diff --git a/.github/workflows/coverage.yml b/.github/workflows/coverage.yml
new file mode 100644
index 000000000..40c5739e6
--- /dev/null
+++ b/.github/workflows/coverage.yml
@@ -0,0 +1,33 @@
+name: Coverage
+
+# Measures coverage for every package on main. The numbers themselves are not
+# asserted here — the point is to populate the Nx Cloud cache, so that when a
+# PR re-measures its merge-base the `test:coverage` task hashes already exist
+# remotely and the summaries are restored instead of recomputed.
+on:
+  push:
+    branches: [main]
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+env:
+  NX_CLOUD_ACCESS_TOKEN: ${{ secrets.NX_CLOUD_ACCESS_TOKEN }}
+
+permissions:
+  contents: read
+
+jobs:
+  coverage:
+    name: Warm Coverage Cache
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+        with:
+          persist-credentials: false
+      - name: Setup Tools
+        uses: TanStack/config/.github/setup@190f659075ff0845850e330883eb26d7ffd0671f # main
+      - name: Run Coverage
+        run: pnpm run test:coverage:all
diff --git a/.github/workflows/pr.yml b/.github/workflows/pr.yml
index a83eb33a8..265e2136b 100644
--- a/.github/workflows/pr.yml
+++ b/.github/workflows/pr.yml
@@ -31,6 +31,78 @@ jobs:
           main-branch-name: main
       - name: Run Checks
         run: pnpm run test:pr
+  coverage:
+    name: Coverage
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+        with:
+          fetch-depth: 0
+          persist-credentials: false
+      - name: Setup Tools
+        uses: TanStack/config/.github/setup@190f659075ff0845850e330883eb26d7ffd0671f # main
+      - name: Get base and head commits for `nx affected`
+        uses: nrwl/nx-set-shas@3e9ad7370203c1e93d109be57f3b72eb0eb511b1 # v4.4.0
+        with:
+          main-branch-name: main
+      # Stashed outside the working tree because checking out the merge base
+      # would otherwise remove it — the script may be added or changed by the
+      # very PR under test. Both runs and the comparison use this copy, so all
+      # three agree on the version.
+      - name: Stage the coverage script
+        run: cp scripts/coverage-check.mjs "$RUNNER_TEMP/coverage-check.mjs"
+      # The set of packages to measure is decided once, here, so that the base
+      # run measures exactly what the head run does. `--with-target` keeps out
+      # projects that have no test:coverage script, so a docs-only PR resolves
+      # to an empty list rather than erroring in run-many.
+      - name: Resolve affected packages
+        id: affected
+        run: |
+          list=$(pnpm exec nx show projects --affected --with-target=test:coverage --exclude='examples/**,testing/**' --json | jq -r 'sort | join(",")')
+          echo "list=$list" >> "$GITHUB_OUTPUT"
+          echo "Measuring: ${list:-<none>}"
+      - name: Coverage for this PR
+        if: steps.affected.outputs.list != ''
+        # Via env, not `${{ }}` inside run: project names come from the PR's own
+        # files, so interpolating them into the shell would be injectable.
+        env:
+          PROJECTS: ${{ steps.affected.outputs.list }}
+        run: |
+          rm -rf packages/*/coverage
+          pnpm exec nx run-many --targets=test:coverage --projects="$PROJECTS"
+          node "$RUNNER_TEMP/coverage-check.mjs" --collect "$RUNNER_TEMP/cov-head"
+      # Re-measuring the merge-base is normally a set of Nx Cloud cache
+      # restores seeded by the Coverage workflow on main: the task hashes are
+      # identical, and `test:coverage` declares its coverage dir as an output,
+      # so the summaries come back without the tests re-running.
+      - name: Coverage for the merge base
+        if: steps.affected.outputs.list != ''
+        env:
+          PROJECTS: ${{ steps.affected.outputs.list }}
+        run: |
+          git checkout --force --detach "$NX_BASE"
+          # coverage/ is gitignored, so the checkout leaves this PR's results in
+          # place — without this the base collection silently re-reads head's
+          # numbers and every delta comes out as 0.00.
+          rm -rf packages/*/coverage
+          pnpm install --frozen-lockfile
+          # Packages added by this PR do not exist on the base commit.
+          printf '%s\n' "$PROJECTS" | tr ',' '\n' | sort > /tmp/wanted
+          pnpm exec nx show projects --with-target=test:coverage --json | jq -r '.[]' | sort > /tmp/available
+          base_list=$(comm -12 /tmp/wanted /tmp/available | paste -sd, -)
+          if [ -z "$base_list" ]; then
+            echo "No affected package exists on the base commit; nothing to compare against."
+            exit 0
+          fi
+          pnpm exec nx run-many --targets=test:coverage --projects="$base_list"
+          node "$RUNNER_TEMP/coverage-check.mjs" --collect "$RUNNER_TEMP/cov-base"
+      - name: Compare
+        if: steps.affected.outputs.list != ''
+        run: |
+          node "$RUNNER_TEMP/coverage-check.mjs" \
+            --base "$RUNNER_TEMP/cov-base" \
+            --head "$RUNNER_TEMP/cov-head"
   preview:
     name: Preview
     runs-on: ubuntu-latest
diff --git a/CLAUDE.md b/CLAUDE.md
index 4a34c8978..955c8f666 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -42,7 +42,7 @@ pnpm test:lib:dev          # Watch mode for unit tests
 pnpm test:oxlint           # Lint affected packages (oxlint, incl. type-aware)
 pnpm test:types            # Type check affected packages
 pnpm test:build            # Verify build artifacts with publint
-pnpm test:coverage         # Generate coverage reports
+# Coverage is CI-only — see the Coverage section below. Don't run it locally.
 pnpm test:knip             # Check for unused dependencies
 pnpm test:sherif           # Check pnpm workspace consistency
 pnpm test:docs             # Verify documentation links
@@ -306,7 +306,16 @@ Each package uses `exports` field in package.json for subpath exports (e.g., `@t
 
 - Unit tests in `*.test.ts` files alongside source
 - Uses Vitest with happy-dom for DOM testing
-- Test coverage via `pnpm test:coverage`
+- **Coverage is CI-only.** Don't run it locally and don't add it to local
+  gates — it is deliberately absent from `test`, `test:pr`, `test:ci` and the
+  git hooks. The `Coverage` job on each PR measures every affected package
+  twice, on the PR head and on its merge-base with `main`, and fails when a
+  metric drops more than 0.5pp between them. There is **no baseline file** —
+  don't reintroduce one, it was removed precisely because it needed manual
+  syncing and was platform-sensitive. The only remedy for a drop is tests.
+  `.github/workflows/coverage.yml` runs coverage on pushes to `main` purely to
+  warm the Nx Cloud cache so the base-side run is mostly cache restores. See
+  CONTRIBUTING.md.
