ccw(self) -> Self {
+        Self(Unit::new_unchecked(NVec2::new(-self.0.y, self.0.x)))
+    }
+
+    #[must_use]
+    pub fn into_vec(self, length: Length) -> Vec2 {
+        let mm_value = length.get::<millimeter>();
+        Vec2(self.0.into_inner() * mm_value)
+    }
+}
+
+impl core::fmt::Display for UnitVec2 {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        write!(f, "[{}, {}]", self.0.x, self.0.y)
+    }
+}
+
+#[derive(Copy, Clone, PartialEq, Serialize, Deserialize)]
+#[serde(from = "Point3Wire", into = "Point3Wire")]
+pub struct Point3(NPoint3<f64>);
+
+#[derive(Copy, Clone, Debug, PartialEq, Serialize, Deserialize)]
+#[serde(rename = "Point3", deny_unknown_fields)]
+struct Point3Wire {
+    x: f64,
+    y: f64,
+    z: f64,
+}
+
+impl From<Point3> for Point3Wire {
+    fn from(p: Point3) -> Self {
+        Self {
+            x: p.0.x,
+            y: p.0.y,
+            z: p.0.z,
+        }
+    }
+}
+
+impl From<Point3Wire> for Point3 {
+    fn from(w: Point3Wire) -> Self {
+        Self(NPoint3::new(w.x, w.y, w.z))
+    }
+}
+
+impl Point3 {
+    #[must_use]
+    pub fn origin() -> Self {
+        Self(NPoint3::origin())
+    }
+
+    #[must_use]
+    pub fn from_mm(x: f64, y: f64, z: f64) -> Self {
+        Self(NPoint3::new(x, y, z))
+    }
+
+    #[must_use]
+    pub fn from_lengths(x: Length, y: Length, z: Length) -> Self {
+        Self(NPoint3::new(
+            x.get::<millimeter>(),
+            y.get::<millimeter>(),
+            z.get::<millimeter>(),
+        ))
+    }
+
+    #[must_use]
+    pub fn x(self) -> Length {
+        Length::new::<millimeter>(self.0.x)
+    }
+
+    #[must_use]
+    pub fn y(self) -> Length {
+        Length::new::<millimeter>(self.0.y)
+    }
+
+    #[must_use]
+    pub fn z(self) -> Length {
+        Length::new::<millimeter>(self.0.z)
+    }
+}
+
+impl core::fmt::Debug for Point3 {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        write!(
+            f,
+            "Point3({} mm, {} mm, {} mm)",
+            self.0.x, self.0.y, self.0.z,
+        )
+    }
+}
+
+impl core::fmt::Display for Point3 {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        write!(f, "({} mm, {} mm, {} mm)", self.0.x, self.0.y, self.0.z)
+    }
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, Serialize, Deserialize)]
+#[serde(try_from = "UnitVec3Wire", into = "UnitVec3Wire")]
+pub struct UnitVec3(Unit<NVec3<f64>>);
+
+#[derive(Copy, Clone, Debug, PartialEq, Serialize, Deserialize)]
+#[serde(rename = "UnitVec3", deny_unknown_fields)]
+struct UnitVec3Wire {
+    x: f64,
+    y: f64,
+    z: f64,
+}
+
+impl From<UnitVec3> for UnitVec3Wire {
+    fn from(u: UnitVec3) -> Self {
+        Self {
+            x: u.0.x,
+            y: u.0.y,
+            z: u.0.z,
+        }
+    }
+}
+
+impl TryFrom<UnitVec3Wire> for UnitVec3 {
+    type Error = TypesError;
+    fn try_from(w: UnitVec3Wire) -> Result<Self> {
+        Self::try_from_components(w.x, w.y, w.z)
+    }
+}
+
+impl UnitVec3 {
+    #[must_use]
+    pub fn x_axis() -> Self {
+        Self(Unit::new_unchecked(NVec3::new(1.0, 0.0, 0.0)))
+    }
+
+    #[must_use]
+    pub fn y_axis() -> Self {
+        Self(Unit::new_unchecked(NVec3::new(0.0, 1.0, 0.0)))
+    }
+
+    #[must_use]
+    pub fn z_axis() -> Self {
+        Self(Unit::new_unchecked(NVec3::new(0.0, 0.0, 1.0)))
+    }
+
