diff --git a/tests/ci-concurrency.test.mjs b/tests/ci-concurrency.test.mjs
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+// Guards the property that keeps the pull_request lane from racing itself:
+// that every PR-triggered workflow supersedes its own stale runs instead of
+// letting them finish.
+//
+// Without a `concurrency` group, pushing three times to a branch leaves three
+// full runs of `.github/workflows/ci.yml` alive at once. Each one holds its
+// runners for the whole matrix, and the two that are already obsolete report
+// their results onto the same commit status as the one that matters. On a
+// saturated queue that is the difference between a PR being checked and a PR
+// waiting behind its own superseded attempts.
+//
+// The group expression matters as much as the flag. `concurrency: ci` is a
+// single constant key for the whole repository, so it does not cancel a stale
+// run of the same PR — it serialises *every* PR into one lane and cancels
+// other people's work. The per-PR keying tested below is what makes
+// `cancel-in-progress: true` safe.
+//
+// Nothing else in the suite sees any of this. tests/ci-supply-chain.test.mjs
+// covers pinning, permissions, runners and timeouts, and its timeout rationale
+// only mentions concurrency in passing as the reason a stuck job is not
+// superseded. tests/workflow-scripts.test.mjs asks which commands a workflow
+// runs. tests/automation-workflow-ordering.test.mjs covers step ordering in the
+// generator workflows, which are not PR-triggered. A new PR-triggered workflow
+// can be added today with no concurrency block at all and the suite stays
+// green.
+//
+// SERIALISED_WORKFLOWS below follows the retiring-baseline convention used by
+// KNOWN_PERSISTING_CHECKOUTS and KNOWN_UNBOUNDED_JOBS in
+// tests/ci-supply-chain.test.mjs: a workflow that genuinely must not be
+// cancelled mid-flight stays possible, but it has to be argued for here, and
+// the companion staleness test forces the entry to be deleted once the
+// workflow stops needing it.
+import assert from 'node:assert/strict';
+import { readdirSync, readFileSync } from 'node:fs';
+import { join } from 'node:path';
+import test from 'node:test';
+import { parse } from 'yaml';
+
+const workflowDir = new URL('../.github/workflows/', import.meta.url).pathname;
+
+const workflows = readdirSync(workflowDir)
+  .filter((name) => /\.ya?ml$/.test(name))
+  .sort()
+  .map((name) => ({
+    name,
+    doc: parse(readFileSync(join(workflowDir, name), 'utf8')),
+  }));
+
+// `on` is parsed as the boolean true by YAML 1.1 compatibility rules, so the
+// trigger block is read back through both keys. Mirrors the helper in
+// tests/ci-supply-chain.test.mjs.
+function triggers(doc) {
+  const on = doc?.on ?? doc?.[true];
+  if (!on) return [];
+  if (typeof on === 'string') return [on];
+  if (Array.isArray(on)) return on;
+  return Object.keys(on);
+}
+
+// `pull_request_target` is banned outright by ci-supply-chain.test.mjs, but it
+// is included here so that this contract does not become the hole through
+// which an uncancelled PR-triggered workflow arrives if that ban is ever
+// relaxed.
+const PR_TRIGGERS = ['pull_request', 'pull_request_target'];
+
+function isPullRequestTriggered(doc) {
+  return triggers(doc).some((trigger) => PR_TRIGGERS.includes(trigger));
+}
+
+// Workflows that declare a concurrency group but must NOT cancel the run in
+// flight, with the reason. Keyed by file name.
+const SERIALISED_WORKFLOWS = new Map([
+  [
+    'deploy-gh-pages.yml',
+    'Publishes the live site. Cancelling a deploy halfway can leave GitHub ' +
+      'Pages serving a partially uploaded artifact, so overlapping runs are ' +
+      'queued behind the constant "pages" group rather than cancelled. Not ' +
+      'PR-triggered, so it is outside the cancel-in-progress contract below ' +
+      'and only needs the constant-group exemption.',
+  ],
+]);
+
+// A group is per-PR if it varies with the event. `${{ ... }}` referencing any
+// of these contexts gives each pull request its own key; a group with no
+// expression at all is a single repository-wide lane.
+const PER_PR_CONTEXTS = [
+  'github.event.pull_request.number',
+  'github.head_ref',
+  'github.ref',
+];
+
+function groupExpression(doc) {
+  const group = doc?.concurrency;
+  if (group === undefined || group === null) return undefined;
+  // `concurrency:` accepts either a bare string or a map with `group:`.