 - **E2E tests are mandatory** — see E2E Testing section below
 
 ### E2E Testing (REQUIRED)
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 021698cae..609125b2c 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -68,7 +68,7 @@ All commands are run from the repo root. Nx handles affected detection and cachi
 | Run unit tests (affected)     | `pnpm test:lib`     |
 | Watch unit tests              | `pnpm test:lib:dev` |
 | Type-check (affected)         | `pnpm test:types`   |
-| Lint (affected)               | `pnpm test:eslint`  |
+| Lint (affected)               | `pnpm test:oxlint`  |
 | Verify build artifacts        | `pnpm test:build`   |
 | Format the repo               | `pnpm format`       |
 | Build (affected)              | `pnpm build`        |
@@ -80,6 +80,29 @@ All commands are run from the repo root. Nx handles affected detection and cachi
 
 Working on a single package? `cd packages/<pkg>` and use its scripts directly (`pnpm test:lib`, `pnpm test:types`, etc.).
 
+## Coverage
+
+**Coverage runs in CI only. It is not part of `pnpm test`, `pnpm test:pr`, or any git hook, and you are not expected to run it locally.**
+
+The `Coverage` job on every PR measures each affected package **twice** — once on your branch and once on its merge-base with `main` — and compares the two. A drop of more than 0.5 percentage points in any metric (statements, branches, functions, lines) fails the job. Packages your PR didn't affect are never measured.
+
+There is no baseline file to keep in sync, and nothing to update when a package is added or removed: both numbers come from the same job on the same runner. There are also no target percentages to hit — the gate only catches coverage getting _worse_ in what you touched, and never blocks a PR for being below some repo-wide bar.
+
+Read the numbers from the PR's Checks tab: open the `Coverage` job and its summary has a per-package table with deltas, on every run whether it passed or failed. It is not posted as a PR comment.
+
+Re-measuring the merge-base is usually close to free. A separate `Coverage` workflow runs `test:coverage:all` on every push to `main`, which populates the Nx Cloud cache; because `test:coverage` declares its `coverage/` directory as a task output, the base-side run generally restores cached summaries rather than re-running any tests.
+
+### If the job says coverage dropped
+
+Add tests covering the code you changed. That's the whole remedy — there is no number to override. If you genuinely deleted well-tested code and the drop is expected, say so in the PR and a maintainer can merge past the failing check.
+
+Two known limitations:
+
+- Uncovered `.tsx` files can't be remapped by the coverage provider and are dropped from the report with a `Failed to parse ... Excluding it from coverage` warning. `.tsx` files that tests _do_ load are measured normally, so the UI packages read higher than their real coverage.
+- `preact-ai-devtools`, `react-ai-devtools`, and `solid-ai-devtools` have no tests and sit at 0%.
+
+A package your PR adds shows as `new` and can't fail the job — there is no base-commit coverage to compare it against. The same is true for a package whose suite can't run on the base commit.
+
 ## TypeScript configuration
 
 There is a single `tsconfig.base.json` at the repo root with the shared `compilerOptions`. Every package extends it and overrides only what's unique to that package (e.g. `outDir`, JSX runtime, framework lib).
@@ -104,7 +127,7 @@ Tests are included in typecheck. `vite.config.ts` / `vitest.config.ts` are not 
 
 - Place tests under `packages/<pkg>/tests/` with the suffix `.test.ts` (or `.test.tsx` for JSX).
 - Vitest's defaults discover anything matching `**/*.{test,spec}.?(c|m)[jt]s?(x)` — no per-package config is needed.
-- Tests are typechecked by `tsc` and linted by ESLint.
+- Tests are typechecked by `tsc` and linted by oxlint.
 
 ## Adding E2E test coverage (required)
 
@@ -146,7 +169,7 @@ The defensive `ignore` list in `.changeset/config.json` blocks accidental public
 ## Pull request flow
 
 1. Push your branch and open a PR against `main`.
-2. CI runs: `pnpm test:pr` (sherif workspace check, knip dead-code, docs link verification, ESLint, unit tests, typecheck, build artifacts, build) + the full E2E suite.
+2. CI runs: `pnpm test:pr` (sherif workspace check, knip dead-code, docs link verification, oxlint, unit tests, typecheck, build artifacts, build), the `Coverage` regression gate, and the full E2E suite.
 3. Address review comments.
 4. A maintainer merges. Releases are cut via Changesets — your changeset entry lands in the next release.
 
diff --git a/package.json b/package.json
index 446793d8c..d9cb2e643 100644
--- a/package.json
+++ b/package.json
@@ -21,6 +21,7 @@
     "test:lib": "nx affected --targets=test:lib --exclude=examples/**,testing/**",
     "test:lib:dev": "pnpm test:lib && nx watch --all -- pnpm test:lib",
     "test:coverage": "nx affected --targets=test:coverage --exclude=examples/**,testing/**",
+    "test:coverage:all": "nx run-many --targets=test:coverage --exclude=examples/**,testing/**",
     "test:build": "nx affected --target=test:build --exclude=examples/**,testing/**",
     "test:types": "nx affected --targets=test:types --exclude=examples/**,testing/**",
     "test:knip": "knip",
diff --git a/packages/ai-acp/package.json b/packages/ai-acp/package.json
index 9da225b75..881aab573 100644
--- a/packages/ai-acp/package.json
+++ b/packages/ai-acp/package.json
@@ -38,6 +38,7 @@
     "lint:fix": "oxlint src --type-aware --fix",
     "test:build": "publint --strict",
     "test:oxlint": "oxlint src --type-aware",
+    "test:coverage": "vitest run --coverage --coverage.include='src/**' --coverage.reporter=text-summary --coverage.reporter=json-summary",
     "test:lib": "vitest run --testTimeout=30000",
     "test:lib:dev": "vitest --watch",
     "test:types": "tsc"
diff --git a/packages/ai-angular/package.json b/packages/ai-angular/package.json
index 0ae199a25..ef7b66c3d 100644
--- a/packages/ai-angular/package.json
+++ b/packages/ai-angular/package.json
@@ -46,6 +46,7 @@
   "scripts": {
