diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
new file mode 100644
index 0000000..4ee7690
--- /dev/null
+++ b/.github/workflows/ci.yml
@@ -0,0 +1,37 @@
+# Mirrors the root package.json scripts step for step: keep the two in
+# correspondence when adding a check to either side.
+name: CI
+
+on:
+  pull_request:
+  push:
+    branches: [main]
+
+permissions:
+  contents: read
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  verify:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 22
+          cache: npm
+
+      - run: npm ci
+
+      - run: npm run typecheck
+
+      - run: npm test
+
+      - run: npm run build
+
+      - name: Version lockstep
+        run: npm run check-version
diff --git a/README.md b/README.md
index d563660..66851a0 100644
--- a/README.md
+++ b/README.md
@@ -42,14 +42,31 @@ npm test
 npm run build
 ```
 
-Releases bump every package to one version, then publish them all:
+Releases bump every package to one version, then publish them all. `npm version`
+fans the bump out through `scripts/set-version.mjs` (workspace manifests, internal
+ranges, embedded `VERSION` constants) and refreshes the lockfile. It also creates
+a release commit and a `v<version>` tag, and refuses to run on a dirty tree, so
+commit or stash first. Pass `-m` to keep the `chore(release): vX.Y.Z` commit
+convention:
 
 ```sh
-npm run set-version -- 0.2.0
+npm version 0.2.0 -m 'chore(release): v%s'
 npm run build
 npm publish --workspaces
 ```
 
+Add `--no-git-tag-version` to skip the commit and tag; the bump is then left
+staged for you to commit yourself.
+
+To bump without `npm version`, run `npm run set-version -- 0.2.0` followed by
+`npm install --package-lock-only`; `check-version` does not inspect the lockfile,
+so a bare `set-version` leaves `package-lock.json` behind unnoticed.
+
+`npm run check-version` verifies the lockstep without changing anything:
+every manifest on the root version, every internal dependency range at
+`^<version>`, every embedded `VERSION` constant matching. CI runs it alongside
+typecheck, test, and build on every pull request and push to `main`.
+
 ## Getting started (operators)
 
 ```sh
diff --git a/okf-bundle/interfaces/packages.md b/okf-bundle/interfaces/packages.md
index 9a47e89..3cf6f1f 100644
--- a/okf-bundle/interfaces/packages.md
+++ b/okf-bundle/interfaces/packages.md
@@ -30,11 +30,19 @@ between "millwright's own deployment is CDK" and "your workflows are not CDK".
 ```sh
 npm install && npm run typecheck && npm test && npm run build
 
-npm run set-version -- 0.2.0    # bump every package to one version
+npm version 0.2.0 -m 'chore(release): v%s'   # bump every package, commit, tag v0.2.0
 npm run build
 npm publish --workspaces
 ```
 
+`npm run check-version` (`scripts/set-version.mjs --check`) asserts the lockstep without writing:
+every manifest on the root version, every `@copperbox/millwright-*` range at `^<version>`, every
+`src/version.ts` `VERSION` matching. `.github/workflows/ci.yml` runs typecheck, test, build, and
+this check on pull requests and pushes to `main`. The root `version` lifecycle script fans an
+`npm version` bump through `set-version.mjs` and refreshes the lockfile. `npm version` then
+commits and tags `v<version>`, and it refuses to run on a dirty tree; `--no-git-tag-version`
+leaves the bump staged without a commit or tag. Publishing stays manual.
