diff --git a/.github/workflows/engine-parity.yml b/.github/workflows/engine-parity.yml
new file mode 100644
index 0000000..6e356c4
--- /dev/null
+++ b/.github/workflows/engine-parity.yml
@@ -0,0 +1,27 @@
+name: Engine parity
+
+# The scaffold's generated devDependencies must match the engine release its `neo.mjs` pin resolves to, read from npm.
+# See scripts/check-engine-parity.mjs for the scope and why the engine is never read from a checkout.
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+permissions:
+  contents: read
+
+jobs:
+  parity:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v6
+
+      - uses: actions/setup-node@v6
+        with:
+          node-version: '24'
+
+      - name: Parity check contract
+        run: node scripts/test-check-engine-parity.mjs
+
+      - name: Generated devDependencies match the pinned engine release
+        run: node scripts/check-engine-parity.mjs
diff --git a/.npmignore b/.npmignore
index 011bf01..278acd0 100644
--- a/.npmignore
+++ b/.npmignore
@@ -3,6 +3,7 @@
 .github/ISSUE_ARCHIVE
 .github/RELEASE_NOTES
 .github/.sync-metadata.json
+scripts
 
 # Original content of the .gitignore file
 # See http://help.github.com/ignore-files/ for more about ignoring files.
diff --git a/package.json b/package.json
index f01d670..2a78fd2 100644
--- a/package.json
+++ b/package.json
@@ -10,7 +10,7 @@
     "scripts"     : {
         "ai:mcp-server-github-workflow": "node ./node_modules/neo.mjs/ai/mcp/server/github-workflow/mcp-stdio.mjs -c ./ai/mcp/server/github-workflow/config.json",
         "neo-app"                      : "node ./tasks/neo-app.mjs",
-        "test"                         : "echo \"Error: no test specified\" && exit 1"
+        "test"                         : "node scripts/test-check-engine-parity.mjs && node scripts/check-engine-parity.mjs"
     },
     "bin"         : {
         "neo-app": "./tasks/neo-app.mjs"
diff --git a/scripts/check-engine-parity.mjs b/scripts/check-engine-parity.mjs
new file mode 100644
index 0000000..6dd5d1b
--- /dev/null
+++ b/scripts/check-engine-parity.mjs
@@ -0,0 +1,80 @@
+#!/usr/bin/env node
+/**
+ * @summary Fails when a devDependency the scaffold generates differs from the engine release its `neo.mjs` pin resolves
+ * to.
+ *
+ * `tasks/createPackageJson.mjs` writes every scaffolded app's devDependencies, which advertise the engine's own ranges.
+ * They once fell a whole major behind with nothing noticing. Whether a scaffolded app's bundle scripts build from these
+ * hoisted copies, or from the engine's nested ones after its `postinstall`, is not settled here (#34's release-gate
+ * run reports it): the declared ranges should agree either way.
+ *
+ * Scope: every devDependency BOTH sides declare, compared exactly. The engine is read from npm, never from a checkout,
+ * because a scaffolded app installs the published package and a checkout can be ahead of it. The pin decides which
+ * release: the newest published version its range allows. A package only the scaffold declares has no engine
+ * counterpart to drift from, so it is listed and not compared.
+ */
+
+import {execFileSync}      from 'node:child_process';
+import {realpathSync}      from 'node:fs';
+import os                  from 'node:os';
+import path                from 'node:path';
+import process             from 'node:process';
+import {fileURLToPath}     from 'node:url';
+import createPackageJson   from '../tasks/createPackageJson.mjs';
+
+/**
+ * @summary The package.json the scaffold writes, captured without touching a disk.
+ * @returns {Object}
+ */
+export function generatedPackageJson() {
+    let packageJson = null;
+
+    createPackageJson.init('parity-check', 'parity-check', {writeFileSync: (file, text) => {packageJson = JSON.parse(text)}}, os, path);
+
+    return packageJson
+}
+
+/**
+ * @summary Compares the scaffold's devDependencies with the engine's.