     "clean": "premove ./build ./dist",
     "test:oxlint": "oxlint src --type-aware",
+    "test:coverage": "vitest run --coverage --coverage.include='src/**' --coverage.reporter=text-summary --coverage.reporter=json-summary",
     "test:lib": "vitest run",
     "test:lib:dev": "vitest",
     "test:types": "tsc",
diff --git a/packages/ai-anthropic/package.json b/packages/ai-anthropic/package.json
index 84a66285a..f3787c0e4 100644
--- a/packages/ai-anthropic/package.json
+++ b/packages/ai-anthropic/package.json
@@ -53,6 +53,7 @@
     "lint:fix": "oxlint src --type-aware --fix",
     "test:build": "publint --strict",
     "test:oxlint": "oxlint src --type-aware",
+    "test:coverage": "vitest run --coverage --coverage.include='src/**' --coverage.reporter=text-summary --coverage.reporter=json-summary",
     "test:lib": "vitest",
     "test:lib:dev": "pnpm test:lib --watch",
     "test:types": "tsc"
diff --git a/packages/ai-bedrock/package.json b/packages/ai-bedrock/package.json
index ad929a557..1eb8be9cf 100644
--- a/packages/ai-bedrock/package.json
+++ b/packages/ai-bedrock/package.json
@@ -28,6 +28,7 @@
     "lint:fix": "oxlint src --type-aware --fix",
     "test:build": "publint --strict",
     "test:oxlint": "oxlint src --type-aware",
+    "test:coverage": "vitest run --coverage --coverage.include='src/**' --coverage.reporter=text-summary --coverage.reporter=json-summary",
     "test:lib": "vitest run",
     "test:lib:dev": "pnpm test:lib --watch",
     "test:types": "tsc"
diff --git a/packages/ai-claude-code/package.json b/packages/ai-claude-code/package.json
index feed08af8..b97ccffd8 100644
--- a/packages/ai-claude-code/package.json
+++ b/packages/ai-claude-code/package.json
@@ -42,6 +42,7 @@
     "lint:fix": "oxlint src --type-aware --fix",
     "test:build": "publint --strict",
     "test:oxlint": "oxlint src --type-aware",
+    "test:coverage": "vitest run --coverage --coverage.include='src/**' --coverage.reporter=text-summary --coverage.reporter=json-summary",
     "test:lib": "vitest",
     "test:lib:dev": "pnpm test:lib --watch",
     "test:types": "tsc"
diff --git a/packages/ai-client/package.json b/packages/ai-client/package.json
index 81acd1427..7df06aa8c 100644
--- a/packages/ai-client/package.json
+++ b/packages/ai-client/package.json
@@ -53,9 +53,9 @@
     "clean": "premove ./build ./dist",
     "lint:fix": "oxlint src --type-aware --fix",
     "test:build": "publint --strict",
-    "test:coverage": "vitest run --coverage",
-    "test:coverage:watch": "vitest --coverage --watch",
     "test:oxlint": "oxlint src --type-aware",
+    "test:coverage": "vitest run --coverage --coverage.include='src/**' --coverage.reporter=text-summary --coverage.reporter=json-summary",
+    "test:coverage:watch": "vitest --coverage --watch",
     "test:lib": "vitest",
     "test:lib:dev": "pnpm test:lib --watch",
     "test:types": "tsc"
diff --git a/packages/ai-code-mode-snippets/package.json b/packages/ai-code-mode-snippets/package.json
index 727153954..c113bdd1d 100644
--- a/packages/ai-code-mode-snippets/package.json
+++ b/packages/ai-code-mode-snippets/package.json
@@ -44,6 +44,7 @@
     "lint:fix": "oxlint src --type-aware --fix",
     "test:build": "publint --strict",
     "test:oxlint": "oxlint src --type-aware",
+    "test:coverage": "vitest run --passWithNoTests --coverage --coverage.include='src/**' --coverage.reporter=text-summary --coverage.reporter=json-summary",
     "test:lib": "vitest --passWithNoTests",
     "test:lib:dev": "pnpm test:lib --watch",
     "test:types": "tsc",
diff --git a/packages/ai-code-mode/package.json b/packages/ai-code-mode/package.json
index ca4572c8d..c46505ec0 100644
--- a/packages/ai-code-mode/package.json
+++ b/packages/ai-code-mode/package.json
@@ -41,6 +41,7 @@
     "lint:fix": "oxlint src --type-aware --fix",
     "test:build": "publint --strict",
     "test:oxlint": "oxlint src --type-aware",
+    "test:coverage": "vitest run --coverage --coverage.include='src/**' --coverage.reporter=text-summary --coverage.reporter=json-summary",
     "test:lib": "vitest",
     "test:lib:dev": "pnpm test:lib --watch",
     "test:types": "tsc"
diff --git a/packages/ai-codex/package.json b/packages/ai-codex/package.json
index ac07eccb1..3cdb0c690 100644
--- a/packages/ai-codex/package.json
+++ b/packages/ai-codex/package.json
@@ -41,6 +41,7 @@
     "lint:fix": "oxlint src --type-aware --fix",
     "test:build": "publint --strict",
     "test:oxlint": "oxlint src --type-aware",
+    "test:coverage": "vitest run --coverage --coverage.include='src/**' --coverage.reporter=text-summary --coverage.reporter=json-summary",
     "test:lib": "vitest",
     "test:lib:dev": "pnpm test:lib --watch",
     "test:types": "tsc"
diff --git a/packages/ai-devtools/package.json b/packages/ai-devtools/package.json
index 87760494c..59c2242b2 100644
--- a/packages/ai-devtools/package.json
+++ b/packages/ai-devtools/package.json
@@ -73,6 +73,7 @@
     "lint:fix": "oxlint src --type-aware --fix",
     "test:build": "publint --strict",
     "test:oxlint": "oxlint src --type-aware",
+    "test:coverage": "vitest run --passWithNoTests --coverage --coverage.include='src/**' --coverage.reporter=text-summary --coverage.reporter=json-summary",
     "test:lib": "vitest --passWithNoTests",
     "test:lib:dev": "pnpm test:lib --watch",
     "test:types": "tsc"
diff --git a/packages/ai-durable-stream/package.json b/packages/ai-durable-stream/package.json
index 69f2161b8..95ba04d60 100644
--- a/packages/ai-durable-stream/package.json
+++ b/packages/ai-durable-stream/package.json
@@ -36,6 +36,7 @@
     "lint:fix": "oxlint src --type-aware --fix",
     "test:build": "publint --strict",
     "test:oxlint": "oxlint src --type-aware",
+    "test:coverage": "vitest run --coverage --coverage.include='src/**' --coverage.reporter=text-summary --coverage.reporter=json-summary",
     "test:lib": "vitest run",
     "test:lib:dev": "pnpm test:lib --watch",
     "test:types": "tsc"
diff --git a/packages/ai-elevenlabs/package.json b/packages/ai-elevenlabs/package.json
index 437447b9e..20b10cc20 100644
--- a/packages/ai-elevenlabs/package.json
