diff --git a/.tangled/workflows/test.yml b/.tangled/workflows/test.yml
index 430af44..6b76179 100644
--- a/.tangled/workflows/test.yml
+++ b/.tangled/workflows/test.yml
@@ -16,6 +16,7 @@ dependencies:
     - erlang_28
     - rebar3
     - nodejs_22
+    - git
 
 steps:
   # Tangled injects repository secrets into every step. Gleam 1.18 and later
@@ -28,6 +29,44 @@ steps:
       gleam format --check src test
       gleam test
 
+  - name: "Check every extension"
+    command: |
+      set -euo pipefail
+      : "${HEXPM_READ_API_KEY:?Configure HEXPM_READ_API_KEY in Tangled repository secrets}"
+      shopt -s globstar nullglob
+
+      check_package() {
+        local package="$1"
+        echo "Checking ${package}"
+        (
+          cd "${package}"
+          source_directories=(src)
+          [[ -d test ]] && source_directories+=(test)
+          [[ -d dev ]] && source_directories+=(dev)
+          gleam format --check "${source_directories[@]}"
+          if [[ -d test ]]; then
+            gleam test
+          else
+            gleam check
+          fi
+        )
+      }
+
+      extension_manifests=(extensions/*/gleam.toml)
+      (( ${#extension_manifests[@]} > 0 ))
+      for manifest in "${extension_manifests[@]}"; do
+        check_package "${manifest%/gleam.toml}"
+      done
+
+      example_manifests=()
+      for manifest in extensions/*/examples/**/gleam.toml; do
+        [[ "${manifest}" == */build/* ]] || example_manifests+=("${manifest}")
+      done
+      (( ${#example_manifests[@]} > 0 ))
+      for manifest in "${example_manifests[@]}"; do
+        check_package "${manifest%/gleam.toml}"
+      done
+
   - name: "Check every example"
     command: |
       set -euo pipefail
diff --git a/src/tiramisu/dev/extension.gleam b/src/tiramisu/dev/extension.gleam
index d1808c8..e045313 100644
--- a/src/tiramisu/dev/extension.gleam
+++ b/src/tiramisu/dev/extension.gleam
@@ -171,6 +171,7 @@
 ////     { heading: "Entities and Commands", subheading: "Entity Capabilities", name: "root", anchor: "root", summary: "Return the stable hierarchy root for this entity incarnation." },
 ////     { heading: "Entities and Commands", subheading: "Entity Capabilities", name: "object", anchor: "object", summary: "Find the object currently attached to a named entity slot." },
 ////     { heading: "Entities and Commands", subheading: "System Capabilities", name: "system_scene", anchor: "system_scene", summary: "Return the Savoiardi scene owned by this system's renderer." },
+////     { heading: "Entities and Commands", subheading: "System Capabilities", name: "system_active_camera", anchor: "system_active_camera", summary: "Return the active camera selected within this system's renderer." },
 ////     { heading: "Entities and Commands", subheading: "System Capabilities", name: "system_renderer", anchor: "system_renderer", summary: "Return the Savoiardi renderer owned by this system instance." },
 ////     { heading: "Entities and Commands", subheading: "System Capabilities", name: "find_entity", anchor: "find_entity", summary: "Find a currently mounted entity by id within this system's renderer." },
 ////     { heading: "Entities and Commands", subheading: "Command Types", name: "ObjectChange", anchor: "ObjectChange", summary: "Describes a change to one named object slot." },
@@ -348,6 +349,7 @@ import gleam/time/duration
 import gleam/time/timestamp
 import lustre
 import lustre/attribute.{type Attribute}
+import savoiardi/camera.{type Camera}
 import savoiardi/object.{type Object3D}
 import savoiardi/renderer.{type Renderer}
 import savoiardi/scene.{type Scene}
@@ -744,6 +746,7 @@ pub opaque type SystemContext {
   SystemContext(
     scene: Scene,
     renderer: Renderer,
+    active_camera: fn() -> Result(Camera, Nil),
     find_entity: fn(String) -> Result(Entity, Nil),
     entities: fn() -> List(Entity),
   )
@@ -810,6 +813,16 @@ pub fn system_renderer(context: SystemContext) -> Renderer {
   renderer
 }
 
+/// @superdocs Entities and Commands - System Capabilities
+/// Return the active camera selected within this system's renderer.
+///
+/// Returns `Error(Nil)` until a mounted camera component has claimed the
+/// renderer-local active-camera role.
+pub fn system_active_camera(context: SystemContext) -> Result(Camera, Nil) {
+  let SystemContext(active_camera:, ..) = context
+  active_camera()
+}
+
 /// @superdocs Entities and Commands - System Capabilities
 /// Find a currently mounted entity by id within this system's renderer.
 ///
@@ -2286,10 +2299,11 @@ pub fn new_context(entity: Entity, instance: Option(InstanceKey)) -> Context {
 pub fn new_system_context(
   scene: Scene,
   renderer: Renderer,
+  active_camera: fn() -> Result(Camera, Nil),
   find_entity: fn(String) -> Result(Entity, Nil),
   entities: fn() -> List(Entity),
 ) -> SystemContext {
-  SystemContext(scene:, renderer:, find_entity:, entities:)
+  SystemContext(scene:, renderer:, active_camera:, find_entity:, entities:)
 }
 
