diff --git a/Cargo.lock b/Cargo.lock
new file mode 100644
--- /dev/null
+++ b/Cargo.lock
@@ -0,0 +1,198 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
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+dependencies = [
+ "windows-link",
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diff --git a/Cargo.toml b/Cargo.toml
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -1,3 +1,38 @@
 [workspace]
 resolver = "2"
 members = ["crates/cli", "crates/core"]
+
+[workspace.lints.clippy]
+bool_comparison = "deny"
+duplicate_mod = "deny"
+inconsistent_struct_constructor = "deny"
+invalid_regex = "deny"
+mem_forget = "deny"
+mixed_case_hex_literals = "deny"
+suspicious_arithmetic_impl = "deny"
+uninit_assumed_init = "deny"
+suspicious_else_formatting = "deny"
+suspicious_op_assign_impl = "deny"
+suspicious_to_owned = "deny"
+cmp_owned = "deny"
+cmp_null = "deny"
+manual_map = "deny"
+
+too_many_arguments = "warn"
+cognitive_complexity = "warn"
+large_enum_variant = "warn"
+needless_borrow = "warn"
+needless_pass_by_value = "warn"
+redundant_clone = "warn"
+unnecessary_cast = "warn"
+inefficient_to_string = "warn"
+or_fun_call = "warn"
+unnecessary_to_owned = "warn"
+map_clone = "warn"
+flat_map_identity = "warn"
+needless_collect = "warn"
+vec_init_then_push = "warn"
+
+len_zero = "allow"
+range_plus_one = "allow"
+manual_range_contains = "allow"
diff --git a/rustfmt.toml b/rustfmt.toml
new file mode 100644
--- /dev/null
+++ b/rustfmt.toml
@@ -0,0 +1,6 @@
+max_width = 120
+fn_params_layout = "Compressed"
+single_line_if_else_max_width = 100
+single_line_let_else_max_width = 100
+use_field_init_shorthand = true
+struct_lit_width = 100
diff --git a/crates/cli/Cargo.toml b/crates/cli/Cargo.toml
--- a/crates/cli/Cargo.toml
+++ b/crates/cli/Cargo.toml
@@ -4,3 +4,6 @@
 edition = "2024"
 
 [dependencies]
+clap = { version = "4.5.40", features = ["derive"] }
+compiler_core = { path = "../core" }
+owo-colors = "4.2.1"
diff --git a/docs/src/SUMMARY.md b/docs/src/SUMMARY.md
--- a/docs/src/SUMMARY.md
+++ b/docs/src/SUMMARY.md
@@ -4,6 +4,7 @@
 