+++ b/packages/ai-elevenlabs/package.json
@@ -53,6 +53,7 @@
     "lint:fix": "oxlint src --type-aware --fix",
     "test:build": "publint --strict",
     "test:oxlint": "oxlint src --type-aware",
+    "test:coverage": "vitest run --passWithNoTests --coverage --coverage.include='src/**' --coverage.reporter=text-summary --coverage.reporter=json-summary",
     "test:lib": "vitest --passWithNoTests",
     "test:lib:dev": "pnpm test:lib --watch",
     "test:types": "tsc"
diff --git a/packages/ai-event-client/package.json b/packages/ai-event-client/package.json
index ae1c70b01..df8ebfbbb 100644
--- a/packages/ai-event-client/package.json
+++ b/packages/ai-event-client/package.json
@@ -37,6 +37,7 @@
     "lint:fix": "oxlint src --type-aware --fix",
     "test:build": "publint --strict",
     "test:oxlint": "oxlint src --type-aware",
+    "test:coverage": "vitest run --passWithNoTests --coverage --coverage.include='src/**' --coverage.reporter=text-summary --coverage.reporter=json-summary",
     "test:lib": "vitest --passWithNoTests",
     "test:lib:dev": "pnpm test:lib --watch",
     "test:types": "tsc"
diff --git a/packages/ai-fal/package.json b/packages/ai-fal/package.json
index 7e9bd7413..e36ffc619 100644
--- a/packages/ai-fal/package.json
+++ b/packages/ai-fal/package.json
@@ -36,6 +36,7 @@
     "lint:fix": "oxlint src --type-aware --fix",
     "test:build": "publint --strict",
     "test:oxlint": "oxlint src --type-aware",
+    "test:coverage": "vitest run --coverage --coverage.include='src/**' --coverage.reporter=text-summary --coverage.reporter=json-summary",
     "test:lib": "vitest run",
     "test:lib:dev": "pnpm test:lib --watch",
     "test:types": "tsc"
diff --git a/packages/ai-gemini/package.json b/packages/ai-gemini/package.json
index 55e02d00c..291ade6bb 100644
--- a/packages/ai-gemini/package.json
+++ b/packages/ai-gemini/package.json
@@ -44,6 +44,7 @@
     "lint:fix": "oxlint src --type-aware --fix",
     "test:build": "publint --strict",
     "test:oxlint": "oxlint src --type-aware",
+    "test:coverage": "vitest run --coverage --coverage.include='src/**' --coverage.reporter=text-summary --coverage.reporter=json-summary",
     "test:lib": "vitest",
     "test:lib:dev": "pnpm test:lib --watch",
     "test:types": "tsc"
diff --git a/packages/ai-grok-build/package.json b/packages/ai-grok-build/package.json
index 0c95e4c5f..6e09554f1 100644
--- a/packages/ai-grok-build/package.json
+++ b/packages/ai-grok-build/package.json
@@ -42,6 +42,7 @@
     "lint:fix": "oxlint src --type-aware --fix",
     "test:build": "publint --strict",
     "test:oxlint": "oxlint src --type-aware",
+    "test:coverage": "vitest run --coverage --coverage.include='src/**' --coverage.reporter=text-summary --coverage.reporter=json-summary",
     "test:lib": "vitest",
     "test:lib:dev": "pnpm test:lib --watch",
     "test:types": "tsc"
diff --git a/packages/ai-grok/package.json b/packages/ai-grok/package.json
index 38d245914..c12e8c611 100644
--- a/packages/ai-grok/package.json
+++ b/packages/ai-grok/package.json
@@ -40,6 +40,7 @@
     "lint:fix": "oxlint src --type-aware --fix",
     "test:build": "publint --strict",
     "test:oxlint": "oxlint src --type-aware",
+    "test:coverage": "vitest run --coverage --coverage.include='src/**' --coverage.reporter=text-summary --coverage.reporter=json-summary",
     "test:lib": "vitest run",
     "test:lib:dev": "pnpm test:lib --watch",
     "test:types": "tsc"
diff --git a/packages/ai-groq/package.json b/packages/ai-groq/package.json
index db6e9845a..8c12442df 100644
--- a/packages/ai-groq/package.json
+++ b/packages/ai-groq/package.json
@@ -40,6 +40,7 @@
     "lint:fix": "oxlint src --type-aware --fix",
     "test:build": "publint --strict",
     "test:oxlint": "oxlint src --type-aware",
+    "test:coverage": "vitest run --coverage --coverage.include='src/**' --coverage.reporter=text-summary --coverage.reporter=json-summary",
     "test:lib": "vitest run",
     "test:lib:dev": "pnpm test:lib --watch",
     "test:types": "tsc"
diff --git a/packages/ai-isolate-cloudflare/package.json b/packages/ai-isolate-cloudflare/package.json
index dbc798ac0..fc05f4ec0 100644
--- a/packages/ai-isolate-cloudflare/package.json
+++ b/packages/ai-isolate-cloudflare/package.json
@@ -48,6 +48,7 @@
     "lint:fix": "oxlint src --type-aware --fix",
     "test:build": "publint --strict",
     "test:oxlint": "oxlint src --type-aware",
+    "test:coverage": "vitest run --passWithNoTests --coverage --coverage.include='src/**' --coverage.reporter=text-summary --coverage.reporter=json-summary",
     "test:lib": "vitest --passWithNoTests",
     "test:lib:dev": "pnpm test:lib --watch",
     "test:types": "tsc"
diff --git a/packages/ai-isolate-node/package.json b/packages/ai-isolate-node/package.json
index 5a3d6103f..0ba666184 100644
--- a/packages/ai-isolate-node/package.json
+++ b/packages/ai-isolate-node/package.json
@@ -40,6 +40,7 @@
     "lint:fix": "oxlint src --type-aware --fix",
     "test:build": "publint --strict",
     "test:oxlint": "oxlint src --type-aware",
+    "test:coverage": "vitest run --passWithNoTests --coverage --coverage.include='src/**' --coverage.reporter=text-summary --coverage.reporter=json-summary",
     "test:lib": "vitest --passWithNoTests",
     "test:lib:dev": "pnpm test:lib --watch",
     "test:types": "tsc"
diff --git a/packages/ai-isolate-quickjs/package.json b/packages/ai-isolate-quickjs/package.json
index 6c1acdb34..55b1e2e8b 100644
--- a/packages/ai-isolate-quickjs/package.json
+++ b/packages/ai-isolate-quickjs/package.json
@@ -37,6 +37,7 @@
     "lint:fix": "oxlint src --type-aware --fix",
     "test:build": "publint --strict",
     "test:oxlint": "oxlint src --type-aware",
+    "test:coverage": "vitest run --passWithNoTests --coverage --coverage.include='src/**' --coverage.reporter=text-summary --coverage.reporter=json-summary",
     "test:lib": "vitest --passWithNoTests",
     "test:lib:dev": "pnpm test:lib --watch",
     "test:types": "tsc"
diff --git a/packages/ai-mcp/package.json b/packages/ai-mcp/package.json
index e7719920c..8323ba3e2 100644
--- a/packages/ai-mcp/package.json
+++ b/packages/ai-mcp/package.json