+    pub fn try_from_components(x: f64, y: f64, z: f64) -> Result<Self> {
+        Unit::try_new(NVec3::new(x, y, z), f64::EPSILON)
+            .map(Self)
+            .ok_or(TypesError::ZeroLengthAxis)
+    }
+
+    #[must_use]
+    pub fn components(self) -> (f64, f64, f64) {
+        (self.0.x, self.0.y, self.0.z)
+    }
+
+    #[must_use]
+    pub fn dot(self, other: Self) -> f64 {
+        self.0.dot(&other.0)
+    }
+}
+
+impl core::fmt::Display for UnitVec3 {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        write!(f, "[{}, {}, {}]", self.0.x, self.0.y, self.0.z)
+    }
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, Serialize, Deserialize)]
+#[serde(try_from = "SketchPlaneBasisWire", into = "SketchPlaneBasisWire")]
+pub struct SketchPlaneBasis {
+    origin: Point3,
+    x: UnitVec3,
+    y: UnitVec3,
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, Serialize, Deserialize)]
+#[serde(rename = "SketchPlaneBasis", deny_unknown_fields)]
+struct SketchPlaneBasisWire {
+    origin: Point3,
+    x_axis: UnitVec3,
+    y_axis: UnitVec3,
+}
+
+impl From<SketchPlaneBasis> for SketchPlaneBasisWire {
+    fn from(b: SketchPlaneBasis) -> Self {
+        Self {
+            origin: b.origin,
+            x_axis: b.x,
+            y_axis: b.y,
+        }
+    }
+}
+
+impl TryFrom<SketchPlaneBasisWire> for SketchPlaneBasis {
+    type Error = TypesError;
+    fn try_from(w: SketchPlaneBasisWire) -> Result<Self> {
+        Self::new(w.origin, w.x_axis, w.y_axis, Tolerance::new(1e-9))
+    }
+}
+
+impl SketchPlaneBasis {
+    pub fn new(origin: Point3, x: UnitVec3, y: UnitVec3, tolerance: Tolerance) -> Result<Self> {
+        let dot = x.dot(y);
+        if dot.abs() > tolerance.value() {
+            return Err(TypesError::NonOrthogonalPlaneAxes(dot.abs()));
+        }
+        let y_projected = y.0.into_inner() - x.0.into_inner() * dot;
+        let y_orthonormal =
+            UnitVec3(Unit::try_new(y_projected, f64::EPSILON).ok_or(TypesError::ZeroLengthAxis)?);
+        Ok(Self {
+            origin,
+            x,
+            y: y_orthonormal,
+        })
+    }
+
+    #[must_use]
+    pub fn origin(self) -> Point3 {
+        self.origin
+    }
+
+    #[must_use]
+    pub fn x_axis(self) -> UnitVec3 {
+        self.x
+    }
+
+    #[must_use]
+    pub fn y_axis(self) -> UnitVec3 {
+        self.y
+    }
+
+    #[must_use]
+    pub fn normal(self) -> UnitVec3 {
+        UnitVec3(Unit::new_normalize(self.x.0.cross(&self.y.0)))
+    }
+}
+
+impl core::fmt::Display for SketchPlaneBasis {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        write!(
+            f,
+            "basis{{ o={}, x={}, y={} }}",
+            self.origin, self.x, self.y,
+        )
+    }
+}
diff --git a/crates/bone-types/src/testing.rs b/crates/bone-types/src/testing.rs
new file mode 100644
index 0000000..fe2c50a
--- /dev/null
+++ b/crates/bone-types/src/testing.rs
@@ -0,0 +1,55 @@
+use std::sync::Once;
+
+use tracing_subscriber::EnvFilter;
+
+static INIT: Once = Once::new();
+
+pub fn init_test_tracing() {
+    INIT.call_once(|| {
+        let filter = EnvFilter::try_from_default_env()
+            .unwrap_or_else(|_| EnvFilter::new("bone_types=debug"));
+        let _ = tracing_subscriber::fmt()
+            .with_env_filter(filter)
+            .with_test_writer()
+            .try_init();
+    });
+}
+
+#[derive(Debug, thiserror::Error)]