+
 ## Related
 
 - [Deployment construct](deployment.md) · [Run model](../schemas/run-model.md) for the
diff --git a/package-lock.json b/package-lock.json
index 2511a11..8496681 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -1,12 +1,12 @@
 {
   "name": "millwright",
-  "version": "0.6.3",
+  "version": "0.7.0",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "millwright",
-      "version": "0.6.3",
+      "version": "0.7.0",
       "license": "MIT",
       "workspaces": [
         "packages/*"
@@ -2727,12 +2727,12 @@
     },
     "packages/millwright-cdk": {
       "name": "@copperbox/millwright-cdk",
-      "version": "0.6.2",
+      "version": "0.7.0",
       "license": "MIT",
       "dependencies": {
-        "@copperbox/millwright-cli": "^0.6.2",
-        "@copperbox/millwright-state": "^0.6.2",
-        "@copperbox/millwright-workflows": "^0.6.2",
+        "@copperbox/millwright-cli": "^0.7.0",
+        "@copperbox/millwright-state": "^0.7.0",
+        "@copperbox/millwright-workflows": "^0.7.0",
         "esbuild": "^0.28.0",
         "ssh2": "^1.17.0"
       },
@@ -2760,7 +2760,7 @@
     },
     "packages/millwright-cli": {
       "name": "@copperbox/millwright-cli",
-      "version": "0.6.2",
+      "version": "0.7.0",
       "license": "MIT",
       "dependencies": {
         "@aws-sdk/client-cloudwatch-logs": "^3.1108.0",
@@ -2773,8 +2773,8 @@
         "@aws-sdk/client-sfn": "^3.1108.0",
         "@aws-sdk/client-ssm": "^3.700.0",
         "@aws-sdk/lib-dynamodb": "^3.1108.0",
-        "@copperbox/millwright-state": "^0.6.2",
-        "@copperbox/millwright-workflows": "^0.6.2",
+        "@copperbox/millwright-state": "^0.7.0",
+        "@copperbox/millwright-workflows": "^0.7.0",
         "commander": "^12.1.0",
         "ssh2": "^1.17.0",
         "typescript": "^5.7.0"
@@ -2792,7 +2792,7 @@
     },
     "packages/millwright-state": {
       "name": "@copperbox/millwright-state",
-      "version": "0.6.2",
+      "version": "0.7.0",
       "license": "MIT",
       "engines": {
         "node": ">=20"
@@ -2800,7 +2800,7 @@
     },
     "packages/millwright-workflows": {
       "name": "@copperbox/millwright-workflows",
-      "version": "0.6.2",
+      "version": "0.7.0",
       "license": "MIT",
       "engines": {
         "node": ">=20"
diff --git a/package.json b/package.json
index ad0b294..e47b8da 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "millwright",
-  "version": "0.6.3",
+  "version": "0.7.0",
   "private": true,
   "description": "Millwright monorepo — polling-driven CI/CD in your own AWS account",
   "license": "MIT",
@@ -11,7 +11,9 @@
     "build": "npm run build --workspace @copperbox/millwright-state --workspace @copperbox/millwright-workflows --workspace @copperbox/millwright-cdk --workspace @copperbox/millwright-cli",
     "typecheck": "npm run typecheck --workspaces",
     "test": "vitest run",
-    "set-version": "node scripts/set-version.mjs"
+    "set-version": "node scripts/set-version.mjs",
+    "check-version": "node scripts/set-version.mjs --check",
+    "version": "node scripts/set-version.mjs && npm install --package-lock-only && git add -u package.json packages package-lock.json"
   },
   "devDependencies": {
     "@types/node": "^22.10.0",
diff --git a/packages/millwright-cdk/package.json b/packages/millwright-cdk/package.json
index 9204181..ff3a169 100644
--- a/packages/millwright-cdk/package.json
+++ b/packages/millwright-cdk/package.json
@@ -1,6 +1,6 @@
 {
   "name": "@copperbox/millwright-cdk",
-  "version": "0.6.2",
+  "version": "0.7.0",
   "description": "The Millwright CDK construct — deploys the millwright control plane into your AWS account",
   "license": "MIT",
   "repository": {
@@ -23,9 +23,9 @@
     "test": "vitest run"
   },
   "dependencies": {
-    "@copperbox/millwright-cli": "^0.6.2",
-    "@copperbox/millwright-state": "^0.6.2",
-    "@copperbox/millwright-workflows": "^0.6.2",
+    "@copperbox/millwright-cli": "^0.7.0",
+    "@copperbox/millwright-state": "^0.7.0",
+    "@copperbox/millwright-workflows": "^0.7.0",
     "esbuild": "^0.28.0",
     "ssh2": "^1.17.0"
   },
diff --git a/packages/millwright-cdk/src/version.ts b/packages/millwright-cdk/src/version.ts
index d811ac7..a3545c8 100644
--- a/packages/millwright-cdk/src/version.ts
+++ b/packages/millwright-cdk/src/version.ts
@@ -1,5 +1,5 @@
 // Kept in lockstep with package.json by scripts/set-version.mjs — do not edit by hand.