@@ -56,8 +56,8 @@
     "clean": "premove ./build ./dist",
     "lint:fix": "oxlint src --type-aware --fix",
     "test:build": "publint --strict",
-    "test:coverage": "vitest run --coverage",
     "test:oxlint": "oxlint src --type-aware",
+    "test:coverage": "vitest run --coverage --coverage.include='src/**' --coverage.reporter=text-summary --coverage.reporter=json-summary",
     "test:lib": "vitest",
     "test:lib:dev": "pnpm test:lib --watch",
     "test:types": "tsc"
diff --git a/packages/ai-memory/package.json b/packages/ai-memory/package.json
index d3dafd191..b4518a13d 100644
--- a/packages/ai-memory/package.json
+++ b/packages/ai-memory/package.json
@@ -50,6 +50,7 @@
     "lint:fix": "oxlint src --type-aware --fix",
     "test:build": "publint --strict",
     "test:oxlint": "oxlint src --type-aware",
+    "test:coverage": "vitest run --passWithNoTests --coverage --coverage.include='src/**' --coverage.reporter=text-summary --coverage.reporter=json-summary",
     "test:lib": "vitest --passWithNoTests",
     "test:lib:dev": "pnpm test:lib --watch",
     "test:types": "tsc"
diff --git a/packages/ai-mistral/package.json b/packages/ai-mistral/package.json
index a40cdc5ca..a6455c2ab 100644
--- a/packages/ai-mistral/package.json
+++ b/packages/ai-mistral/package.json
@@ -36,6 +36,7 @@
     "lint:fix": "oxlint src --type-aware --fix",
     "test:build": "publint --strict",
     "test:oxlint": "oxlint src --type-aware",
+    "test:coverage": "vitest run --coverage --coverage.include='src/**' --coverage.reporter=text-summary --coverage.reporter=json-summary",
     "test:lib": "vitest run",
     "test:lib:dev": "pnpm test:lib --watch",
     "test:types": "tsc"
diff --git a/packages/ai-ollama/package.json b/packages/ai-ollama/package.json
index b4950d4c8..ac56d69e7 100644
--- a/packages/ai-ollama/package.json
+++ b/packages/ai-ollama/package.json
@@ -36,6 +36,7 @@
     "lint:fix": "oxlint src --type-aware --fix",
     "test:build": "publint --strict",
     "test:oxlint": "oxlint src --type-aware",
+    "test:coverage": "vitest run --passWithNoTests --coverage --coverage.include='src/**' --coverage.reporter=text-summary --coverage.reporter=json-summary",
     "test:lib": "vitest --passWithNoTests",
     "test:lib:dev": "pnpm test:lib --watch",
     "test:types": "tsc"
diff --git a/packages/ai-openai/package.json b/packages/ai-openai/package.json
index c225f7d8b..9d80cf020 100644
--- a/packages/ai-openai/package.json
+++ b/packages/ai-openai/package.json
@@ -44,6 +44,7 @@
     "lint:fix": "oxlint src --type-aware --fix",
     "test:build": "publint --strict",
     "test:oxlint": "oxlint src --type-aware",
+    "test:coverage": "vitest run --coverage --coverage.include='src/**' --coverage.reporter=text-summary --coverage.reporter=json-summary",
     "test:lib": "vitest run",
     "test:lib:dev": "pnpm test:lib --watch",
     "test:types": "tsc"
diff --git a/packages/ai-opencode/package.json b/packages/ai-opencode/package.json
index b3a72b4b5..4867ef305 100644
--- a/packages/ai-opencode/package.json
+++ b/packages/ai-opencode/package.json
@@ -40,6 +40,7 @@
     "lint:fix": "oxlint src --type-aware --fix",
     "test:build": "publint --strict",
     "test:oxlint": "oxlint src --type-aware",
+    "test:coverage": "vitest run --coverage --coverage.include='src/**' --coverage.reporter=text-summary --coverage.reporter=json-summary",
     "test:lib": "vitest",
     "test:lib:dev": "pnpm test:lib --watch",
     "test:types": "tsc"
diff --git a/packages/ai-openrouter/package.json b/packages/ai-openrouter/package.json
index a37469607..5efc24ffb 100644
--- a/packages/ai-openrouter/package.json
+++ b/packages/ai-openrouter/package.json
@@ -44,6 +44,7 @@
     "lint:fix": "oxlint src --type-aware --fix",
     "test:build": "publint --strict",
     "test:oxlint": "oxlint src --type-aware",
+    "test:coverage": "vitest run --coverage --coverage.include='src/**' --coverage.reporter=text-summary --coverage.reporter=json-summary",
     "test:lib": "vitest run",
     "test:lib:dev": "pnpm test:lib --watch",
     "test:types": "tsc"
diff --git a/packages/ai-persistence/package.json b/packages/ai-persistence/package.json
index 8cbf8ea7d..39cbef309 100644
--- a/packages/ai-persistence/package.json
+++ b/packages/ai-persistence/package.json
@@ -43,6 +43,7 @@
     "clean": "premove ./build ./dist",
     "lint:fix": "oxlint src --type-aware --fix",
     "test:build": "publint --strict",
+    "test:coverage": "vitest run --coverage --coverage.include='src/**' --coverage.reporter=text-summary --coverage.reporter=json-summary",
     "test:lib": "vitest",
     "test:lib:dev": "pnpm test:lib --watch",
     "test:types": "tsc",
diff --git a/packages/ai-preact/package.json b/packages/ai-preact/package.json
index 90fc8f117..e94d2957b 100644
--- a/packages/ai-preact/package.json
+++ b/packages/ai-preact/package.json
@@ -38,6 +38,7 @@
     "clean": "premove ./build ./dist",
     "lint:fix": "oxlint src --type-aware --fix",
     "test:oxlint": "oxlint src --type-aware",
+    "test:coverage": "vitest run --coverage --coverage.include='src/**' --coverage.reporter=text-summary --coverage.reporter=json-summary",
     "test:lib": "vitest run",
     "test:lib:dev": "pnpm test:lib --watch",
     "test:types": "tsc",
diff --git a/packages/ai-react-ui/package.json b/packages/ai-react-ui/package.json
index 28c767708..26ea2e267 100644
--- a/packages/ai-react-ui/package.json
+++ b/packages/ai-react-ui/package.json
@@ -34,6 +34,7 @@
     "clean": "premove ./build ./dist",
     "test:build": "publint --strict",
     "test:oxlint": "oxlint src --type-aware",
+    "test:coverage": "vitest run --passWithNoTests --coverage --coverage.include='src/**' --coverage.reporter=text-summary --coverage.reporter=json-summary",
     "test:lib": "vitest --passWithNoTests",
     "test:lib:dev": "pnpm test:lib --watch",
     "test:types": "tsc"
diff --git a/packages/ai-react/package.json b/packages/ai-react/package.json
index 5eb097cb7..c833076c0 100644
--- a/packages/ai-react/package.json
+++ b/packages/ai-react/package.json
@@ -38,6 +38,7 @@
     "clean": "premove ./build ./dist",