+ * @param {Object} scaffold The generated devDependencies
+ * @param {Object} engine   The engine release's devDependencies
+ * @returns {{drift: Object[], scaffoldOnly: String[]}} `drift` holds `{name, scaffold, engine}` for each differing range
+ */
+export function findDrift(scaffold, engine) {
+    const names = Object.keys(scaffold);
+
+    return {
+        drift       : names.filter(name => name in engine && scaffold[name] !== engine[name])
+            .map(name => ({name, scaffold: scaffold[name], engine: engine[name]})),
+        scaffoldOnly: names.filter(name => !(name in engine))
+    }
+}
+
+/**
+ * @param {...String} args
+ * @returns {*} the parsed `npm view … --json` answer
+ */
+const npmView = (...args) => JSON.parse(execFileSync('npm', ['view', ...args, '--json'], {encoding: 'utf8'}));
+
+if (process.argv[1] && realpathSync(process.argv[1]) === realpathSync(fileURLToPath(import.meta.url))) {
+    const
+        packageJson = generatedPackageJson(),
+        range       = packageJson.dependencies['neo.mjs'],
+        // One matching version answers as a string, several as an ascending list
+        version     = [].concat(npmView(`neo.mjs@${range}`, 'version')).at(-1),
+        {drift, scaffoldOnly} = findDrift(packageJson.devDependencies, npmView(`neo.mjs@${version}`, 'devDependencies'));
+
+    if (scaffoldOnly.length > 0) {
+        console.log(`check-engine-parity: not compared, no engine counterpart: ${scaffoldOnly.join(', ')}`)
+    }
+
+    if (drift.length > 0) {
+        console.error(`check-engine-parity: ${drift.length} generated devDependencies differ from neo.mjs@${version} ` +
+            `(the newest release \`${range}\` allows):`);
+        drift.forEach(({name, scaffold, engine}) => console.error(`  ${name}  scaffold ${scaffold}  engine ${engine}`));
+        console.error('Move them in tasks/createPackageJson.mjs to the engine\'s values.');
+        process.exitCode = 1
+    } else {
+        console.log(`check-engine-parity: every devDependency the scaffold shares with neo.mjs@${version} matches.`)
+    }
+}
diff --git a/scripts/test-check-engine-parity.mjs b/scripts/test-check-engine-parity.mjs
new file mode 100644
index 0000000..1dda489
--- /dev/null
+++ b/scripts/test-check-engine-parity.mjs
@@ -0,0 +1,39 @@
+#!/usr/bin/env node
+/**
+ * @summary Contract checks for the engine-parity check: it reads what the scaffold really generates, reports a planted
+ * divergence, stays quiet on a match, and lists what it does not compare.
+ */
+
+import assert                              from 'node:assert/strict';
+import {findDrift, generatedPackageJson} from './check-engine-parity.mjs';
+
+// ── It reads the package.json the scaffold writes ──────────────────────────────────────────────────
+{
+    const packageJson = generatedPackageJson();
+
+    assert.match(packageJson.dependencies['neo.mjs'], /^\^\d+\.\d+\.\d+$/, 'the engine pin is read from the generator');
+    assert.ok(Object.keys(packageJson.devDependencies).length > 10, 'and so are the devDependencies');
+}
+
+// ── A planted divergence is reported, a match is not ───────────────────────────────────────────────
+{
+    const engine = {marked: '^18.0.13', 'monaco-editor': '0.56.0', sass: '^1.104.1'};
+
+    assert.deepEqual(findDrift({marked: '^18.0.13', 'monaco-editor': '0.56.0'}, engine).drift, [], 'matching ranges pass');
+
+    assert.deepEqual(findDrift({marked: '^17.0.4', 'monaco-editor': '0.56.0'}, engine).drift,
+        [{name: 'marked', scaffold: '^17.0.4', engine: '^18.0.13'}], 'a range a major behind is drift');
+
+    assert.deepEqual(findDrift({'monaco-editor': '^0.56.0'}, engine).drift,
+        [{name: 'monaco-editor', scaffold: '^0.56.0', engine: '0.56.0'}], 'an exact pin widened to a caret is drift too');
+}
+
+// ── What only the scaffold declares is listed, never compared ──────────────────────────────────────
+{
+    const {drift, scaffoldOnly} = findDrift({chromadb: '^3.3.1', marked: '^18.0.13'}, {marked: '^18.0.13'});
+
+    assert.deepEqual(drift, []);
+    assert.deepEqual(scaffoldOnly, ['chromadb'])
+}
+
+console.log('check-engine-parity: contract arms green.');