+#[error("non-deterministic output across two invocations of the same closure")]
+pub struct NonDeterministic;
+
+pub fn check_bit_identical<T: Eq, F: Fn() -> T>(f: F) -> Result<(), NonDeterministic> {
+    let first = f();
+    let second = f();
+    if first == second {
+        Ok(())
+    } else {
+        Err(NonDeterministic)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::{check_bit_identical, init_test_tracing};
+
+    #[test]
+    fn deterministic_closure_is_ok() {
+        assert!(check_bit_identical(|| 42_u64).is_ok());
+        assert!(check_bit_identical(|| (1_u32, 2_u32, 3_u32)).is_ok());
+    }
+
+    #[test]
+    fn non_deterministic_closure_is_err() {
+        let counter = std::sync::atomic::AtomicU64::new(0);
+        let result =
+            check_bit_identical(|| counter.fetch_add(1, std::sync::atomic::Ordering::Relaxed));
+        assert!(result.is_err());
+    }
+
+    #[test]
+    fn init_test_tracing_idempotent() {
+        init_test_tracing();
+        init_test_tracing();
+    }
+}
diff --git a/crates/bone-types/tests/snapshots/surface__dimensioned_surface.snap b/crates/bone-types/tests/snapshots/surface__dimensioned_surface.snap
new file mode 100644
index 0000000..4124f7c
--- /dev/null
+++ b/crates/bone-types/tests/snapshots/surface__dimensioned_surface.snap
@@ -0,0 +1,16 @@
+---
+source: crates/bone-types/tests/surface.rs
+expression: surface
+---
+length_mm_in=12.5
+length_mm_out=12.5
+length_m_out=0.0125
+angle_deg_in=180
+angle_deg_out=180
+angle_rad_out=3.1415926535897931
+tolerance_value=0.000001
+tolerance_display=tol=0.000001
+angle_tol_direct_rad=0.0001
+angle_tol_direct_display=tol_angle=0.0001 rad
+angle_tol_scaled_rad=0.0000001
+angle_tol_zero_rad=0
diff --git a/crates/bone-types/tests/snapshots/surface__id_debug_surface.snap b/crates/bone-types/tests/snapshots/surface__id_debug_surface.snap
new file mode 100644
index 0000000..fd6cdb4
--- /dev/null
+++ b/crates/bone-types/tests/snapshots/surface__id_debug_surface.snap
@@ -0,0 +1,23 @@
+---
+source: crates/bone-types/tests/surface.rs
+expression: surface
+---
+FaceId::null = FaceId(null)
+EdgeId::null = EdgeId(null)
+VertexId::null = VertexId(null)
+ShellId::null = ShellId(null)
+SolidId::null = SolidId(null)
+LoopId::null = LoopId(null)
+WireId::null = WireId(null)
+FeatureId::null = FeatureId(null)
+NodeId::null = NodeId(null)
+DocumentId::null = DocumentId(null)
+SketchId::null = SketchId(null)
+SketchEntityId::null = SketchEntityId(null)
+SketchRelationId::null = SketchRelationId(null)
+SketchDimensionId::null = SketchDimensionId(null)
+SketchParameterId::null = SketchParameterId(null)
+FaceId::default = FaceId(null)
+FaceId::null.is_null = true
+EdgeId::null.is_null = true
+SketchEntityId::null.is_null = true
diff --git a/crates/bone-types/tests/snapshots/surface__solver_surface.snap b/crates/bone-types/tests/snapshots/surface__solver_surface.snap
new file mode 100644
index 0000000..3a932a2
--- /dev/null
+++ b/crates/bone-types/tests/snapshots/surface__solver_surface.snap
@@ -0,0 +1,26 @@
+---
+source: crates/bone-types/tests/surface.rs
+expression: surface
+---
+parameter_debug   = Parameter(0.25)
+parameter_display = param=0.25
+parameter_value   = 0.25
+dof_debug         = DegreesOfFreedom(3)
+dof_display       = dof=3
+dof_value         = 3
+residual_debug    = SolverResidual(1.5e-6)
+residual_display  = res=0.0000015