     "lint:fix": "oxlint src --type-aware --fix",
     "test:oxlint": "oxlint src --type-aware",
+    "test:coverage": "vitest run --coverage --coverage.include='src/**' --coverage.reporter=text-summary --coverage.reporter=json-summary",
     "test:lib": "vitest run",
     "test:lib:dev": "pnpm test:lib --watch",
     "test:types": "tsc",
diff --git a/packages/ai-sandbox-cloudflare/package.json b/packages/ai-sandbox-cloudflare/package.json
index 9b966de96..9809db8df 100644
--- a/packages/ai-sandbox-cloudflare/package.json
+++ b/packages/ai-sandbox-cloudflare/package.json
@@ -46,6 +46,7 @@
     "lint:fix": "oxlint src --type-aware --fix",
     "test:build": "publint --strict",
     "test:oxlint": "oxlint src --type-aware",
+    "test:coverage": "vitest run --coverage --coverage.include='src/**' --coverage.reporter=text-summary --coverage.reporter=json-summary",
     "test:lib": "vitest",
     "test:lib:dev": "pnpm test:lib --watch",
     "test:types": "tsc"
diff --git a/packages/ai-sandbox-daytona/package.json b/packages/ai-sandbox-daytona/package.json
index 097ad1222..4a780a337 100644
--- a/packages/ai-sandbox-daytona/package.json
+++ b/packages/ai-sandbox-daytona/package.json
@@ -37,6 +37,7 @@
     "lint:fix": "oxlint src --type-aware --fix",
     "test:build": "publint --strict",
     "test:oxlint": "oxlint src --type-aware",
+    "test:coverage": "vitest run --coverage --coverage.include='src/**' --coverage.reporter=text-summary --coverage.reporter=json-summary",
     "test:lib": "vitest",
     "test:lib:dev": "pnpm test:lib --watch",
     "test:types": "tsc"
diff --git a/packages/ai-sandbox-docker/package.json b/packages/ai-sandbox-docker/package.json
index 2864f03a5..df3516648 100644
--- a/packages/ai-sandbox-docker/package.json
+++ b/packages/ai-sandbox-docker/package.json
@@ -38,6 +38,7 @@
     "lint:fix": "oxlint src --type-aware --fix",
     "test:build": "publint --strict",
     "test:oxlint": "oxlint src --type-aware",
+    "test:coverage": "vitest run --coverage --coverage.include='src/**' --coverage.reporter=text-summary --coverage.reporter=json-summary",
     "test:lib": "vitest",
     "test:lib:dev": "pnpm test:lib --watch",
     "test:types": "tsc"
diff --git a/packages/ai-sandbox-local-process/package.json b/packages/ai-sandbox-local-process/package.json
index 236575f15..8211525e3 100644
--- a/packages/ai-sandbox-local-process/package.json
+++ b/packages/ai-sandbox-local-process/package.json
@@ -36,6 +36,7 @@
     "lint:fix": "oxlint src --type-aware --fix",
     "test:build": "publint --strict",
     "test:oxlint": "oxlint src --type-aware",
+    "test:coverage": "vitest run --coverage --coverage.include='src/**' --coverage.reporter=text-summary --coverage.reporter=json-summary",
     "test:lib": "vitest",
     "test:lib:dev": "pnpm test:lib --watch",
     "test:types": "tsc"
diff --git a/packages/ai-sandbox-sprites/package.json b/packages/ai-sandbox-sprites/package.json
index cc8fd3531..b58e32129 100644
--- a/packages/ai-sandbox-sprites/package.json
+++ b/packages/ai-sandbox-sprites/package.json
@@ -38,6 +38,7 @@
     "lint:fix": "oxlint src --type-aware --fix",
     "test:build": "publint --strict",
     "test:oxlint": "oxlint src --type-aware",
+    "test:coverage": "vitest run --coverage --coverage.include='src/**' --coverage.reporter=text-summary --coverage.reporter=json-summary",
     "test:lib": "vitest",
     "test:lib:dev": "pnpm test:lib --watch",
     "test:types": "tsc"
diff --git a/packages/ai-sandbox-vercel/package.json b/packages/ai-sandbox-vercel/package.json
index 4c1de18c0..d76bce12c 100644
--- a/packages/ai-sandbox-vercel/package.json
+++ b/packages/ai-sandbox-vercel/package.json
@@ -37,6 +37,7 @@
     "lint:fix": "oxlint src --type-aware --fix",
     "test:build": "publint --strict",
     "test:oxlint": "oxlint src --type-aware",
+    "test:coverage": "vitest run --coverage --coverage.include='src/**' --coverage.reporter=text-summary --coverage.reporter=json-summary",
     "test:lib": "vitest",
     "test:lib:dev": "pnpm test:lib --watch",
     "test:types": "tsc"
diff --git a/packages/ai-sandbox/package.json b/packages/ai-sandbox/package.json
index 4177c32f3..46f1b11e9 100644
--- a/packages/ai-sandbox/package.json
+++ b/packages/ai-sandbox/package.json
@@ -50,6 +50,7 @@
     "lint:fix": "oxlint src --type-aware --fix",
     "test:build": "publint --strict",
     "test:oxlint": "oxlint src --type-aware",
+    "test:coverage": "vitest run --coverage --coverage.include='src/**' --coverage.reporter=text-summary --coverage.reporter=json-summary",
     "test:lib": "vitest",
     "test:lib:dev": "pnpm test:lib --watch",
     "test:types": "tsc"
diff --git a/packages/ai-solid-ui/package.json b/packages/ai-solid-ui/package.json
index 13c56355b..ebf0d2688 100644
--- a/packages/ai-solid-ui/package.json
+++ b/packages/ai-solid-ui/package.json
@@ -36,6 +36,7 @@
     "clean": "premove ./build ./dist",
     "test:build": "publint --strict",
     "test:oxlint": "oxlint src --type-aware",
+    "test:coverage": "vitest run --passWithNoTests --coverage --coverage.include='src/**' --coverage.reporter=text-summary --coverage.reporter=json-summary",
     "test:lib": "vitest --passWithNoTests",
     "test:lib:dev": "pnpm test:lib --watch",
     "test:types": "tsc"
diff --git a/packages/ai-solid/package.json b/packages/ai-solid/package.json
index 88e573808..850df8627 100644
--- a/packages/ai-solid/package.json
+++ b/packages/ai-solid/package.json
@@ -33,6 +33,7 @@
   "scripts": {
     "clean": "premove ./build ./dist",
     "test:oxlint": "oxlint src --type-aware",
+    "test:coverage": "vitest run --coverage --coverage.include='src/**' --coverage.reporter=text-summary --coverage.reporter=json-summary",
     "test:lib": "vitest run",
     "test:lib:dev": "pnpm test:lib --watch",
     "test:types": "tsc",
diff --git a/packages/ai-svelte/package.json b/packages/ai-svelte/package.json
index 12862366b..375f74216 100644
--- a/packages/ai-svelte/package.json
+++ b/packages/ai-svelte/package.json
@@ -36,6 +36,7 @@
     "clean": "rm -rf ./dist",
     "lint:fix": "oxlint src --type-aware --fix",
     "test:oxlint": "oxlint src --type-aware",