 # Internal planning
 
+- [Supported subset](internal/supported_subset.md)
 - [Specs]()
   - [Pipeline overview](internal/specs/00_pipeline_overview.md)
   - [Tree-sitter to AST](internal/specs/01_tree_sitter_ast.md)
diff --git a/crates/cli/src/args.rs b/crates/cli/src/args.rs
new file mode 100644
--- /dev/null
+++ b/crates/cli/src/args.rs
@@ -0,0 +1,20 @@
+use std::path::PathBuf;
+
+use clap::{Parser, Subcommand};
+
+#[derive(Debug, Parser)]
+#[command(name = "gleam-wasm")]
+#[command(about = "Compile Gleam source to WebAssembly")]
+pub struct Args {
+    #[command(subcommand)]
+    pub command: Command,
+}
+
+#[derive(Debug, Subcommand)]
+pub enum Command {
+    /// Run the compiler pipeline for a source file.
+    Compile {
+        /// Gleam source file to compile.
+        input: PathBuf,
+    },
+}
diff --git a/crates/cli/src/commands.rs b/crates/cli/src/commands.rs
new file mode 100644
--- /dev/null
+++ b/crates/cli/src/commands.rs
@@ -0,0 +1,39 @@
+use std::{fs, process::ExitCode};
+
+use compiler_core::{self, source::SourceFile, source::SourceFileId};
+
+use crate::{args::Command, echo};
+
+pub fn run(command: Command) -> ExitCode {
+    match command {
+        Command::Compile { input } => compile(input),
+    }
+}
+
+fn compile(input: std::path::PathBuf) -> ExitCode {
+    let source = match fs::read_to_string(&input) {
+        Ok(source) => source,
+        Err(error) => {
+            echo::error(format!("could not read {}: {error}", input.display()));
+            return ExitCode::FAILURE;
+        }
+    };
+
+    let source = SourceFile::with_path(SourceFileId(0), input.clone(), source);
+    match compiler_core::compile_source(source) {
+        Ok(output) => {
+            echo::status(
+                "compiled",
+                format!("{} ({} bytes)", input.display(), output.wasm.bytes.len()),
+            );
+            ExitCode::SUCCESS
+        }
+        Err(diagnostics) => {
+            echo::error(format!("could not compile {}", input.display()));
+            for diagnostic in diagnostics {
+                echo::diagnostic(diagnostic.message);
+            }
+            ExitCode::FAILURE
+        }
+    }
+}
diff --git a/crates/cli/src/echo.rs b/crates/cli/src/echo.rs
new file mode 100644
--- /dev/null
+++ b/crates/cli/src/echo.rs
@@ -0,0 +1,13 @@
+use owo_colors::OwoColorize;
+
+pub fn status(label: &str, message: impl AsRef<str>) {
+    eprintln!("{} {}", label.bright_magenta().bold(), message.as_ref());
+}
+
+pub fn error(message: impl AsRef<str>) {
+    eprintln!("{} {}", "error".bright_red().bold(), message.as_ref());
+}
+
+pub fn diagnostic(message: impl AsRef<str>) {
+    eprintln!("{} {}", "diagnostic".bright_yellow().bold(), message.as_ref());
+}
diff --git a/crates/cli/src/main.rs b/crates/cli/src/main.rs
--- a/crates/cli/src/main.rs
+++ b/crates/cli/src/main.rs
@@ -1,3 +1,10 @@
-fn main() {
-    println!("Hello, world!");
+mod args;
+mod commands;
+mod echo;
+
+use clap::Parser;
+
+fn main() -> std::process::ExitCode {
+    let args = args::Args::parse();
+    commands::run(args.command)
 }
diff --git a/crates/core/src/ast.rs b/crates/core/src/ast.rs
new file mode 100644
--- /dev/null
+++ b/crates/core/src/ast.rs
@@ -0,0 +1,11 @@
+use crate::{diagnostic::Diagnostics, parse::ConcreteSyntaxTree, source::Span};
+
+/// Compiler-owned AST module.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct Module {
+    pub span: Span,
+}
+
