diff --git a/.github/workflows/coverage.yml b/.github/workflows/coverage.yml
index 1ea4a3d..13892e4 100644
--- a/.github/workflows/coverage.yml
+++ b/.github/workflows/coverage.yml
@@ -38,6 +38,8 @@ jobs:
 
       - name: Validate error normalization
         run: pnpm errors:normalize:test && pnpm errors:normalize:check
+      - name: Validate workflow pnpm version matches packageManager
+        run: pnpm ci:pnpm-version:test && pnpm ci:pnpm-version:check
 
       - name: Mainnet readiness matches the committed manifest (recorded fixture, no RPC, no secrets)
         run: |
diff --git a/docs/CI.md b/docs/CI.md
index b9a7295..a5a3f82 100644
--- a/docs/CI.md
+++ b/docs/CI.md
@@ -60,6 +60,34 @@ ratio after including unexecuted `src/` files. Lower the threshold only when
 the existing codebase cannot meet a higher bar yet; prefer adding tests instead
 of weakening the gate.
 
+## pnpm version manifest check
+
+CI fails if any GitHub Actions workflow's `pnpm/action-setup` step declares
+a different pnpm version than the root `package.json` `packageManager` field
+(currently `pnpm@10.13.1`). This keeps every workflow on the exact pnpm
+version developers and the committed lockfile use.
+
+### Run locally
+
+```bash
+pnpm ci:pnpm-version:test   # unit tests for the checker itself
+pnpm ci:pnpm-version:check  # scans .github/workflows/*.yml
+```
+
+Checker source: `scripts/check-pnpm-version.mjs` (tests in
+`scripts/check-pnpm-version.test.mjs`).
+
+### CI workflow
+
+Wired into `.github/workflows/coverage.yml` (Node-only, runs on every push
+and pull request to `main`).
+
+### Changing the pnpm version
+
+Bump `packageManager` in `package.json` and every `pnpm/action-setup`
+`version:` in `.github/workflows/*.yml` together in the same PR; the check
+fails loudly if they drift apart.
+
 ## Bindings worktree cleanliness gate
 