+    "test:coverage": "vitest run --coverage --coverage.include='src/**' --coverage.reporter=text-summary --coverage.reporter=json-summary",
     "test:lib": "vitest run",
     "test:lib:dev": "pnpm test:lib --watch",
     "test:types": "svelte-check",
diff --git a/packages/ai-utils/package.json b/packages/ai-utils/package.json
index f3c738318..46c4384e9 100644
--- a/packages/ai-utils/package.json
+++ b/packages/ai-utils/package.json
@@ -36,6 +36,7 @@
     "lint:fix": "oxlint src --type-aware --fix",
     "test:build": "publint --strict",
     "test:oxlint": "oxlint src --type-aware",
+    "test:coverage": "vitest run --coverage --coverage.include='src/**' --coverage.reporter=text-summary --coverage.reporter=json-summary",
     "test:lib": "vitest run",
     "test:lib:dev": "pnpm test:lib --watch",
     "test:types": "tsc"
diff --git a/packages/ai-vue-ui/package.json b/packages/ai-vue-ui/package.json
index 05d7979f0..c46e45e16 100644
--- a/packages/ai-vue-ui/package.json
+++ b/packages/ai-vue-ui/package.json
@@ -35,6 +35,7 @@
     "clean": "premove ./build ./dist",
     "test:build": "publint --strict",
     "test:oxlint": "oxlint src --type-aware",
+    "test:coverage": "vitest run --passWithNoTests --coverage --coverage.include='src/**' --coverage.reporter=text-summary --coverage.reporter=json-summary",
     "test:lib": "vitest --passWithNoTests",
     "test:lib:dev": "pnpm test:lib --watch",
     "test:types": "vue-tsc --noEmit"
diff --git a/packages/ai-vue/package.json b/packages/ai-vue/package.json
index f7c4f1062..1d902b1b2 100644
--- a/packages/ai-vue/package.json
+++ b/packages/ai-vue/package.json
@@ -33,6 +33,7 @@
   "scripts": {
     "clean": "premove ./build ./dist",
     "test:oxlint": "oxlint src --type-aware",
+    "test:coverage": "vitest run --coverage --coverage.include='src/**' --coverage.reporter=text-summary --coverage.reporter=json-summary",
     "test:lib": "vitest run",
     "test:lib:dev": "pnpm test:lib --watch",
     "test:types": "tsc",
diff --git a/packages/ai/package.json b/packages/ai/package.json
index e11254775..6b797688b 100644
--- a/packages/ai/package.json
+++ b/packages/ai/package.json
@@ -64,9 +64,9 @@
     "clean": "premove ./build ./dist",
     "lint:fix": "oxlint src --type-aware --fix",
     "test:build": "publint --strict",
-    "test:coverage": "vitest run --coverage",
-    "test:coverage:watch": "vitest --coverage --watch",
     "test:oxlint": "oxlint src --type-aware",
+    "test:coverage": "vitest run --coverage --coverage.include='src/**' --coverage.reporter=text-summary --coverage.reporter=json-summary",
+    "test:coverage:watch": "vitest --coverage --watch",
     "test:lib": "vitest",
     "test:lib:dev": "pnpm test:lib --watch",
     "test:types": "tsc"
diff --git a/packages/openai-base/package.json b/packages/openai-base/package.json
index 5f3112a21..eb5a05eba 100644
--- a/packages/openai-base/package.json
+++ b/packages/openai-base/package.json
@@ -36,6 +36,7 @@
     "lint:fix": "oxlint src --type-aware --fix",
     "test:build": "publint --strict",
     "test:oxlint": "oxlint src --type-aware",
+    "test:coverage": "vitest run --coverage --coverage.include='src/**' --coverage.reporter=text-summary --coverage.reporter=json-summary",
     "test:lib": "vitest run",
     "test:lib:dev": "pnpm test:lib --watch",
     "test:types": "tsc"
diff --git a/packages/preact-ai-devtools/package.json b/packages/preact-ai-devtools/package.json
index e6226f65a..62f65aff4 100644
--- a/packages/preact-ai-devtools/package.json
+++ b/packages/preact-ai-devtools/package.json
@@ -21,6 +21,7 @@
     "clean": "premove ./build ./dist",
     "lint:fix": "oxlint src --type-aware --fix",
     "test:oxlint": "oxlint src --type-aware",
+    "test:coverage": "vitest run --passWithNoTests --coverage --coverage.include='src/**' --coverage.reporter=text-summary --coverage.reporter=json-summary",
     "test:lib": "vitest --passWithNoTests",
     "test:lib:dev": "pnpm test:lib --watch",
     "test:types": "tsc",
diff --git a/packages/react-ai-devtools/package.json b/packages/react-ai-devtools/package.json
index 0330960b8..8761296fa 100644
--- a/packages/react-ai-devtools/package.json
+++ b/packages/react-ai-devtools/package.json
@@ -21,6 +21,7 @@
     "clean": "premove ./build ./dist",
     "lint:fix": "oxlint src --type-aware --fix",
     "test:oxlint": "oxlint src --type-aware",
+    "test:coverage": "vitest run --passWithNoTests --coverage --coverage.include='src/**' --coverage.reporter=text-summary --coverage.reporter=json-summary",
     "test:lib": "vitest --passWithNoTests",
     "test:lib:dev": "pnpm test:lib --watch",
     "test:types": "tsc",
diff --git a/packages/solid-ai-devtools/package.json b/packages/solid-ai-devtools/package.json
index 0583e859d..8a8c1117b 100644
--- a/packages/solid-ai-devtools/package.json
+++ b/packages/solid-ai-devtools/package.json
@@ -42,6 +42,7 @@
     "lint:fix": "oxlint src --type-aware --fix",
     "test:build": "publint --strict",
     "test:oxlint": "oxlint src --type-aware",
+    "test:coverage": "vitest run --passWithNoTests --coverage --coverage.include='src/**' --coverage.reporter=text-summary --coverage.reporter=json-summary",
     "test:lib": "vitest --passWithNoTests",
     "test:lib:dev": "pnpm test:lib --watch",
     "test:types": "tsc"
diff --git a/scripts/coverage-check.mjs b/scripts/coverage-check.mjs
new file mode 100644
index 000000000..6f8ac6dbf
--- /dev/null
+++ b/scripts/coverage-check.mjs
@@ -0,0 +1,216 @@
+// Compares coverage between two runs and fails when a package regressed.
+//
+// Both sides are measured in the same CI job — the PR head and its merge-base
+// with main — so there is no baseline file to keep in sync, no per-platform
+// skew, and packages can be added or removed without anyone updating a
+// checked-in number.
+//
+// Usage:
+//   node scripts/coverage-check.mjs --collect <dir>          # after a coverage run
+//   node scripts/coverage-check.mjs --base <dir> --head <dir>
+//
+// ponytail: two directories of json-summary files and a subtraction. No
+// coverage service, no history, no baseline to maintain.