+  const raw = typeof group === 'string' ? group : group.group;
+  if (typeof raw !== 'string') return undefined;
+  // Prettier wraps long expressions across lines; YAML folds them back with
+  // newlines that would otherwise break substring matching.
+  return raw.replace(/\s+/g, ' ').trim();
+}
+
+function cancelsInProgress(doc) {
+  const group = doc?.concurrency;
+  if (typeof group !== 'object' || group === null) return false;
+  return group['cancel-in-progress'] === true;
+}
+
+function isPerPullRequest(expression) {
+  if (typeof expression !== 'string') return false;
+  if (!expression.includes('${{')) return false;
+  return PER_PR_CONTEXTS.some((context) => expression.includes(context));
+}
+
+// The scanners below take the parsed workflows as an argument rather than
+// closing over the repository's own, so each failure path can be exercised
+// against a fixture. A guard whose failure branch never runs is a guard nobody
+// has checked.
+function withoutConcurrency(parsedWorkflows) {
+  return parsedWorkflows
+    .filter(({ doc }) => isPullRequestTriggered(doc))
+    .filter(({ doc }) => groupExpression(doc) === undefined)
+    .map(({ name }) => name)
+    .sort();
+}
+
+function withoutCancellation(parsedWorkflows, allowlist) {
+  return parsedWorkflows
+    .filter(({ doc }) => isPullRequestTriggered(doc))
+    .filter(({ name }) => !allowlist.has(name))
+    .filter(({ doc }) => groupExpression(doc) !== undefined)
+    .filter(({ doc }) => !cancelsInProgress(doc))
+    .map(({ name }) => name)
+    .sort();
+}
+
+function withConstantGroup(parsedWorkflows, allowlist) {
+  return parsedWorkflows
+    .filter(({ doc }) => isPullRequestTriggered(doc))
+    .filter(({ name }) => !allowlist.has(name))
+    .filter(({ doc }) => groupExpression(doc) !== undefined)
+    .filter(({ doc }) => !isPerPullRequest(groupExpression(doc)))
+    .map(({ name, doc }) => `${name}: ${groupExpression(doc)}`)
+    .sort();
+}
+
+// The constant-group rule applies to every workflow, not just PR-triggered
+// ones: a repository-wide key queues unrelated scheduled runs behind each
+// other just as effectively.
+function unexplainedConstantGroups(parsedWorkflows, allowlist) {
+  return parsedWorkflows
+    .filter(({ name }) => !allowlist.has(name))
+    .filter(({ doc }) => groupExpression(doc) !== undefined)
+    .filter(({ doc }) => !isPerPullRequest(groupExpression(doc)))
+    .map(({ name }) => name)
+    .sort();
+}
+
+// An entry in SERIALISED_WORKFLOWS is still earning its place while the
+// workflow either refuses to cancel or keys its group on something constant.
+// Once it does both of the things the contract asks for, the exemption is
+// describing nothing and has to go.