+pub fn build(cst: ConcreteSyntaxTree) -> Result<Module, Diagnostics> {
+    Ok(Module { span: cst.source.whole_span() })
+}
diff --git a/crates/core/src/diagnostic.rs b/crates/core/src/diagnostic.rs
new file mode 100644
--- /dev/null
+++ b/crates/core/src/diagnostic.rs
@@ -0,0 +1,61 @@
+use crate::source::Span;
+
+/// A stable diagnostic code.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub enum DiagnosticCode {
+    Unsupported,
+    ParseError,
+    AstError,
+    ResolveError,
+    TypeError,
+    LoweringError,
+    WasmError,
+}
+
+/// A source location attached to a diagnostic message.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct Label {
+    pub span: Span,
+    pub message: Option<String>,
+}
+
+impl Label {
+    pub fn primary(span: Span, message: impl Into<String>) -> Self {
+        Self { span, message: Some(message.into()) }
+    }
+}
+
+/// A compiler diagnostic that can be rendered by the CLI or tests.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct Diagnostic {
+    pub code: DiagnosticCode,
+    pub message: String,
+    pub labels: Vec<Label>,
+    pub notes: Vec<String>,
+}
+
+impl Diagnostic {
+    pub fn new(code: DiagnosticCode, message: impl Into<String>) -> Self {
+        Self { code, message: message.into(), labels: Vec::new(), notes: Vec::new() }
+    }
+
+    pub fn with_label(mut self, label: Label) -> Self {
+        self.labels.push(label);
+        self
+    }
+
+    pub fn with_note(mut self, note: impl Into<String>) -> Self {
+        self.notes.push(note.into());
+        self
+    }
+
+    pub fn unsupported(phase: &str, span: Span) -> Self {
+        Self::new(
+            DiagnosticCode::Unsupported,
+            format!("{phase} is not implemented for this input yet"),
+        )
+        .with_label(Label::primary(span, "unsupported here"))
+    }
+}
+
+pub type Diagnostics = Vec<Diagnostic>;
diff --git a/crates/core/src/ir.rs b/crates/core/src/ir.rs
new file mode 100644
--- /dev/null
+++ b/crates/core/src/ir.rs
@@ -0,0 +1,11 @@
+use crate::{diagnostic::Diagnostics, types::TypedModule};
+
+/// Core IR module.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct Module {
+    pub typed: TypedModule,
+}
+
+pub fn lower(module: TypedModule) -> Result<Module, Diagnostics> {
+    Ok(Module { typed: module })
+}
diff --git a/crates/core/src/lib.rs b/crates/core/src/lib.rs
--- a/crates/core/src/lib.rs
+++ b/crates/core/src/lib.rs
@@ -1,5 +1,37 @@
-pub fn add(left: u64, right: u64) -> u64 {
-    left + right
+pub mod ast;
+pub mod diagnostic;
+pub mod ir;
+pub mod parse;
+pub mod resolve;
+pub mod source;
+pub mod types;
+pub mod wasm;
+
+use diagnostic::Diagnostics;
+use source::{SourceFile, SourceFileId};
+
+/// Output from a full compile pipeline run.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct CompileOutput {
+    pub wasm: wasm::WasmModule,
+}
+
+/// Compile a Gleam source string.
+pub fn compile(source: impl Into<String>) -> Result<CompileOutput, Diagnostics> {
+    let source = SourceFile::new(SourceFileId(0), source);
+    compile_source(source)
+}
+
+/// Compile an already-created source file.
+pub fn compile_source(source: SourceFile) -> Result<CompileOutput, Diagnostics> {
+    let cst = parse::parse(source)?;
+    let ast = ast::build(cst)?;
+    let resolved = resolve::resolve(ast)?;
+    let typed = types::check(resolved)?;
+    let ir = ir::lower(typed)?;
+    let wasm = wasm::emit(ir)?;
+
+    Ok(CompileOutput { wasm })
 }
 