+residual_value    = 0.0000015
+damping_display   = damping=0.000000000001
+damping_value     = 0.000000000001
+step_tol_display  = step_tol=0.00000000000001
+step_tol_value    = 0.00000000000001
+parent_debug      = ParentIndex(4)
+parent_display    = parent#4
+parent_value      = 4
+budget_debug      = BudgetCeiling(16ms)
+budget_display    = budget=16ms
+budget_duration   = 16ms
+item_relation     = relation(SketchRelationId(null))
+item_dimension    = dimension(SketchDimensionId(null))
+item_entity       = entity(SketchEntityId(null))
diff --git a/crates/bone-types/tests/snapshots/surface__space_surface.snap b/crates/bone-types/tests/snapshots/surface__space_surface.snap
new file mode 100644
index 0000000..3fec6b8
--- /dev/null
+++ b/crates/bone-types/tests/snapshots/surface__space_surface.snap
@@ -0,0 +1,31 @@
+---
+source: crates/bone-types/tests/surface.rs
+expression: surface
+---
+point2_debug   = Point2(1.5 mm, -2.5 mm)
+point2_display = (1.5 mm, -2.5 mm)
+point2_x_mm    = 1.5
+point2_y_mm    = -2.5
+vec2_debug     = Vec2(3 mm, 4 mm)
+vec2_display   = <3 mm, 4 mm>
+vec2_x_mm      = 3
+vec2_y_mm      = 4
+point2_sub_mm  = (9.000000000000002, 5)
+vec2_perp_ccw  = <-4 mm, 3 mm>
+vec2_norm_mm   = 5
+unit2_x_debug  = UnitVec2([[1.0, 0.0]])
+unit2_x_display= [1, 0]
+unit2_y_display= [0, 1]
+unit2_perp_y   = (-0, 1)
+point3_debug   = Point3(1 mm, 2 mm, 3 mm)
+point3_display = (1 mm, 2 mm, 3 mm)
+point3_x_mm    = 1
+point3_y_mm    = 2
+point3_z_mm    = 3
+unit_x_debug   = UnitVec3([[1.0, 0.0, 0.0]])
+unit_x_display = [1, 0, 0]
+unit_y_display = [0, 1, 0]
+unit_z_display = [0, 0, 1]
+basis_debug    = SketchPlaneBasis { origin: Point3(1 mm, 2 mm, 3 mm), x: UnitVec3([[1.0, 0.0, 0.0]]), y: UnitVec3([[0.0, 1.0, 0.0]]) }
+basis_display  = basis{ o=(1 mm, 2 mm, 3 mm), x=[1, 0, 0], y=[0, 1, 0] }
+basis_normal   = [0, 0, 1]
diff --git a/crates/bone-types/tests/surface.rs b/crates/bone-types/tests/surface.rs
new file mode 100644
index 0000000..a5286e2
--- /dev/null
+++ b/crates/bone-types/tests/surface.rs
@@ -0,0 +1,197 @@
+use bone_types::{
+    Angle, AngleTolerance, BudgetCeiling, DegreesOfFreedom, DocumentId, EdgeId, FaceId, FeatureId,
+    Length, LoopId, NewtonDamping, NewtonStepTolerance, NodeId, Parameter, ParentIndex, Point2,
+    Point3, ShellId, SketchDimensionId, SketchEntityId, SketchId, SketchItemId, SketchParameterId,
+    SketchPlaneBasis, SketchRelationId, SolidId, SolverResidual, Tolerance, UnitVec2, UnitVec3,
+    Vec2, VertexId, WireId, degree, millimeter, radian,
+};
+use slotmap::Key;
+
+#[test]
+fn id_debug_surface() {
+    let surface = format!(
+        "FaceId::null = {:?}\n\
+         EdgeId::null = {:?}\n\
+         VertexId::null = {:?}\n\
+         ShellId::null = {:?}\n\
+         SolidId::null = {:?}\n\
+         LoopId::null = {:?}\n\
+         WireId::null = {:?}\n\
+         FeatureId::null = {:?}\n\
+         NodeId::null = {:?}\n\
+         DocumentId::null = {:?}\n\
+         SketchId::null = {:?}\n\
+         SketchEntityId::null = {:?}\n\
+         SketchRelationId::null = {:?}\n\
+         SketchDimensionId::null = {:?}\n\
+         SketchParameterId::null = {:?}\n\
+         FaceId::default = {:?}\n\
+         FaceId::null.is_null = {}\n\
+         EdgeId::null.is_null = {}\n\
+         SketchEntityId::null.is_null = {}",
+        FaceId::null(),
+        EdgeId::null(),
+        VertexId::null(),