+function stillNeedsSerialisation(doc) {
+  return !cancelsInProgress(doc) || !isPerPullRequest(groupExpression(doc));
+}
+
+test('the pull_request trigger detection is not vacuous', () => {
+  assert.ok(workflows.length > 0, 'no workflow files found');
+  const prWorkflows = workflows
+    .filter(({ doc }) => isPullRequestTriggered(doc))
+    .map(({ name }) => name);
+  assert.ok(
+    prWorkflows.length > 0,
+    'no PR-triggered workflow was detected, so every test below would pass over an empty set',
+  );
+});
+
+test('every PR-triggered workflow declares a concurrency group', () => {
+  const offenders = withoutConcurrency(workflows);
+  assert.deepEqual(
+    offenders,
+    [],
+    `a pull_request workflow without 'concurrency:' leaves every superseded push running to completion and reporting onto the same commit:\n${offenders.join('\n')}`,
+  );
+});
+
+test('every PR-triggered concurrency group cancels the superseded run', () => {
+  const offenders = withoutCancellation(workflows, SERIALISED_WORKFLOWS);
+  assert.deepEqual(
+    offenders,
+    [],
+    `set 'cancel-in-progress: true', or record the workflow in SERIALISED_WORKFLOWS with the reason it must finish:\n${offenders.join('\n')}`,
+  );
+});
+
+test('every PR-triggered concurrency group is keyed per pull request', () => {
+  const offenders = withConstantGroup(workflows, SERIALISED_WORKFLOWS);
+  assert.deepEqual(
+    offenders,
+    [],
+    `a constant group serialises every pull request into one lane and cancels unrelated work; key the group on one of ${PER_PR_CONTEXTS.join(', ')}:\n${offenders.join('\n')}`,
+  );
+});
+
+test('every workflow with a constant concurrency group is recorded as deliberate', () => {
+  const unexplained = unexplainedConstantGroups(
+    workflows,
+    SERIALISED_WORKFLOWS,
+  );
+  assert.deepEqual(
+    unexplained,
+    [],
+    `a repository-wide constant group queues unrelated runs behind each other; key it per event or record it in SERIALISED_WORKFLOWS with the reason:\n${unexplained.join('\n')}`,
+  );
+});
+
+test('the serialised-workflow baseline retires itself', () => {
+  for (const [name, reason] of SERIALISED_WORKFLOWS) {
+    const entry = workflows.find((workflow) => workflow.name === name);
+    assert.ok(
+      entry,
+      `${name} is recorded in SERIALISED_WORKFLOWS but no such workflow exists; remove the entry`,
+    );
+    assert.ok(
+      typeof reason === 'string' && reason.trim().length > 0,
+      `${name}: record why this workflow must not be cancelled`,
+    );
+    assert.notEqual(
+      groupExpression(entry.doc),
+      undefined,
+      `${name} no longer declares a concurrency group, so the exemption describes nothing; remove it from SERIALISED_WORKFLOWS`,
+    );
+    assert.ok(
+      stillNeedsSerialisation(entry.doc),
+      `${name} now cancels in progress on a per-PR group; remove it from SERIALISED_WORKFLOWS so the gap cannot reopen`,
+    );
+  }
+});
+
+// Fixtures. Each one drives a scanner down the branch the repository's own
+// workflows never take, so the guards above cannot pass by never firing.
+const fixture = (name, doc) => [{ name, doc }];
+
+test('the concurrency scanners report the violations they exist to catch', () => {
+  const missing = fixture('new-ci.yml', {
+    on: { pull_request: null },
+    jobs: {},
+  });
+  assert.deepEqual(withoutConcurrency(missing), ['new-ci.yml']);
+
+  const uncancelled = fixture('new-ci.yml', {
+    on: { pull_request: null },
+    concurrency: { group: 'x-${{ github.ref }}' },
+  });
+  assert.deepEqual(withoutCancellation(uncancelled, new Map()), ['new-ci.yml']);
+  assert.deepEqual(
+    withoutCancellation(uncancelled, SERIALISED_WORKFLOWS),
+    ['new-ci.yml'],
+    'an unrelated allowlist entry must not excuse a different workflow',
+  );
+
+  const constant = fixture('new-ci.yml', {
+    on: { pull_request: null },
+    concurrency: { group: 'ci', 'cancel-in-progress': true },
+  });
+  assert.deepEqual(withConstantGroup(constant, new Map()), ['new-ci.yml: ci']);
+  assert.deepEqual(withConstantGroup(constant, SERIALISED_WORKFLOWS), [
+    'new-ci.yml: ci',
+  ]);
+});
+
+test('the scanners accept a compliant workflow and skip non-PR triggers', () => {
+  const compliant = fixture('new-ci.yml', {
+    on: { pull_request: null },
+    concurrency: {
+      group: 'new-ci-${{ github.event.pull_request.number || github.ref }}',
+      'cancel-in-progress': true,
+    },
+  });
+  assert.deepEqual(withoutConcurrency(compliant), []);
+  assert.deepEqual(withoutCancellation(compliant, new Map()), []);
+  assert.deepEqual(withConstantGroup(compliant, new Map()), []);
+
+  const scheduled = fixture('nightly.yml', {
+    on: { schedule: [{ cron: '0 0 * * *' }] },
+    jobs: {},
+  });
+  assert.deepEqual(withoutConcurrency(scheduled), []);
+  assert.deepEqual(withoutCancellation(scheduled, new Map()), []);
+  assert.deepEqual(withConstantGroup(scheduled, new Map()), []);
+});
+
+test('the trigger reader handles every form `on:` can take', () => {
+  assert.ok(isPullRequestTriggered({ on: 'pull_request' }));
+  assert.ok(isPullRequestTriggered({ on: ['push', 'pull_request'] }));
+  assert.ok(isPullRequestTriggered({ on: { pull_request: { branches: [] } } }));
+  // YAML 1.1 folds the bare key `on` to the boolean true.