-export const VERSION = '0.6.2';
+export const VERSION = '0.7.0';
 
 /**
  * Highest run-model schemaVersion this control plane accepts. Synth fails
diff --git a/packages/millwright-cdk/test/millwright.test.ts b/packages/millwright-cdk/test/millwright.test.ts
index 02d94ea..65fb8d8 100644
--- a/packages/millwright-cdk/test/millwright.test.ts
+++ b/packages/millwright-cdk/test/millwright.test.ts
@@ -3,12 +3,6 @@ import { Annotations, Match, Template } from 'aws-cdk-lib/assertions';
 import * as iam from 'aws-cdk-lib/aws-iam';
 import { describe, expect, it } from 'vitest';
 import { Boundary, Millwright, MillwrightProps, SUPPORTED_SCHEMA_VERSION, VERSION } from '../src';
-import cdkPkg from '../package.json';
-import rootPkg from '../../../package.json';
-import cliPkg from '../../millwright-cli/package.json';
-import statePkg from '../../millwright-state/package.json';
-import workflowsPkg from '../../millwright-workflows/package.json';
-import { VERSION as CLI_VERSION } from '../../millwright-cli/src/version';
 
 const BOUNDARY_ARN = 'arn:aws:iam::123456789012:policy/team-boundary';
 
@@ -166,19 +160,3 @@ describe('run executor wiring', () => {
     expect(millwright.buildProject.projectName).toBe(millwright.runExecutor.buildProjectName);
   });
 });
-
-describe('lockstep version', () => {
-  it('keeps the embedded VERSION constants in sync with their manifests', () => {
-    expect(VERSION).toBe(cdkPkg.version);
-    expect(CLI_VERSION).toBe(cliPkg.version);
-  });
-
-  it('keeps every workspace manifest on the root version', () => {
-    // A hand-edited bump that skips `npm run set-version` moves some subset
-    // of the five manifests; pinning all four workspaces to the root catches
-    // any divergence, not just the cdk one.
-    for (const pkg of [cdkPkg, cliPkg, statePkg, workflowsPkg]) {
-      expect(pkg.version, pkg.name).toBe(rootPkg.version);
-    }
-  });
-});
diff --git a/packages/millwright-cli/package.json b/packages/millwright-cli/package.json
index 7bd81c8..9aae3a6 100644
--- a/packages/millwright-cli/package.json
+++ b/packages/millwright-cli/package.json
@@ -1,6 +1,6 @@
 {
   "name": "@copperbox/millwright-cli",
-  "version": "0.6.2",
+  "version": "0.7.0",
   "description": "millwright CLI — operate a millwright deployment from operator and developer machines",
   "license": "MIT",
   "repository": {
@@ -33,8 +33,8 @@
     "@aws-sdk/client-sfn": "^3.1108.0",
     "@aws-sdk/client-ssm": "^3.700.0",
     "@aws-sdk/lib-dynamodb": "^3.1108.0",
-    "@copperbox/millwright-state": "^0.6.2",
-    "@copperbox/millwright-workflows": "^0.6.2",
+    "@copperbox/millwright-state": "^0.7.0",
+    "@copperbox/millwright-workflows": "^0.7.0",
     "commander": "^12.1.0",
     "ssh2": "^1.17.0",
     "typescript": "^5.7.0"
diff --git a/packages/millwright-cli/src/version.ts b/packages/millwright-cli/src/version.ts
index cf52011..88a9716 100644
--- a/packages/millwright-cli/src/version.ts
+++ b/packages/millwright-cli/src/version.ts
@@ -1,2 +1,2 @@
 // Kept in lockstep with package.json by scripts/set-version.mjs — do not edit by hand.