+        ShellId::null(),
+        SolidId::null(),
+        LoopId::null(),
+        WireId::null(),
+        FeatureId::null(),
+        NodeId::null(),
+        DocumentId::null(),
+        SketchId::null(),
+        SketchEntityId::null(),
+        SketchRelationId::null(),
+        SketchDimensionId::null(),
+        SketchParameterId::null(),
+        FaceId::default(),
+        FaceId::null().is_null(),
+        EdgeId::null().is_null(),
+        SketchEntityId::null().is_null(),
+    );
+    insta::assert_snapshot!(surface);
+}
+
+#[test]
+fn dimensioned_surface() {
+    use uom::si::length::meter;
+    let l = Length::new::<millimeter>(12.5);
+    let a = Angle::new::<degree>(180.0);
+    let t = Tolerance::new(1e-6);
+    let at_direct = AngleTolerance::from_radians(1e-4);
+    let at_scaled = AngleTolerance::from_arc_length(t, Length::new::<millimeter>(10.0));
+    let at_zero = AngleTolerance::ZERO;
+    let surface = format!(
+        "length_mm_in={mm_in}\n\
+         length_mm_out={mm_out}\n\
+         length_m_out={m_out}\n\
+         angle_deg_in=180\n\
+         angle_deg_out={deg_out}\n\
+         angle_rad_out={rad_out:.16}\n\
+         tolerance_value={tol_val}\n\
+         tolerance_display={tol_disp}\n\
+         angle_tol_direct_rad={atd_rad}\n\
+         angle_tol_direct_display={atd_disp}\n\
+         angle_tol_scaled_rad={ats_rad}\n\
+         angle_tol_zero_rad={atz_rad}",
+        mm_in = 12.5,
+        mm_out = l.get::<millimeter>(),
+        m_out = l.get::<meter>(),
+        deg_out = a.get::<degree>(),
+        rad_out = a.get::<radian>(),
+        tol_val = t.value(),
+        tol_disp = t,
+        atd_rad = at_direct.radians(),
+        atd_disp = at_direct,
+        ats_rad = at_scaled.radians(),
+        atz_rad = at_zero.radians(),
+    );
+    insta::assert_snapshot!(surface);
+}
+
+#[test]
+fn space_surface() {
+    let p2 = Point2::from_mm(1.5, -2.5);
+    let v2 = Vec2::from_mm(3.0, 4.0);
+    let p3 = Point3::from_mm(1.0, 2.0, 3.0);
+    let u2x = UnitVec2::x_axis();
+    let u2y = UnitVec2::y_axis();
+    let ux = UnitVec3::x_axis();
+    let uy = UnitVec3::y_axis();
+    let uz = UnitVec3::z_axis();
+    let Ok(basis) = SketchPlaneBasis::new(p3, ux, uy, Tolerance::new(1e-9)) else {
+        panic!("x and y unit axes are orthogonal");
+    };
+    let diff = Point2::from_mm(10.0, 7.0) - Point2::from_mm(1.0, 2.0);
+
+    let surface = format!(
+        "point2_debug   = {p2:?}\n\
+         point2_display = {p2}\n\
+         point2_x_mm    = {p2x}\n\
+         point2_y_mm    = {p2y}\n\
+         vec2_debug     = {v2:?}\n\
+         vec2_display   = {v2}\n\
+         vec2_x_mm      = {v2x}\n\
+         vec2_y_mm      = {v2y}\n\
+         point2_sub_mm  = ({dx}, {dy})\n\
+         vec2_perp_ccw  = {vperp}\n\
+         vec2_norm_mm   = {vnorm}\n\
+         unit2_x_debug  = {u2x:?}\n\
+         unit2_x_display= {u2x}\n\
+         unit2_y_display= {u2y}\n\
+         unit2_perp_y   = ({u2px}, {u2py})\n\
+         point3_debug   = {p3:?}\n\
+         point3_display = {p3}\n\
+         point3_x_mm    = {p3x}\n\
+         point3_y_mm    = {p3y}\n\
+         point3_z_mm    = {p3z}\n\
+         unit_x_debug   = {ux:?}\n\
+         unit_x_display = {ux}\n\
+         unit_y_display = {uy}\n\
+         unit_z_display = {uz}\n\
+         basis_debug    = {basis:?}\n\
+         basis_display  = {basis}\n\