+import {
+  appendFileSync,
+  mkdirSync,
+  readFileSync,
+  readdirSync,
+  writeFileSync,
+} from 'node:fs'
+import { join } from 'node:path'
+
+const METRICS = ['statements', 'branches', 'functions', 'lines']
+
+// Coverage percentages wobble slightly between runs (v8 attributes some bytes
+// differently depending on JIT timing), so require a real drop.
+const TOLERANCE = 0.5
+
+function arg(name) {
+  const i = process.argv.indexOf(name)
+  return i === -1 ? undefined : process.argv[i + 1]
+}
+
+/** Reduce a vitest json-summary to the four totals we compare. */
+function totals(summary) {
+  // A package with no source loaded reports totals of 0/0 as 100%; that is not
+  // a number worth comparing.
+  if (summary.total.statements.total === 0) return undefined
+  return Object.fromEntries(
+    METRICS.map((metric) => [metric, summary.total[metric].pct]),
+  )
+}
+
+/**
+ * Copy this run's summaries into `dir`, one small file per package, so they
+ * survive the `git checkout` that happens between the two coverage runs.
+ */
+function collect(dir) {
+  mkdirSync(dir, { recursive: true })
+  let n = 0
+  for (const entry of readdirSync('packages', { withFileTypes: true })) {
+    if (!entry.isDirectory()) continue
+    let summary
+    try {
+      summary = JSON.parse(
+        readFileSync(
+          join('packages', entry.name, 'coverage', 'coverage-summary.json'),
+          'utf8',
+        ),
+      )
+    } catch (error) {
+      // No summary means the package was not measured in this run, which is
+      // different from it measuring 0%.
+      if (error.code === 'ENOENT') continue
+      throw error
+    }
+    const metrics = totals(summary)
+    if (!metrics) continue
+    writeFileSync(join(dir, `${entry.name}.json`), JSON.stringify(metrics))
+    n++
+  }
+  console.log(`Collected ${n} coverage summary/summaries into ${dir}`)
+}
+
+function read(dir) {
+  let files
+  try {
+    files = readdirSync(dir)
+  } catch (error) {
+    // A missing directory means that side measured nothing at all.
+    if (error.code === 'ENOENT') return {}
+    throw error
+  }
+  return Object.fromEntries(
+    files
+      .filter((file) => file.endsWith('.json'))
+      .map((file) => [
+        file.replace(/\.json$/, ''),
+        JSON.parse(readFileSync(join(dir, file), 'utf8')),
+      ]),
+  )
+}
+
+const collectDir = arg('--collect')
+if (collectDir) {
+  collect(collectDir)
+  process.exit(0)
+}
+
+const baseDir = arg('--base')
+const headDir = arg('--head')
+if (!baseDir || !headDir) {
+  console.error(
+    'Usage: coverage-check.mjs --collect <dir> | --base <dir> --head <dir>',
+  )
+  process.exit(2)
+}
+
+const base = read(baseDir)
+const head = read(headDir)
+const names = Object.keys(head).sort()
+
+if (names.length === 0) {
+  console.log('No packages were measured — nothing to compare.')
+  process.exit(0)
+}
+
+const regressions = []
+const additions = []
+const rows = []
+
+for (const name of names) {
+  const before = base[name]
+  const after = head[name]
+  if (!before) {
+    // New package, or one whose suite could not be measured on the base
+    // commit. There is nothing to regress against.
+    additions.push(name)
+    rows.push([
+      name,
+      ...METRICS.map((metric) => `${after[metric].toFixed(2)}%`),
+      'new',
+    ])
+    continue
+  }
+  const deltas = METRICS.map((metric) => after[metric] - before[metric])
+  const dropped = METRICS.filter((metric, i) => deltas[i] < -TOLERANCE)
+  if (dropped.length > 0) regressions.push({ name, before, after, dropped })
+  rows.push([
+    name,
+    ...METRICS.map((metric, i) => {
+      const sign = deltas[i] > 0 ? '+' : ''
+      return `${after[metric].toFixed(2)}% (${sign}${deltas[i].toFixed(2)})`
+    }),
+    dropped.length > 0 ? 'DROP' : 'ok',
+  ])
+}
+
+const removed = Object.keys(base)
+  .filter((name) => !head[name])
+  .sort()
+
+const header = ['package', ...METRICS, '']
+const widths = header.map((_, column) =>
+  Math.max(header[column].length, ...rows.map((row) => row[column].length)),
+)
+const line = (cells) =>
+  cells
+    .map((cell, i) => cell.padEnd(widths[i]))
+    .join('  ')
+    .trimEnd()
+
+console.log(line(header))
+console.log(widths.map((width) => '-'.repeat(width)).join('  '))
+for (const row of rows) console.log(line(row))
+
+if (additions.length > 0) {
+  console.log(
+    `\n${additions.length} package(s) had no coverage on the base commit` +
+      ` (new, or unmeasurable there): ${additions.join(', ')}`,
+  )
+}
+if (removed.length > 0) {
+  console.log(`\nNot measured on this PR: ${removed.join(', ')}`)
+}
+
+if (process.env.GITHUB_STEP_SUMMARY) {
+  const md = [
+    `## Coverage`,
+    ``,
+    regressions.length > 0
+      ? `❌ Coverage dropped in ${regressions.length} package(s).`
+      : `✅ Coverage held across ${names.length} measured package(s).`,
+    ``,
+    `Each package is measured twice in this job — on this PR and on its merge-base` +
+      ` with \`main\` — and compared. A drop of more than ${TOLERANCE}pp in any metric fails.` +
+      ` Packages your PR doesn't affect are not measured and not listed.`,
+    ``,
+    `| package | ${METRICS.join(' | ')} | |`,
+    `| --- | ${METRICS.map(() => '---:').join(' | ')} | --- |`,
+    ...rows.map((row) => `| ${row.join(' | ')} |`),
+  ]
+  if (additions.length > 0) {
+    md.push(
+      ``,
+      `> No coverage on the base commit, so nothing to compare against:` +
+        ` ${additions.join(', ')}.`,
+    )
+  }
+  appendFileSync(process.env.GITHUB_STEP_SUMMARY, `${md.join('\n')}\n`)
+}
+
+if (regressions.length > 0) {
+  console.error(`\nCoverage dropped in ${regressions.length} package(s):`)
+  for (const { name, before, after, dropped } of regressions) {
+    for (const metric of dropped) {
+      console.error(
+        `  ${name} ${metric}: ${before[metric].toFixed(2)}% -> ${after[metric].toFixed(2)}%`,
+      )
+    }
+  }
+  console.error(`\nAdd tests covering the code this PR changed.`)
+  process.exit(1)
+}
+
+console.log(`\nCoverage held across ${names.length} measured package(s).`)