-export const VERSION = '0.6.2';
+export const VERSION = '0.7.0';
diff --git a/packages/millwright-state/package.json b/packages/millwright-state/package.json
index 1039b19..813e381 100644
--- a/packages/millwright-state/package.json
+++ b/packages/millwright-state/package.json
@@ -1,6 +1,6 @@
 {
   "name": "@copperbox/millwright-state",
-  "version": "0.6.2",
+  "version": "0.7.0",
   "description": "Millwright's shared data-plane helpers — state/polling table item accessors, SSM config-plane paths, S3 layout",
   "license": "MIT",
   "repository": {
diff --git a/packages/millwright-workflows/package.json b/packages/millwright-workflows/package.json
index 870a7dc..f52ef08 100644
--- a/packages/millwright-workflows/package.json
+++ b/packages/millwright-workflows/package.json
@@ -1,6 +1,6 @@
 {
   "name": "@copperbox/millwright-workflows",
-  "version": "0.6.2",
+  "version": "0.7.0",
   "description": "Millwright workflow definition library — the only install in watched repos",
   "license": "MIT",
   "repository": {
diff --git a/scripts/set-version.mjs b/scripts/set-version.mjs
index 448e58a..ceb06e1 100644
--- a/scripts/set-version.mjs
+++ b/scripts/set-version.mjs
@@ -1,22 +1,23 @@
 #!/usr/bin/env node
-// Lockstep version bump: all three packages (and the root) always share one
-// version, and the embedded src/version.ts constants track package.json.
+// Lockstep version bump: all four packages (and the root) always share one
+// version, every internal @copperbox/millwright-* dependency range is
+// ^<version>, and the embedded src/version.ts constants track package.json.
 //
-//   npm run set-version -- 0.2.0
+//   npm version 0.2.0              # bump everything to 0.2.0 (via the
+//                                  # root `version` lifecycle script)
+//   npm run set-version -- 0.2.0   # the same bump without npm version
+//   npm run set-version -- --check # verify the tree is in lockstep (CI)
+//
+// Under `npm version` the script takes no argument: npm exports the new
+// version as npm_package_version for every lifecycle script on every
+// platform, whereas `$npm_package_version` in package.json only expands
+// under a POSIX shell (cmd.exe hands it over as a literal).
 
 import { readFileSync, writeFileSync, existsSync } from 'node:fs';
 import { join, dirname } from 'node:path';
-import { fileURLToPath } from 'node:url';
-
-const root = join(dirname(fileURLToPath(import.meta.url)), '..');
-const version = process.argv[2];
+import { fileURLToPath, pathToFileURL } from 'node:url';
 
-if (!version || !/^\d+\.\d+\.\d+(-[0-9A-Za-z-.]+)?$/.test(version)) {
-  console.error('Usage: npm run set-version -- <semver>');
-  process.exit(1);
-}
-
-const packageDirs = [
+export const packageDirs = [
   '.',
   'packages/millwright-state',
   'packages/millwright-workflows',
@@ -24,24 +25,122 @@ const packageDirs = [
   'packages/millwright-cli',
 ];
 
-for (const dir of packageDirs) {
-  const manifestPath = join(root, dir, 'package.json');
-  const manifest = JSON.parse(readFileSync(manifestPath, 'utf8'));
-  manifest.version = version;
-  for (const block of ['dependencies', 'devDependencies', 'peerDependencies', 'optionalDependencies']) {
-    for (const dep of Object.keys(manifest[block] ?? {})) {
-      if (dep.startsWith('@copperbox/millwright-')) manifest[block][dep] = `^${version}`;
+const dependencyBlocks = ['dependencies', 'devDependencies', 'peerDependencies', 'optionalDependencies'];
+const internalScope = '@copperbox/millwright-';
+const versionPattern = /^\d+\.\d+\.\d+(-[0-9A-Za-z-.]+)?$/;
+const versionConstant = /export const VERSION = '([^']*)';/;
+
+function readManifest(root, dir) {
+  return JSON.parse(readFileSync(join(root, dir, 'package.json'), 'utf8'));
+}
+
+function versionTsPath(root, dir) {
+  const path = join(root, dir, 'src', 'version.ts');
+  return existsSync(path) ? path : undefined;
+}
+
+/**
+ * Rewrites every manifest and embedded VERSION constant under `root` to
+ * `version`. Returns one log line per manifest touched.
