diff --git a/.github/workflows/mutation.yml b/.github/workflows/mutation.yml
new file mode 100644
index 0000000..821ae29
--- /dev/null
+++ b/.github/workflows/mutation.yml
@@ -0,0 +1,90 @@
+name: Mutation Testing
+
+# Mutation testing is slow (it re-runs the test suite per mutant), so it runs
+# nightly and on demand — never on pull requests — to avoid PR latency. It is
+# informational/non-blocking: the score is reported to the run summary and the
+# HTML report is uploaded as an artifact.
+
+on:
+  schedule:
+    - cron: "0 4 * * *" # nightly, 04:00 UTC
+  workflow_dispatch:
+    inputs:
+      mutate:
+        description: "Optional glob to scope mutation (e.g. src/tokens/**/*.ts)"
+        required: false
+        type: string
+
+concurrency:
+  group: mutation-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  stryker:
+    name: Stryker (packages/pipeline)
+    runs-on: ubuntu-latest
+    timeout-minutes: 90
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Setup Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: "20"
+
+      - name: Install pnpm
+        uses: pnpm/action-setup@v4
+        with:
+          version: 9
+
+      - name: Install dependencies
+        run: pnpm install --frozen-lockfile
+
+      - name: Run Stryker mutation testing
+        working-directory: packages/pipeline
+        run: |
+          # Run from packages/pipeline so the pnpm-linked node_modules resolve in
+          # the Stryker sandbox; the config lives at the repo root.
+          if [ -n "${{ github.event.inputs.mutate }}" ]; then
+            npx stryker run ../../stryker.conf.json --mutate "${{ github.event.inputs.mutate }}"
+          else
+            npx stryker run ../../stryker.conf.json
+          fi
+
+      - name: Mutation score summary
+        if: always()
+        working-directory: packages/pipeline
+        run: |
+          node -e '
+            const fs = require("fs");
+            const path = "reports/mutation/mutation-report.json";
+            if (!fs.existsSync(path)) { console.log("No mutation report produced."); process.exit(0); }
+            const report = JSON.parse(fs.readFileSync(path, "utf-8"));
+            const counts = { Killed: 0, Survived: 0, Timeout: 0, NoCoverage: 0, RuntimeError: 0, CompileError: 0, Ignored: 0 };
+            for (const file of Object.values(report.files || {})) {
+              for (const m of file.mutants || []) counts[m.status] = (counts[m.status] || 0) + 1;
+            }
+            const detected = counts.Killed + counts.Timeout;
+            const valid = detected + counts.Survived + counts.NoCoverage;
+            const score = valid ? ((detected / valid) * 100).toFixed(2) : "n/a";
+            const out = [
+              "## Mutation testing (packages/pipeline)",
+              "",
+              `**Mutation score: ${score}%**` + (score === "n/a" ? "" : ` (target 80%, opt-in)`),
+              "",
+              "| Status | Count |",
+              "| --- | --- |",
+              ...Object.entries(counts).map(([k, v]) => `| ${k} | ${v} |`),
+            ].join("\n");
+            fs.appendFileSync(process.env.GITHUB_STEP_SUMMARY || "/dev/stdout", out + "\n");
+            console.log(`Mutation score: ${score}%`);
+          '
+
+      - name: Upload mutation report
+        if: always()
+        uses: actions/upload-artifact@v4
+        with:
+          name: mutation-report
+          path: packages/pipeline/reports/mutation/
+          if-no-files-found: ignore
+          retention-days: 14
diff --git a/README.md b/README.md
index 4a29f63..c34c9f5 100644
--- a/README.md
+++ b/README.md
@@ -241,6 +241,33 @@ Full catalog: `.claude/skills/README.md`
 ./scripts/setup-playwright.sh      # One-time browser engine setup
 ```
 
+### Mutation testing (Stryker)
+
+Mutation testing measures how well the test suite actually catches bugs. The
+framework's own TypeScript code (`packages/pipeline`) is mutation-tested via
+[Stryker](https://stryker-mutator.io/); `stryker.conf.json` lives at the repo
+root but **runs from `packages/pipeline`** so the pnpm-linked dependencies resolve
+inside Stryker's sandbox.
