diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index a0a7fd8..5b3d937 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -39,6 +39,10 @@ jobs:
           --health-retries 5
     steps:
       - uses: actions/checkout@v4
+        with:
+          # Full history so coverage:diff can resolve `origin/<base-ref>...HEAD`
+          # against the PR's base branch (ADR-020 L1).
+          fetch-depth: 0
       - uses: pnpm/action-setup@v4
         with:
           version: 9
@@ -58,12 +62,26 @@ jobs:
           DATABASE_URL: postgres://postgres:postgres@localhost:5432/cms_test
           PAYLOAD_SECRET: test-secret-do-not-use-in-prod
         run: pnpm test -- --coverage
+      # L2 — merge per-package lcovs to coverage/lcov.info + emit
+      # coverage/summary.json (ADR-020). Runs even on test failure so the
+      # artifact still captures what was produced.
+      - name: Coverage — aggregate (L2)
+        if: always()
+        run: pnpm coverage:aggregate
+      # L1 — cover-the-diff gate. Only meaningful on PRs (push-to-main has
+      # no base ref to diff against). Compares against the PR's base branch.
+      - name: Coverage — diff (L1)
+        if: github.event_name == 'pull_request'
+        run: pnpm coverage:diff -- --base origin/${{ github.base_ref }}
       - run: pnpm build
       - uses: actions/upload-artifact@v4
         if: always()
         with:
           name: coverage
-          path: "**/coverage/lcov.info"
+          path: |
+            coverage/lcov.info
+            coverage/summary.json
+            **/coverage/lcov.info
           retention-days: 7
 
   e2e:
diff --git a/.github/workflows/coverage-snapshot.yml b/.github/workflows/coverage-snapshot.yml
new file mode 100644
index 0000000..51924fc
--- /dev/null
+++ b/.github/workflows/coverage-snapshot.yml
@@ -0,0 +1,72 @@
+# Coverage snapshot — commits coverage/summary.json back to main after each
+# merge so the trend history accumulates in `git log -- coverage/summary.json`
+# (ADR-020 L2). This is the only workflow that needs `contents: write`.
+#
+# Skipped if summary.json hasn't changed since the previous commit.
+
+name: Coverage snapshot
+
+on:
+  push:
+    branches: [main]
+
+# Avoid two snapshot runs racing each other on rapid-fire merges.
+concurrency:
+  group: coverage-snapshot
+  cancel-in-progress: false
+
+permissions:
+  contents: write
+
+jobs:
+  snapshot:
+    runs-on: ubuntu-latest
+    services:
+      postgres:
+        image: postgres:16-alpine
+        env:
+          POSTGRES_PASSWORD: postgres
+          POSTGRES_USER: postgres
+          POSTGRES_DB: cms_test
+        ports:
+          - 5432:5432
+        options: >-
+          --health-cmd "pg_isready -U postgres"
+          --health-interval 10s
+          --health-timeout 5s
+          --health-retries 5
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          # Token with write scope so we can push the snapshot back.
+          token: ${{ secrets.GITHUB_TOKEN }}
+      - uses: pnpm/action-setup@v4
+        with:
+          version: 9
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 22
+          cache: pnpm
+      - run: pnpm install --frozen-lockfile
+      - name: Test with coverage
+        env:
+          DATABASE_URL: postgres://postgres:postgres@localhost:5432/cms_test
+          PAYLOAD_SECRET: test-secret-do-not-use-in-prod
+        run: pnpm test -- --coverage
+      - name: Aggregate
+        run: pnpm coverage:aggregate
+      - name: Commit summary.json if changed
+        run: |
+          if git diff --quiet --exit-code coverage/summary.json; then
+            echo "No summary.json change; skipping commit."
+            exit 0
+          fi
+          git config user.name "github-actions[bot]"
+          git config user.email "41898282+github-actions[bot]@users.noreply.github.com"
+          git add coverage/summary.json
+          git commit -m "chore(coverage): snapshot ${GITHUB_SHA::7}
+
+          Auto-generated by .github/workflows/coverage-snapshot.yml.
+
+          [skip ci]"
+          git push origin HEAD:main