diff --git a/.github/workflows/native-bench.yml b/.github/workflows/native-bench.yml
index e5522e0..e98f90f 100644
--- a/.github/workflows/native-bench.yml
+++ b/.github/workflows/native-bench.yml
@@ -30,3 +30,11 @@ jobs:
       - run: python bench/mypyc_run.py --runs 5
         env:
           PYFUN_BIN: target/release/pyfun
+      # A multi-module project through the same build, checked against `pyfun run`.
+      - name: Build a project natively
+        run: |
+          target/release/pyfun build --native examples/modules/main.pyfun -o /tmp/modules-native
+          target/release/pyfun run examples/modules/main.pyfun > /tmp/plain.txt
+          python /tmp/modules-native > /tmp/native.txt
+          diff /tmp/plain.txt /tmp/native.txt
+          ls /tmp/modules-native
diff --git a/DESIGN.md b/DESIGN.md
index 0bd03af..a1e0156 100644
--- a/DESIGN.md
+++ b/DESIGN.md
@@ -440,8 +440,9 @@ redefinition rule (`--allow-redefinition-new --local-partial-types`): a Python l
 match arms can hold differently narrowed types in different arms, which the default
 first-assignment rule rejects. `pyfun build --native <file> -o <dir>` does the whole trip: it
 compiles natively, runs mypyc with those flags, leaves the extension and a `__main__.py` in `<dir>`
-(so the program runs as `python <dir>`), and needs `mypy` plus a C compiler in the environment
-(single file for now). With both, `bench/mypyc_run.py` builds every benchmark that way: 1.3x to 13x faster than the same file on CPython, and on the
+(so the program runs as `python <dir>`), and needs `mypy` plus a C compiler in the environment. A
+project builds the same way: every module is annotated from its own types and compiled by mypyc,
+and the shared `_pyfun_rt.py` stays Python, imported by the compiled modules like any other. With both, `bench/mypyc_run.py` builds every benchmark that way: 1.3x to 13x faster than the same file on CPython, and on the
 arithmetic-heavy one about 12x faster than hand-written Python. Native mode is opt-in and applies to every module of a project alike; the default emitter keeps
 `match`/`case` because it reads as the program was written. Every example and the whole end-to-end suite produce identical
 output in both modes.
diff --git a/README.md b/README.md
index 6c42185..2babf86 100644
--- a/README.md
+++ b/README.md
@@ -468,7 +468,7 @@ On the repository's benchmarks (CPython 3.12, `bench/mypyc_run.py`), the mypyc b
 13x faster than the same program as plain Python, and the arithmetic-heavy one runs about 12x
 faster than the hand-written Python it's measured against. `pyfun compile --native` alone, without
 mypyc, is already faster on match-heavy code, because CPython skips its pattern-matching
-machinery. Native builds are single-file for now; [`DESIGN.md` §5.6](https://github.com/simontreanor/Pyfun/blob/main/DESIGN.md)
+machinery. A multi-module project builds the same way. [`DESIGN.md` §5.6](https://github.com/simontreanor/Pyfun/blob/main/DESIGN.md)
 has the details.
 
 ---
diff --git a/ROADMAP.md b/ROADMAP.md
index 8b72f57..ccaa35a 100644
--- a/ROADMAP.md
+++ b/ROADMAP.md
@@ -515,7 +515,8 @@ map_build 1.64× vs hand-written.