+
+```bash
+# Full run (slow — re-runs the suite per mutant)
+cd packages/pipeline && npx stryker run ../../stryker.conf.json
+
+# Scope to one area while iterating
+cd packages/pipeline && npx stryker run ../../stryker.conf.json --mutate "src/tokens/**/*.ts"
+```
+
+The HTML report is written to `packages/pipeline/reports/mutation/index.html`.
+
+- **Threshold (opt-in).** The target mutation score lives in
+  `.claude/pipeline.config.json` → `qualityGate.mutationScore`
+  (`{ "enabled": false, "threshold": 80, "blocking": false }`). It is **off by
+  default**; set `enabled: true` to treat it as a quality gate.
+- **CI.** `.github/workflows/mutation.yml` runs Stryker **nightly** and
+  **on demand** (`workflow_dispatch`) — never on PRs, to avoid latency. It is
+  informational: the score is posted to the run summary and the HTML report is
+  uploaded as an artifact.
+
 ### Pipeline Verification
 ```bash
 ./scripts/verify-tokens.sh         # Catch hardcoded design values
diff --git a/packages/pipeline/vitest.stryker.config.ts b/packages/pipeline/vitest.stryker.config.ts
new file mode 100644
index 0000000..cccc125
--- /dev/null
+++ b/packages/pipeline/vitest.stryker.config.ts
@@ -0,0 +1,17 @@
+import { configDefaults, defineConfig } from "vitest/config";
+
+/**
+ * Vitest config used by Stryker mutation runs (see ../../stryker.conf.json).
+ *
+ * Identical to vitest.config.ts except it excludes the end-to-end test, which
+ * reads a fixture at the repo root — outside the Stryker sandbox (cwd =
+ * packages/pipeline) — and would fail the initial test run. Unit tests provide
+ * the mutant-killing signal.
+ */
+export default defineConfig({
+  test: {
+    include: ["src/**/*.test.ts"],
+    environment: "node",
+    exclude: [...configDefaults.exclude, "**/__tests__/e2e.test.ts"],
+  },
+});
diff --git a/stryker.conf.json b/stryker.conf.json
new file mode 100644
index 0000000..322e7db
--- /dev/null
+++ b/stryker.conf.json
@@ -0,0 +1,28 @@
+{
+  "$schema": "https://raw.githubusercontent.com/stryker-mutator/stryker/master/packages/core/schema/stryker-schema.json",
+  "_comment": "Mutation testing for the framework's own TypeScript code in packages/pipeline. Run from packages/pipeline (see .github/workflows/mutation.yml and the README) so the pnpm-linked node_modules resolve inside the Stryker sandbox.",
+  "packageManager": "pnpm",
+  "plugins": ["@stryker-mutator/vitest-runner", "@stryker-mutator/typescript-checker"],
+  "testRunner": "vitest",
+  "vitest": { "configFile": "vitest.stryker.config.ts" },
+  "checkers": ["typescript"],
+  "tsconfigFile": "tsconfig.json",
+  "mutate": [
+    "src/**/*.ts",
+    "!src/**/*.test.ts",
+    "!src/**/__tests__/**",
+    "!src/**/*.d.ts",
+    "!src/**/index.ts",
+    "!src/**/cli.ts",
+    "!src/**/pipeline-cli.ts"
+  ],
+  "reporters": ["clear-text", "json", "html"],
+  "htmlReporter": { "fileName": "reports/mutation/index.html" },
+  "jsonReporter": { "fileName": "reports/mutation/mutation-report.json" },
+  "thresholds": { "high": 80, "low": 60, "break": null },
+  "concurrency": 2,
+  "timeoutMS": 60000,
+  "tempDirName": ".stryker-tmp",
+  "cleanTempDir": true,
+  "ignoreStatic": true
+}