+         basis_normal   = {normal}",
+        p2x = p2.x().get::<millimeter>(),
+        p2y = p2.y().get::<millimeter>(),
+        v2x = v2.x().get::<millimeter>(),
+        v2y = v2.y().get::<millimeter>(),
+        dx = diff.x().get::<millimeter>(),
+        dy = diff.y().get::<millimeter>(),
+        vperp = v2.perp_ccw(),
+        vnorm = v2.norm_mm(),
+        u2px = u2x.perp_ccw().components().0,
+        u2py = u2x.perp_ccw().components().1,
+        p3x = p3.x().get::<millimeter>(),
+        p3y = p3.y().get::<millimeter>(),
+        p3z = p3.z().get::<millimeter>(),
+        normal = basis.normal(),
+    );
+    insta::assert_snapshot!(surface);
+}
+
+#[test]
+fn solver_surface() {
+    let p = Parameter::new(0.25);
+    let d = DegreesOfFreedom::new(3);
+    let r = SolverResidual::new(1.5e-6);
+    let damp = NewtonDamping::DEFAULT;
+    let step_tol = NewtonStepTolerance::DEFAULT;
+    let parent = ParentIndex::new(4);
+    let budget = BudgetCeiling::FRAME_16MS;
+    let item_rel = SketchItemId::Relation(SketchRelationId::default());
+    let item_dim = SketchItemId::Dimension(SketchDimensionId::default());
+    let item_ent = SketchItemId::Entity(SketchEntityId::default());
+    let surface = format!(
+        "parameter_debug   = {p:?}\n\
+         parameter_display = {p}\n\
+         parameter_value   = {pv}\n\
+         dof_debug         = {d:?}\n\
+         dof_display       = {d}\n\
+         dof_value         = {dv}\n\
+         residual_debug    = {r:?}\n\
+         residual_display  = {r}\n\
+         residual_value    = {rv}\n\
+         damping_display   = {damp}\n\
+         damping_value     = {damp_v}\n\
+         step_tol_display  = {step_tol}\n\
+         step_tol_value    = {step_tol_v}\n\
+         parent_debug      = {parent:?}\n\
+         parent_display    = {parent}\n\
+         parent_value      = {parent_v}\n\
+         budget_debug      = {budget:?}\n\
+         budget_display    = {budget}\n\
+         budget_duration   = {budget_d:?}\n\
+         item_relation     = {item_rel}\n\
+         item_dimension    = {item_dim}\n\
+         item_entity       = {item_ent}",
+        pv = p.value(),
+        dv = d.value(),
+        rv = r.value(),
+        damp_v = damp.value(),
+        step_tol_v = step_tol.value(),
+        parent_v = parent.value(),
+        budget_d = budget.duration(),
+    );
+    insta::assert_snapshot!(surface);
+}
diff --git a/justfile b/justfile
new file mode 100644
index 0000000..041e8b7
--- /dev/null
+++ b/justfile
@@ -0,0 +1,37 @@
+default:
+    @just --list
+
+check:
+    cargo check --workspace --all-targets --all-features
+
+build:
+    cargo build --workspace
+
+build-release:
+    cargo build --workspace --release
+
+run:
+    cargo run -p bone-app
+
+test:
+    cargo test --workspace --all-features
+
+clippy:
+    cargo clippy --workspace --all-targets --all-features -- -D warnings
+
+snap:
+    cargo insta test --workspace --all-features
+
+snap-review:
+    cargo insta review
+
+snap-accept:
+    cargo insta accept
+
+fmt:
+    cargo fmt --all
+
+fmt-check:
+    cargo fmt --all -- --check
+
+lint: fmt-check clippy
diff --git a/rust-toolchain.toml b/rust-toolchain.toml
new file mode 100644
index 0000000..6360c18
--- /dev/null
+++ b/rust-toolchain.toml
@@ -0,0 +1,3 @@
+[toolchain]
+channel = "1.95.0"
+components = ["rustfmt", "clippy"]