+ */
+export function applyVersion(root, version) {
+  const log = [];
+  for (const dir of packageDirs) {
+    const manifest = readManifest(root, dir);
+    manifest.version = version;
+    for (const block of dependencyBlocks) {
+      for (const dep of Object.keys(manifest[block] ?? {})) {
+        if (dep.startsWith(internalScope)) manifest[block][dep] = `^${version}`;
+      }
+    }
+    writeFileSync(join(root, dir, 'package.json'), `${JSON.stringify(manifest, null, 2)}\n`);
+    log.push(`${manifest.name} -> ${version}`);
+
+    const versionTs = versionTsPath(root, dir);
+    if (versionTs) {
+      const source = readFileSync(versionTs, 'utf8').replace(
+        versionConstant,
+        `export const VERSION = '${version}';`,
+      );
+      writeFileSync(versionTs, source);
     }
   }
-  writeFileSync(manifestPath, `${JSON.stringify(manifest, null, 2)}\n`);
-  console.log(`${manifest.name} -> ${version}`);
-
-  const versionTs = join(root, dir, 'src', 'version.ts');
-  if (existsSync(versionTs)) {
-    const source = readFileSync(versionTs, 'utf8').replace(
-      /export const VERSION = '[^']*';/,
-      `export const VERSION = '${version}';`,
-    );
-    writeFileSync(versionTs, source);
+  return log;
+}
+
+/**
+ * Compares every manifest and embedded VERSION constant under `root` against
+ * the root manifest's version. Returns the root version and one line per
+ * mismatch; an empty list means the tree is in lockstep.
+ */
+export function checkLockstep(root) {
+  const { version } = readManifest(root, '.');
+  const mismatches = [];
+  for (const dir of packageDirs) {
+    const manifestPath = join(dir, 'package.json');
+    const manifest = readManifest(root, dir);
+    if (manifest.version !== version) {
+      mismatches.push(`${manifestPath}: version is ${manifest.version}, expected ${version}`);
+    }
+    for (const block of dependencyBlocks) {
+      for (const [dep, range] of Object.entries(manifest[block] ?? {})) {
+        if (dep.startsWith(internalScope) && range !== `^${version}`) {
+          mismatches.push(`${manifestPath}: ${block}.${dep} is ${range}, expected ^${version}`);
+        }
+      }
+    }
+
+    const versionTs = versionTsPath(root, dir);
+    if (versionTs) {
+      const found = readFileSync(versionTs, 'utf8').match(versionConstant)?.[1];
+      if (found !== version) {
+        mismatches.push(`${join(dir, 'src', 'version.ts')}: VERSION is ${found ?? 'missing'}, expected ${version}`);
+      }
+    }
+  }
+  return { version, mismatches };
+}
+
+function usage() {
+  console.error('Usage: npm run set-version -- <semver>');
+  console.error('       npm run set-version -- --check');
+  console.error('       npm version <semver>   (reads npm_package_version)');
+  process.exit(1);
+}
+
+/**
+ * The version to apply: the positional argument, or, when running as npm's
+ * `version` lifecycle script, the freshly bumped npm_package_version.