   the same job measured mypyc at 1.56x / **13.2x** / 1.32x faster than the native file on CPython,
   landing at **1.40x / 0.08x / 1.34x of hand-written Python**: collatz, all int arithmetic, runs
   about 12x faster than the hand-written baseline. `pyfun build --native <file> -o <dir>` (compile, mypyc, runnable
-  directory) followed too. Remaining, in order: typed emit and `build` for projects and for block-local functions, or-patterns that bind
+  directory) followed too. Typed emit and `build` for projects followed too. Remaining, in
+  order: typed emit for block-local functions and for block-local functions, or-patterns that bind
   names, then the closure/`seq`/`_pyfun_rt` audit (project mode followed the same night) —
   the checker knows every binding's inferred type, so the emitter could produce fully annotated
   Python whose annotations cannot lie, then compile it with mypyc into a C extension — native speed
diff --git a/src/lowering/mod.rs b/src/lowering/mod.rs
index 383025a..6d62de3 100644
--- a/src/lowering/mod.rs
+++ b/src/lowering/mod.rs
@@ -204,9 +204,11 @@ pub fn lower_in_project(
     float_literals: &HashSet<Span>,
     codecs: &crate::types::Codecs,
     native: bool,
+    binding_types: HashMap<Span, crate::types::Ty>,
 ) -> Result<LoweredModule, LowerError> {
     let mut lowerer = Lowerer::new(module);
     lowerer.native = native;
+    lowerer.binding_types = binding_types;
     lowerer.float_literals = float_literals.clone();
     lowerer.codecs = codecs.clone();
     lowerer.imported_modules = ctx.modules.clone();
@@ -809,12 +811,15 @@ impl Lowerer {
         // The Python spelling of each user type: a record is its class, a sum
         // type an alias over its constructors' classes (skipped when a
         // constructor shares the type's name, which the alias would shadow).
+        // Classes this module defines, plus the ones a project imports from the
+        // shared runtime (`Some`, `None_`, `Ok`, `Error`).
         let class_names: HashSet<String> = py
             .body
             .iter()
-            .filter_map(|s| match s {
-                PyStmt::ClassDef { name, .. } => Some(name.clone()),
-                _ => None,
+            .flat_map(|s| match s {
+                PyStmt::ClassDef { name, .. } => vec![name.clone()],
+                PyStmt::ImportFrom { module, names } if module == "_pyfun_rt" => names.clone(),
+                _ => vec![],
             })
             .collect();
         let mut spell: HashMap<String, String> = HashMap::new();
diff --git a/src/native_build.rs b/src/native_build.rs
index 2fde307..bfbeb2a 100644
--- a/src/native_build.rs
+++ b/src/native_build.rs
@@ -55,7 +55,7 @@ fn build(path: &Path, out: &Path) -> ExitCode {
     if let Ok(module) = pyfun::parse(&source)
         && crate::has_imports(&module)
     {
-        return fail("`build --native` supports a single file so far, not a project");
+        return build_project(path, out);
     }
     let python = match pyfun::compile_with(&source, PyTarget::default(), true) {
         Ok((py, notes)) => {
@@ -85,20 +85,64 @@ fn build(path: &Path, out: &Path) -> ExitCode {
     if let Err(e) = std::fs::write(&source_file, &python) {
         return fail(&format!("cannot write {}: {e}", source_file.display()));
     }
+    finish(path, out, &module, std::slice::from_ref(&module))
+}
+
+/// A project: every module compiled natively into `out`, and every one but the
+/// shared `_pyfun_rt.py` built by mypyc (the runtime stays Python, which the
+/// compiled modules import like any other module).
+fn build_project(path: &Path, out: &Path) -> ExitCode {
+    let entry = path.to_string_lossy().to_string();
+    let project = match crate::resolve_project(&entry) {
+        Ok(p) => p,
+        Err(code) => return code,
+    };
+    if !crate::check_project_ok(&project) {
+        return ExitCode::FAILURE;
+    }
+    let files = match crate::lower_project(&project, PyTarget::default(), true) {
+        Ok(f) => f,
+        Err(code) => return code,
+    };
+    if let Err(e) = std::fs::create_dir_all(out) {
+        return fail(&format!("cannot create {}: {e}", out.display()));
+    }
+    let mut modules = Vec::new();
+    for (name, source) in &files {
+        if let Err(e) = std::fs::write(out.join(name), source) {
+            return fail(&format!("cannot write {name}: {e}"));
+        }
+        if let Some(stem) = name.strip_suffix(".py")
+            && stem != "_pyfun_rt"
+        {
+            modules.push(stem.to_string());
+        }
+    }
+    let Some(entry_module) = pyfun::project::module_name_from_path(path).map(|m| m.to_lowercase())
+    else {
+        return fail("the file needs a name");
+    };
+    finish(path, out, &entry_module, &modules)
+}
+
+/// Run mypyc over `modules` in `out`, drop their sources so imports find the
+/// extensions, and write the `__main__.py` that imports `entry`.