 #[cfg(test)]
@@ -7,8 +39,15 @@
     use super::*;
 
     #[test]
-    fn it_works() {
-        let result = add(2, 2);
-        assert_eq!(result, 4);
+    fn phases_are_wired_together() {
+        let output = compile("pub fn main() { Nil }").expect("pipeline should run");
+        assert!(output.wasm.bytes.is_empty());
+    }
+
+    #[test]
+    #[ignore = "requires parser, type checker, lowering, and WASM codegen"]
+    fn compiles_add_function_end_to_end() {
+        let output = compile("pub fn add(a, b) { a + b }").expect("compile add function");
+        assert!(!output.wasm.bytes.is_empty());
     }
 }
diff --git a/crates/core/src/parse.rs b/crates/core/src/parse.rs
new file mode 100644
--- /dev/null
+++ b/crates/core/src/parse.rs
@@ -0,0 +1,11 @@
+use crate::{diagnostic::Diagnostics, source::SourceFile};
+
+/// Tree-sitter concrete syntax tree for a source file.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct ConcreteSyntaxTree {
+    pub source: SourceFile,
+}
+
+pub fn parse(source: SourceFile) -> Result<ConcreteSyntaxTree, Diagnostics> {
+    Ok(ConcreteSyntaxTree { source })
+}
diff --git a/crates/core/src/resolve.rs b/crates/core/src/resolve.rs
new file mode 100644
--- /dev/null
+++ b/crates/core/src/resolve.rs
@@ -0,0 +1,11 @@
+use crate::{ast, diagnostic::Diagnostics};
+
+/// AST after name resolution.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct ResolvedModule {
+    pub ast: ast::Module,
+}
+
+pub fn resolve(module: ast::Module) -> Result<ResolvedModule, Diagnostics> {
+    Ok(ResolvedModule { ast: module })
+}
diff --git a/crates/core/src/source.rs b/crates/core/src/source.rs
new file mode 100644
--- /dev/null
+++ b/crates/core/src/source.rs
@@ -0,0 +1,41 @@
+use std::path::PathBuf;
+
+/// Stable identifier for a source file known to the compiler.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub struct SourceFileId(pub u32);
+
+/// A byte span inside a source file.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub struct Span {
+    pub file_id: SourceFileId,
+    pub start: usize,
+    pub end: usize,
+}
+
+impl Span {
+    pub fn new(file_id: SourceFileId, start: usize, end: usize) -> Self {
+        Self { file_id, start, end }
+    }
+}
+
+/// Source text plus the metadata needed to report diagnostics.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct SourceFile {
+    pub id: SourceFileId,
+    pub path: Option<PathBuf>,
+    pub text: String,
+}
+
+impl SourceFile {
+    pub fn new(id: SourceFileId, text: impl Into<String>) -> Self {
+        Self { id, path: None, text: text.into() }
+    }
+
+    pub fn with_path(id: SourceFileId, path: impl Into<PathBuf>, text: impl Into<String>) -> Self {
+        Self { id, path: Some(path.into()), text: text.into() }
+    }
+
+    pub fn whole_span(&self) -> Span {
+        Span::new(self.id, 0, self.text.len())
+    }
+}
diff --git a/crates/core/src/types.rs b/crates/core/src/types.rs
new file mode 100644
--- /dev/null
+++ b/crates/core/src/types.rs
@@ -0,0 +1,22 @@
+use crate::{diagnostic::Diagnostics, resolve::ResolvedModule};
+
+/// The initial set of compiler types.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum Type {
+    Int,
+    Float,
+    String,
+    Bool,
+    Nil,
+    Function { params: Vec<Type>, return_type: Box<Type> },
+}
+
+/// Resolved module annotated with type information.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct TypedModule {
+    pub resolved: ResolvedModule,
+}
+
+pub fn check(module: ResolvedModule) -> Result<TypedModule, Diagnostics> {
+    Ok(TypedModule { resolved: module })
+}
diff --git a/crates/core/src/wasm.rs b/crates/core/src/wasm.rs
new file mode 100644
--- /dev/null
+++ b/crates/core/src/wasm.rs
@@ -0,0 +1,11 @@
+use crate::{diagnostic::Diagnostics, ir};
+
+/// WebAssembly output from the backend.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct WasmModule {
+    pub bytes: Vec<u8>,
+}
+
+pub fn emit(_module: ir::Module) -> Result<WasmModule, Diagnostics> {
+    Ok(WasmModule { bytes: Vec::new() })
+}
diff --git a/docs/src/internal/supported_subset.md b/docs/src/internal/supported_subset.md
new file mode 100644
--- /dev/null
+++ b/docs/src/internal/supported_subset.md
@@ -0,0 +1,21 @@
+# Supported subset
+
+The compiler is currently a scaffold. It wires the major phases together, but it
+does not yet parse or compile real Gleam programs to executable WebAssembly.
+
+## Current behavior
+
+- The CLI can read a source file and run the compiler pipeline.
+- Core exposes separate phase modules for parsing, AST building, resolution,
+  typing, lowering, and WASM emission.
+- Source file IDs, spans, and diagnostics are defined.
+- WASM output is an empty placeholder until backend work begins.
+
+## Not supported yet
+
+- Tree-sitter Gleam parsing.
+- Real AST construction.
+- Name resolution.
+- Type checking or Gleam compiler type import.
+- Core IR lowering.
+- Executable `.wasm` generation.
diff --git a/docs/src/internal/tasks/00_project_scaffold.md b/docs/src/internal/tasks/00_project_scaffold.md
--- a/docs/src/internal/tasks/00_project_scaffold.md
+++ b/docs/src/internal/tasks/00_project_scaffold.md
@@ -6,13 +6,13 @@
 
 ## Tasks
 
-- [ ] Define crate modules for `source`, `parse`, `ast`, `resolve`, `types`,
+- [x] Define crate modules for `source`, `parse`, `ast`, `resolve`, `types`,
       `ir`, `wasm`, and `diagnostic`.
-- [ ] Add a top-level compile pipeline function that wires phases together.
-- [ ] Define a shared source file ID and span type.
-- [ ] Define a shared diagnostic type with code, message, and spans.
-- [ ] Add one end-to-end ignored or pending test for `pub fn add(a, b) { a + b }`.
-- [ ] Document the current supported subset in the docs.
+- [x] Add a top-level compile pipeline function that wires phases together.
+- [x] Define a shared source file ID and span type.
+- [x] Define a shared diagnostic type with code, message, and spans.
+- [x] Add one end-to-end ignored test for `pub fn add(a, b) { a + b }`.
+- [x] Document the current supported subset in the docs.
 
 ## Done when