+ */
+export function resolveVersion(positional, env) {
+  if (positional.length > 0) return positional[0];
+  if (env.npm_lifecycle_event === 'version') return env.npm_package_version;
+  return undefined;
+}
+
+function main(args) {
+  const root = join(dirname(fileURLToPath(import.meta.url)), '..');
+  const check = args.includes('--check');
+  const positional = args.filter((arg) => arg !== '--check');
+  if (positional.length > 1 || (check && positional.length > 0)) usage();
+
+  if (check) {
+    const { version, mismatches } = checkLockstep(root);
+    if (mismatches.length > 0) {
+      console.error(`Version lockstep broken (root is ${version}):`);
+      for (const line of mismatches) console.error(`  ${line}`);
+      console.error('Run `npm run set-version -- <semver>` to realign every package.');
+      process.exit(1);
+    }
+    console.log(`All packages in lockstep at ${version}.`);
+    return;
   }
+
+  const version = resolveVersion(positional, process.env);
+  if (!version || !versionPattern.test(version)) usage();
+  for (const line of applyVersion(root, version)) console.log(line);
+}
+
+if (process.argv[1] && import.meta.url === pathToFileURL(process.argv[1]).href) {
+  main(process.argv.slice(2));
 }
diff --git a/scripts/set-version.test.mjs b/scripts/set-version.test.mjs
new file mode 100644
index 0000000..25ce109
--- /dev/null
+++ b/scripts/set-version.test.mjs
@@ -0,0 +1,217 @@
+import { afterEach, describe, expect, it } from 'vitest';
+import { copyFileSync, mkdtempSync, mkdirSync, readFileSync, rmSync, writeFileSync } from 'node:fs';
+import { tmpdir } from 'node:os';
+import { join } from 'node:path';
+import { spawnSync } from 'node:child_process';
+import { applyVersion, checkLockstep, packageDirs, resolveVersion } from './set-version.mjs';
+
+const script = join(import.meta.dirname, 'set-version.mjs');
+
+const names = {
+  '.': 'millwright',
+  'packages/millwright-state': '@copperbox/millwright-state',
+  'packages/millwright-workflows': '@copperbox/millwright-workflows',
+  'packages/millwright-cdk': '@copperbox/millwright-cdk',
+  'packages/millwright-cli': '@copperbox/millwright-cli',
+};
+
+const internalDeps = {
+  'packages/millwright-cdk': {
+    dependencies: ['@copperbox/millwright-state', '@copperbox/millwright-workflows'],
+    devDependencies: ['@copperbox/millwright-cli'],
+  },
+  'packages/millwright-cli': {
+    dependencies: ['@copperbox/millwright-state', '@copperbox/millwright-workflows'],
+  },
+};
+
+const withVersionTs = new Set(['packages/millwright-cdk', 'packages/millwright-cli']);
+
+const tmpdirs = [];
+
+afterEach(() => {
+  for (const dir of tmpdirs.splice(0)) {
+    rmSync(dir, { recursive: true, force: true });
+  }
+});
+
+/** Builds a minimal lockstep tree at `version` and returns its root. */
+function fixture(version) {
+  const root = mkdtempSync(join(tmpdir(), 'set-version-'));
+  tmpdirs.push(root);
+  for (const dir of packageDirs) {
+    mkdirSync(join(root, dir), { recursive: true });
+    const manifest = { name: names[dir], version };
+    for (const [block, deps] of Object.entries(internalDeps[dir] ?? {})) {
+      manifest[block] = Object.fromEntries(deps.map((dep) => [dep, `^${version}`]));
+    }
+    manifest.devDependencies = { ...(manifest.devDependencies ?? {}), typescript: '^5.7.0' };
+    writeFileSync(join(root, dir, 'package.json'), `${JSON.stringify(manifest, null, 2)}\n`);
+    if (withVersionTs.has(dir)) {
+      mkdirSync(join(root, dir, 'src'), { recursive: true });
+      writeFileSync(