 fn option_to_result(value: Option(a)) -> Result(a, Nil) {
diff --git a/src/tiramisu/internal/runtime.gleam b/src/tiramisu/internal/runtime.gleam
index 1e9ce36..f2f2969 100644
--- a/src/tiramisu/internal/runtime.gleam
+++ b/src/tiramisu/internal/runtime.gleam
@@ -764,13 +764,8 @@ fn execute_command(
       )
     extension.SelectActiveCamera(entity) -> select_camera(runtime, entity)
     extension.ClearActiveCamera(entity) -> clear_camera(runtime, entity)
-    extension.MutateObject(entity, slot, mutate) -> {
-      case extension.object(entity, slot) {
-        Ok(value) -> mutate(value)
-        Error(Nil) -> Nil
-      }
-      runtime
-    }
+    extension.MutateObject(entity, slot, mutate) ->
+      mutate_slot(runtime, entity, slot, mutate)
     extension.OwnResource(entity, key, dispose) ->
       own_resource(runtime, origin, entity, key, dispose)
     extension.ReleaseResource(entity, key) ->
@@ -854,6 +849,26 @@ fn drain_pending(runtime: Runtime) -> Runtime {
   }
 }
 
+fn mutate_slot(
+  runtime: Runtime,
+  target: extension.Entity,
+  slot: String,
+  mutate: fn(object.Object3D) -> Nil,
+) -> Runtime {
+  let id = extension.entity_id(target)
+  let target_incarnation = extension.entity_incarnation(target)
+  case get_entity(runtime, id) {
+    Ok(entity) if entity.incarnation == target_incarnation -> {
+      case dict.get(entity.slots, slot) {
+        Ok(value) -> mutate(value.object)
+        Error(Nil) -> Nil
+      }
+      runtime
+    }
+    _ -> runtime
+  }
+}
+
 fn set_slot(
   runtime: Runtime,
   origin: Origin,
@@ -1313,6 +1328,7 @@ fn system_context(runtime: Runtime) -> extension.SystemContext {
   extension.new_system_context(
     runtime.scene,
     runtime.renderer,
+    fn() { active_camera(runtime) },
     fn(id) {
       get_entity(runtime, id)
       |> result.map(to_entity)
diff --git a/test/tiramisu/entity_runtime_test.gleam b/test/tiramisu/entity_runtime_test.gleam
index 3b4e607..9577807 100644
--- a/test/tiramisu/entity_runtime_test.gleam
+++ b/test/tiramisu/entity_runtime_test.gleam
@@ -60,6 +60,24 @@ fn tracked_component(name: String, label: String) -> extension.Component {
   |> extension.build_component
 }
 
+fn mutate_after_set_component() -> extension.Component {
+  let component_kind = kind("mutation-owner")
+  extension.component(kind: component_kind, mount: fn(context, _data) {
+    let entity = extension.entity(context)
+    #(Nil, [
+      extension.set_object(
+        entity,
+        "mesh",
+        test_support.tracked_object("mutation-object"),
+      ),
+      extension.mutate_object(entity, "mesh", fn(_object) {
+        test_support.record_disposal("mutated-after-set")
+      }),
+    ])
+  })
+  |> extension.build_component
+}
+
 fn scene_only_component() -> extension.Component {
   extension.component(
     kind: scene_kind("scene-only"),
@@ -149,6 +167,16 @@ fn root(children: List(scene_tree.Node)) -> scene_tree.Node {
   )
 }
 
+pub fn mutation_command_sees_slot_set_earlier_in_mount_test() {
+  test_support.reset_tracking()
+  let runtime = new_runtime([mutate_after_set_component()])
+  let entity = entity_node("entity", "scene", [value("mutation-owner")], [])
+
+  let _runtime = entity_runtime.reconcile(runtime, root([entity]))
+
+  test_support.disposal_count("mutated-after-set") |> should.equal(1)
+}
+
 pub fn slot_conflict_and_entity_teardown_dispose_once_test() {
   test_support.reset_tracking()
   let runtime =
diff --git a/test/tiramisu/system_runtime_test.gleam b/test/tiramisu/system_runtime_test.gleam
index 60b9992..65a3c79 100644
--- a/test/tiramisu/system_runtime_test.gleam
+++ b/test/tiramisu/system_runtime_test.gleam
@@ -44,6 +44,7 @@ fn context(entities: List(extension.Entity)) -> extension.SystemContext {
   extension.new_system_context(
     test_support.fake_scene(),
     test_support.fake_renderer(),
+    fn() { Error(Nil) },
     fn(_id) { Error(Nil) },
     fn() { entities },
   )