diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 3edaa2a..7e04996 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -14,17 +14,27 @@ jobs:
       - uses: actions/setup-node@48b55a011bda9f5d6aeb4c2d9c7362e8dae4041e    # v6.4.0
         with: { node-version: 24, cache: pnpm }
       - uses: actions-rust-lang/setup-rust-toolchain@46268bd060767258de96ed93c1251119784f2ab6  # v1
-        with: { rustflags: "" }
+        with: { rustflags: "", components: "llvm-tools-preview" }
       - uses: Swatinem/rust-cache@e18b497796c12c097a38f9edb9d0641fb99eee32   # v2
       - run: rustup target add wasm32-unknown-unknown
       - name: Install wasm-pack 0.14.0
         run: cargo install wasm-pack --version 0.14.0 --locked
+      - name: Install cargo-llvm-cov
+        uses: taiki-e/install-action@25435dc8dd3baed7417e0c96d3fe89013a5b2e09  # v2.81.3
+        with: { tool: cargo-llvm-cov }
       - run: pnpm install --frozen-lockfile
       - run: pnpm -r build
       - run: pnpm -r lint
       - run: pnpm -r test
       - run: cargo build --manifest-path packages/codec/Cargo.toml --release
-      - run: cargo test --manifest-path packages/codec/Cargo.toml
+      - name: Rust coverage gate (Constitution X — 80% line coverage)
+        run: cargo llvm-cov --manifest-path packages/codec/Cargo.toml --all-features --fail-under-lines 80 --lcov --output-path rust-lcov.info
+      - name: Upload coverage to Codecov
+        uses: codecov/codecov-action@e79a6962e0d4c0c17b229090214935d2e33f8354  # v6.0.1
+        with:
+          files: rust-lcov.info,packages/codec/coverage/lcov.info
+          flags: rust,ts
+          fail_ci_if_error: false
       - name: Assert size budgets
         run: bash scripts/assert-size.sh
         working-directory: packages/codec
diff --git a/codecov.yml b/codecov.yml
new file mode 100644
index 0000000..d5def11
--- /dev/null
+++ b/codecov.yml
@@ -0,0 +1,16 @@
+coverage:
+  status:
+    patch:
+      rust:
+        informational: true
+    project:
+      rust:
+        informational: true
+
+flags:
+  rust:
+    paths:
+      - packages/codec/src/
+  ts:
+    paths:
+      - packages/codec/src/
diff --git a/packages/codec/vitest.config.ts b/packages/codec/vitest.config.ts
index 0cb1996..bf4a462 100644
--- a/packages/codec/vitest.config.ts
+++ b/packages/codec/vitest.config.ts
@@ -16,7 +16,8 @@ export default defineConfig({
       // enabled: true → `vitest run` always collects + gates coverage, so the
       // 80% threshold (Constitution X) is enforced by plain `pnpm -r test` in CI.
       enabled: true,
-      include: ['src/**'],
+      include: ['src/**/*.ts'],
+      reporter: ['text', 'lcov'],
       exclude: [
         'target/**',
         'pkg/**',