+        join(root, dir, 'src', 'version.ts'),
+        `// Kept in lockstep with package.json by scripts/set-version.mjs — do not edit by hand.\nexport const VERSION = '${version}';\n`,
+      );
+    }
+  }
+  return root;
+}
+
+function readManifest(root, dir) {
+  return JSON.parse(readFileSync(join(root, dir, 'package.json'), 'utf8'));
+}
+
+function writeManifest(root, dir, manifest) {
+  writeFileSync(join(root, dir, 'package.json'), `${JSON.stringify(manifest, null, 2)}\n`);
+}
+
+describe('checkLockstep', () => {
+  it('reports no mismatches for a tree in lockstep', () => {
+    const root = fixture('0.6.3');
+    expect(checkLockstep(root)).toEqual({ version: '0.6.3', mismatches: [] });
+  });
+
+  it('names a package whose version drifted from the root', () => {
+    const root = fixture('0.6.3');
+    const manifest = readManifest(root, 'packages/millwright-cli');
+    manifest.version = '0.6.2';
+    writeManifest(root, 'packages/millwright-cli', manifest);
+
+    const { mismatches } = checkLockstep(root);
+    expect(mismatches).toHaveLength(1);
+    expect(mismatches[0]).toContain('packages/millwright-cli/package.json');
+    expect(mismatches[0]).toContain('0.6.2');
+    expect(mismatches[0]).toContain('0.6.3');
+  });
+
+  it('names an internal dependency range that is not ^<root version>', () => {
+    const root = fixture('0.6.3');
+    const manifest = readManifest(root, 'packages/millwright-cdk');
+    manifest.devDependencies['@copperbox/millwright-cli'] = '^0.6.2';
+    writeManifest(root, 'packages/millwright-cdk', manifest);
+
+    const { mismatches } = checkLockstep(root);
+    expect(mismatches).toHaveLength(1);
+    expect(mismatches[0]).toContain('packages/millwright-cdk/package.json');
+    expect(mismatches[0]).toContain('devDependencies');
+    expect(mismatches[0]).toContain('@copperbox/millwright-cli');
+    expect(mismatches[0]).toContain('^0.6.2');
+    expect(mismatches[0]).toContain('^0.6.3');
+  });
+
+  it('ignores external dependency ranges', () => {
+    const root = fixture('0.6.3');
+    const manifest = readManifest(root, 'packages/millwright-cdk');
+    manifest.devDependencies.typescript = '^0.0.1';
+    writeManifest(root, 'packages/millwright-cdk', manifest);
+    expect(checkLockstep(root).mismatches).toEqual([]);
+  });
+
+  it('names an embedded VERSION constant that drifted', () => {
+    const root = fixture('0.6.3');
+    writeFileSync(
+      join(root, 'packages/millwright-cdk/src/version.ts'),
+      "export const VERSION = '0.6.2';\n",
+    );
+
+    const { mismatches } = checkLockstep(root);
+    expect(mismatches).toHaveLength(1);
+    expect(mismatches[0]).toContain('packages/millwright-cdk/src/version.ts');
+    expect(mismatches[0]).toContain('0.6.2');
+    expect(mismatches[0]).toContain('0.6.3');
+  });
+
+  it('lists every mismatch rather than stopping at the first', () => {
+    const root = fixture('0.6.3');
+    for (const dir of ['packages/millwright-state', 'packages/millwright-workflows']) {
+      const manifest = readManifest(root, dir);
+      manifest.version = '0.6.2';
+      writeManifest(root, dir, manifest);
+    }
+    writeFileSync(join(root, 'packages/millwright-cli/src/version.ts'), "export const VERSION = '0.5.0';\n");
+
+    const { mismatches } = checkLockstep(root);
+    expect(mismatches).toHaveLength(3);
+  });
+});
+
+describe('applyVersion', () => {
+  it('leaves a drifted tree in lockstep at the new version', () => {
+    const root = fixture('0.6.2');