+  assert.ok(isPullRequestTriggered({ [true]: { pull_request: null } }));
+  assert.ok(!isPullRequestTriggered({ on: { push: null } }));
+  assert.ok(!isPullRequestTriggered({}));
+});
+
+test('the group reader normalises both concurrency spellings', () => {
+  assert.equal(groupExpression({ concurrency: 'pages' }), 'pages');
+  assert.equal(groupExpression({ concurrency: { group: 'pages' } }), 'pages');
+  assert.equal(
+    groupExpression({ concurrency: { group: 'ci-${{ github.ref\n }}' } }),
+    'ci-${{ github.ref }}',
+  );
+  assert.equal(groupExpression({}), undefined);
+  assert.equal(
+    groupExpression({ concurrency: { 'cancel-in-progress': true } }),
+    undefined,
+  );
+  // A bare-string concurrency cannot carry cancel-in-progress.
+  assert.equal(cancelsInProgress({ concurrency: 'pages' }), false);
+  assert.equal(
+    cancelsInProgress({
+      concurrency: { group: 'x', 'cancel-in-progress': false },
+    }),
+    false,
+  );
+  assert.equal(
+    cancelsInProgress({
+      concurrency: { group: 'x', 'cancel-in-progress': true },
+    }),
+    true,
+  );
+});
+
+test('per-PR keying requires a real expression, not a literal context name', () => {
+  assert.ok(isPerPullRequest('ci-${{ github.ref }}'));
+  assert.ok(isPerPullRequest('ci-${{ github.event.pull_request.number }}'));
+  assert.ok(isPerPullRequest('ci-${{ github.head_ref }}'));
+  // Text that merely mentions a context is still one constant key.
+  assert.ok(!isPerPullRequest('github.ref'));
+  assert.ok(!isPerPullRequest('ci'));
+  // An expression that varies with nothing PR-specific still serialises PRs.
+  assert.ok(!isPerPullRequest('ci-${{ github.workflow }}'));
+  assert.ok(!isPerPullRequest(undefined));
+});
+
+test('the constant-group scan names a workflow the allowlist does not cover', () => {
+  const constant = fixture('nightly.yml', {
+    on: { schedule: [{ cron: '0 0 * * *' }] },
+    concurrency: { group: 'nightly', 'cancel-in-progress': true },
+  });
+  assert.deepEqual(unexplainedConstantGroups(constant, new Map()), [
+    'nightly.yml',
+  ]);
+  assert.deepEqual(
+    unexplainedConstantGroups(constant, SERIALISED_WORKFLOWS),
+    ['nightly.yml'],
+    'an unrelated allowlist entry must not excuse a different workflow',
+  );
+  assert.deepEqual(
+    unexplainedConstantGroups(constant, new Map([['nightly.yml', 'because']])),
+    [],
+  );
+  // A workflow with no concurrency block at all is the other contract's
+  // problem, not this one's.
+  assert.deepEqual(
+    unexplainedConstantGroups(
+      fixture('nightly.yml', { on: { schedule: [] }, jobs: {} }),
+      new Map(),
+    ),
+    [],
+  );
+});
+
+test('an exemption retires once the workflow satisfies both halves of the contract', () => {
+  // Constant group, cancels: still exempt on the keying half.
+  assert.ok(
+    stillNeedsSerialisation({
+      concurrency: { group: 'pages', 'cancel-in-progress': true },
+    }),
+  );
+  // Per-PR group, refuses to cancel: still exempt on the cancellation half.
+  assert.ok(
+    stillNeedsSerialisation({
+      concurrency: {
+        group: 'x-${{ github.ref }}',
+        'cancel-in-progress': false,
+      },
+    }),
+  );
+  // Both halves satisfied: the entry is describing nothing and must be removed.
+  assert.ok(
+    !stillNeedsSerialisation({
+      concurrency: {
+        group: 'x-${{ github.ref }}',
+        'cancel-in-progress': true,
+      },
+    }),
+  );
+});