 After `pnpm bindings:check` regenerates the round contract's TypeScript
diff --git a/package.json b/package.json
index 66e5b7c..dfcb0bc 100644
--- a/package.json
+++ b/package.json
@@ -86,6 +86,8 @@
     "time:guard:test": "node --test scripts/check-direct-time-access.test.mjs",
     "errors:normalize:check": "node scripts/check-error-normalization.mjs",
     "errors:normalize:test": "pnpm --filter @sub-rosa/logging test && node --test scripts/check-error-normalization.test.mjs",
+    "ci:pnpm-version:check": "node scripts/check-pnpm-version.mjs",
+    "ci:pnpm-version:test": "node --test scripts/check-pnpm-version.test.mjs",
     "line-lengths:check": "node scripts/check-line-lengths.mjs",
     "line-lengths:test": "node --test scripts/check-line-lengths.test.mjs"
   }
diff --git a/scripts/check-pnpm-version.mjs b/scripts/check-pnpm-version.mjs
new file mode 100644
index 0000000..29ab8dd
--- /dev/null
+++ b/scripts/check-pnpm-version.mjs
@@ -0,0 +1,89 @@
+#!/usr/bin/env node
+/**
+ * Verify every GitHub Actions workflow's `pnpm/action-setup` version matches
+ * the root `package.json` `packageManager` field, so CI never silently runs
+ * a different pnpm version than the one developers and lockfiles use.
+ */
+import { readFileSync, readdirSync } from 'node:fs';
+import { join } from 'node:path';
+import { fileURLToPath, pathToFileURL } from 'node:url';
+import { createLogger } from '../packages/logging/src/index.cjs';
+
+const root = fileURLToPath(new URL('..', import.meta.url));
+const logger = createLogger('scripts.check-pnpm-version');
+const WORKFLOWS_DIR = '.github/workflows';
+
+/**
+ * @param {string} packageJsonText
+ * @returns {string} the pnpm version declared by `packageManager`, e.g. "10.13.1"
+ */
+export function parsePackageManagerVersion(packageJsonText) {
+  const pkg = JSON.parse(packageJsonText);
+  const packageManager = pkg.packageManager;
+  if (typeof packageManager !== 'string') {
+    throw new Error('package.json is missing a "packageManager" field');
+  }
+  const match = packageManager.match(/^pnpm@([0-9][0-9A-Za-z.+-]*)$/);
+  if (!match) {
+    throw new Error(`Unrecognized packageManager format: ${packageManager}`);
+  }
+  return match[1];
+}
+
+/**
+ * @typedef {{ file: string, version: string }} WorkflowPnpmVersion
+ * @param {string} workflowDir
+ * @param {(path: string) => string} readFile
+ * @param {(path: string) => string[]} listDir
+ * @returns {WorkflowPnpmVersion[]}
+ */
+export function findWorkflowPnpmVersions(
+  workflowDir,
+  readFile = (path) => readFileSync(path, 'utf8'),
+  listDir = (path) => readdirSync(path),
+) {
+  const results = [];
+  for (const name of listDir(workflowDir)) {
+    if (!/\.ya?ml$/.test(name)) continue;
+    const text = readFile(join(workflowDir, name));
+    const stepPattern = /uses:\s*pnpm\/action-setup@v\d+[\s\S]{0,120}?version:\s*["']?([^"'\s]+)["']?/g;
+    for (const match of text.matchAll(stepPattern)) {
+      results.push({ file: name, version: match[1] });
+    }
+  }
+  return results;
+}
+
+/**
+ * @param {string} expectedVersion
+ * @param {WorkflowPnpmVersion[]} entries
+ * @returns {WorkflowPnpmVersion[]}
+ */
+export function findMismatches(expectedVersion, entries) {
+  return entries.filter((entry) => entry.version !== expectedVersion);
+}
+
+if (process.argv[1] && import.meta.url === pathToFileURL(process.argv[1]).href) {
+  const expected = parsePackageManagerVersion(readFileSync(join(root, 'package.json'), 'utf8'));
+  const entries = findWorkflowPnpmVersions(join(root, WORKFLOWS_DIR));
+  const mismatches = findMismatches(expected, entries);
+
+  if (entries.length === 0) {
+    logger.error('pnpm-version-missing', 'No pnpm/action-setup steps found in .github/workflows', {
+      workflowsDir: WORKFLOWS_DIR,
+    });
+    process.exitCode = 1;
+  } else if (mismatches.length > 0) {
+    logger.error(
+      'pnpm-version-mismatch',
+      `Workflow pnpm/action-setup version must match package.json packageManager (pnpm@${expected})`,
+      { expected, mismatches },
+    );
+    process.exitCode = 1;
+  } else {
+    logger.info(
+      'pnpm-version-check-passed',
+      `All ${entries.length} workflow pnpm/action-setup version(s) match packageManager (pnpm@${expected})`,
+    );
+  }
+}
diff --git a/scripts/check-pnpm-version.test.mjs b/scripts/check-pnpm-version.test.mjs
new file mode 100644
index 0000000..c9917c1
--- /dev/null
+++ b/scripts/check-pnpm-version.test.mjs
@@ -0,0 +1,82 @@
+import assert from 'node:assert/strict';
+import test from 'node:test';
+import {
+  parsePackageManagerVersion,
+  findWorkflowPnpmVersions,
+  findMismatches,
+} from './check-pnpm-version.mjs';
+
+test('parsePackageManagerVersion extracts the version from a pnpm@x.y.z string', () => {
+  const pkg = JSON.stringify({ packageManager: 'pnpm@10.13.1' });
+  assert.equal(parsePackageManagerVersion(pkg), '10.13.1');
+});
+
+test('parsePackageManagerVersion throws when packageManager is missing', () => {
+  assert.throws(() => parsePackageManagerVersion(JSON.stringify({})), /missing/);
+});
+
+test('parsePackageManagerVersion throws for a non-pnpm packageManager', () => {
+  assert.throws(
+    () => parsePackageManagerVersion(JSON.stringify({ packageManager: 'yarn@4.0.0' })),
+    /Unrecognized packageManager format/,
+  );
+});
+
+test('findWorkflowPnpmVersions reads the version from each workflow file', () => {
+  const files = {
+    'a.yml': [
+      'jobs:',
+      '  build:',
+      '    steps:',
+      '      - uses: pnpm/action-setup@v4',
+      '        with:',
+      '          version: 10.13.1',
+    ].join('\n'),
+    'b.yml': [
+      'jobs:',
+      '  test:',
+      '    steps:',
+      "      - uses: pnpm/action-setup@v4",
+      '        with:',
+      '          version: "9.0.0"',
+    ].join('\n'),
+    'not-a-workflow.txt': 'irrelevant',
+  };
+  const result = findWorkflowPnpmVersions(
+    '.github/workflows',
+    (path) => files[path.split('/').pop()],
+    () => Object.keys(files),
+  );
+
+  assert.deepEqual(result, [
+    { file: 'a.yml', version: '10.13.1' },
+    { file: 'b.yml', version: '9.0.0' },
+  ]);
+});
+
+test('findWorkflowPnpmVersions ignores workflows with no pnpm/action-setup step', () => {
+  const files = { 'rust-only.yml': 'jobs:\n  build:\n    steps:\n      - uses: actions/checkout@v4\n' };
+  const result = findWorkflowPnpmVersions(
+    '.github/workflows',
+    (path) => files[path.split('/').pop()],
+    () => Object.keys(files),
+  );
+  assert.deepEqual(result, []);
+});
+
+test('findMismatches returns only entries that differ from the expected version', () => {
+  const entries = [
+    { file: 'a.yml', version: '10.13.1' },
+    { file: 'b.yml', version: '9.0.0' },
+    { file: 'c.yml', version: '10.13.1' },
+  ];
+  assert.deepEqual(findMismatches('10.13.1', entries), [{ file: 'b.yml', version: '9.0.0' }]);
+});
+
+test('findMismatches returns an empty array when every entry matches', () => {
+  const entries = [
+    { file: 'a.yml', version: '10.13.1' },
+    { file: 'b.yml', version: '10.13.1' },
+  ];
+  assert.deepEqual(findMismatches('10.13.1', entries), []);
+});