+    const manifest = readManifest(root, 'packages/millwright-cli');
+    manifest.version = '0.5.0';
+    writeManifest(root, 'packages/millwright-cli', manifest);
+
+    applyVersion(root, '0.6.3');
+
+    expect(checkLockstep(root)).toEqual({ version: '0.6.3', mismatches: [] });
+    expect(readManifest(root, 'packages/millwright-cdk').dependencies['@copperbox/millwright-state']).toBe('^0.6.3');
+    expect(readFileSync(join(root, 'packages/millwright-cli/src/version.ts'), 'utf8')).toContain(
+      "export const VERSION = '0.6.3';",
+    );
+  });
+});
+
+describe('resolveVersion', () => {
+  it('reads npm_package_version only under the version lifecycle', () => {
+    const env = { npm_lifecycle_event: 'version', npm_package_version: '1.2.3' };
+    expect(resolveVersion([], env)).toBe('1.2.3');
+    expect(resolveVersion([], { ...env, npm_lifecycle_event: 'set-version' })).toBeUndefined();
+    expect(resolveVersion([], { npm_package_version: '1.2.3' })).toBeUndefined();
+    expect(resolveVersion(['0.9.0'], env)).toBe('0.9.0');
+  });
+});
+
+describe('command line', () => {
+  it('rejects a missing or malformed version', () => {
+    for (const args of [[], ['1.2']]) {
+      const result = spawnSync(process.execPath, [script, ...args], { encoding: 'utf8' });
+      expect(result.status, args.join(' ')).toBe(1);
+      expect(result.stderr).toContain('Usage:');
+    }
+  });
+
+  it('ignores npm_package_version outside the version lifecycle', () => {
+    const env = { ...process.env, npm_package_version: '1.2.3' };
+    delete env.npm_lifecycle_event;
+    const result = spawnSync(process.execPath, [script], { encoding: 'utf8', env });
+    expect(result.status).toBe(1);
+    expect(result.stderr).toContain('Usage:');
+  });
+
+  it('validates npm_package_version under the version lifecycle', () => {
+    const env = { ...process.env, npm_lifecycle_event: 'version', npm_package_version: 'not-a-version' };
+    const result = spawnSync(process.execPath, [script], { encoding: 'utf8', env });
+    expect(result.status).toBe(1);
+    expect(result.stderr).toContain('Usage:');
+  });
+
+  it('applies npm_package_version under the version lifecycle', () => {
+    // The script resolves its tree from its own location, so a copy inside a
+    // fixture rewrites the fixture rather than this checkout.
+    const root = fixture('0.6.3');
+    mkdirSync(join(root, 'scripts'));
+    copyFileSync(script, join(root, 'scripts', 'set-version.mjs'));
+    const env = { ...process.env, npm_lifecycle_event: 'version', npm_package_version: '0.7.0' };
+    const result = spawnSync(process.execPath, [join(root, 'scripts', 'set-version.mjs')], { encoding: 'utf8', env });
+    expect(result.stderr).toBe('');
+    expect(result.status).toBe(0);
+    expect(checkLockstep(root)).toEqual({ version: '0.7.0', mismatches: [] });
+  });
+
+  it('rejects --check combined with a version', () => {
+    const result = spawnSync(process.execPath, [script, '--check', '1.2.3'], { encoding: 'utf8' });
+    expect(result.status).toBe(1);
+    expect(result.stderr).toContain('Usage:');
+  });
+
+  it('passes --check against this repository', () => {
+    // The real tree is the one CI guards; this test is the only place the
+    // invariant is asserted, so a drift anywhere in the tree fails here.
+    const result = spawnSync(process.execPath, [script, '--check'], { encoding: 'utf8' });
+    expect(result.stderr).toBe('');
+    expect(result.status).toBe(0);
+    expect(result.stdout).toMatch(/lockstep at \d+\.\d+\.\d+/);
+  });
+});