+fn finish(path: &Path, out: &Path, entry: &str, modules: &[String]) -> ExitCode {
     let Some(interpreter) = crate::python_cmd() else {
         return fail("no Python interpreter found (set PYFUN_PYTHON)");
     };
     // Pyfun reuses a Python local across match arms, so a name can hold
     // differently narrowed types in different arms: mypy's newer redefinition
     // rule accepts that where the default first-assignment rule does not.
+    let mut args = vec![
+        "-m".to_string(),
+        "mypyc".to_string(),
+        "--allow-redefinition-new".to_string(),
+        "--local-partial-types".to_string(),
+    ];
+    args.extend(modules.iter().map(|m| format!("{m}.py")));
     let status = Command::new(&interpreter)
-        .args([
-            "-m",
-            "mypyc",
-            "--allow-redefinition-new",
-            "--local-partial-types",
-            &format!("{module}.py"),
-        ])
+        .args(&args)
         .current_dir(out)
         .status();
     match status {
@@ -113,10 +157,12 @@ fn build(path: &Path, out: &Path) -> ExitCode {
     }
     // The extension now sits beside the source; remove the source so an import
     // finds the extension, and give the directory an entry point.
-    let _ = std::fs::remove_file(&source_file);
+    for m in modules {
+        let _ = std::fs::remove_file(out.join(format!("{m}.py")));
+    }
     let _ = std::fs::remove_dir_all(out.join("build"));
     let main = format!(
-        "# Runs the mypyc-compiled Pyfun program `{module}`.\nimport {module}  # noqa: F401\n"
+        "# Runs the mypyc-compiled Pyfun program `{entry}`.\nimport {entry}  # noqa: F401\n"
     );
     if let Err(e) = std::fs::write(out.join("__main__.py"), main) {
         return fail(&format!("cannot write __main__.py: {e}"));
diff --git a/src/project/mod.rs b/src/project/mod.rs
index c29c7f2..3282d0c 100644
--- a/src/project/mod.rs
+++ b/src/project/mod.rs
@@ -295,6 +295,9 @@ pub fn compile_with(
     // (`DESIGN.md` §6.1), which the lowering context below needs.
     let mut exports: HashMap<String, crate::types::ModuleExports> = HashMap::new();
     let mut codecs_by_module: HashMap<String, crate::types::Codecs> = HashMap::new();
+    // Native mode annotates functions from their types (`DESIGN.md` §5.6).
+    let mut types_by_module: HashMap<String, HashMap<crate::lexer::Span, crate::types::Ty>> =
+        HashMap::new();
     let float_spans: HashMap<String, std::collections::HashSet<crate::lexer::Span>> = {
         let mut spans = HashMap::new();
         for module in &project.modules {
@@ -306,6 +309,12 @@ pub fn compile_with(
             let (_errors, types, module_exports, codecs) =
                 crate::types::check_module_collecting(&module.ast, &imports);
             spans.insert(module.name.clone(), crate::float_literal_spans(&types));
+            if native {
+                types_by_module.insert(
+                    module.name.clone(),
+                    types.iter().map(|t| (t.span, t.raw.clone())).collect(),
+                );
+            }
             codecs_by_module.insert(module.name.clone(), codecs);
             exports.insert(module.name.clone(), module_exports);
         }
@@ -374,7 +383,9 @@ pub fn compile_with(
         }
         let floats = float_spans.get(&module.name).unwrap_or(&no_floats);
         let codecs = codecs_by_module.get(&module.name).unwrap_or(&no_codecs);
-        let lowered = lowering::lower_in_project(&module.ast, &ctx, floats, codecs, native)?;
+        let binding_types = types_by_module.remove(&module.name).unwrap_or_default();
+        let lowered =
+            lowering::lower_in_project(&module.ast, &ctx, floats, codecs, native, binding_types)?;
         needs_runtime |= lowered.uses_runtime;
         notes.extend(
             lowered