ci: Build tsc deps and surface vitest stderr in grind workflow (#31543)

Co-authored-by: n8n-cat-bot[bot] <n8n-cat-bot[bot]@users.noreply.github.com>
Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

.github/scripts/grind-changed-tests.mjs

@@ -112,12 +112,14 @@ for (const file of files) {
 		}
 	}
 
+	const stderr = res.stderr ?? '';
+
 	if (!parsed) {
-		rows.push({ file, status: 'no-result' });
+		rows.push({ file, status: 'no-result', stderr });
 		continue;
 	}
 
-	rows.push({ file, status: 'ran', passed: parsed.passed, total: parsed.total });
+	rows.push({ file, status: 'ran', passed: parsed.passed, total: parsed.total, stderr });
 }
 
 // --- Render markdown ---
@@ -131,6 +133,28 @@ const renderRow = ({ file, status, passed, total }) => {
 	return `| \`${file}\` | ${fraction} ⚠️ flaky |`;
 };
 
+const STDERR_EXCERPT_LINES = 20;
+
+const renderDiagnostic = ({ file, stderr }) => {
+	const lines = stderr.split('\n');
+	const truncated = lines.length > STDERR_EXCERPT_LINES;
+	const excerpt = lines.slice(0, STDERR_EXCERPT_LINES).join('\n');
+	const trailer = truncated ? `\n... (${lines.length - STDERR_EXCERPT_LINES} more lines)` : '';
+	return [
+		`<details><summary><code>${file}</code> — first failure stderr</summary>`,
+		'',
+		'```',
+		excerpt + trailer,
+		'```',
+		'',
+		'</details>',
+	].join('\n');
+};
+
+const diagnostics = rows
+	.filter((r) => r.stderr && r.stderr.trim() && (r.status === 'no-result' || (r.status === 'ran' && r.passed < r.total)))
+	.map(renderDiagnostic);
+
 const body = [
 	'<!-- grind-results -->',
 	'## Grind results — pre-merge flake detection (N=' + n + ')',
@@ -139,6 +163,7 @@ const body = [
 	'|---|---|',
 	...rows.map(renderRow),
 	'',
+	...(diagnostics.length ? ['### First-failure diagnostics', '', ...diagnostics, ''] : []),
 	'_Spawn-per-iteration mode. Catches post-teardown async flakes that `vitest --repeat` misses. See [DEVP-198](https://linear.app/n8n/issue/DEVP-198) for design notes._',
 	'',
 ].join('\n');

.github/workflows/grind-changed-tests.yml

@@ -33,7 +33,11 @@ jobs:
       - name: Setup Node.js
         uses: ./.github/actions/setup-nodejs
         with:
-          build-command: ''
+          # Build editor-ui's dependencies (e.g. @n8n/api-types) but not
+          # editor-ui itself — Vitest transforms editor-ui sources on the fly,
+          # while tsc-built deps must exist on disk for import resolution.
+          # Turbo cache makes this near-instant on subsequent runs.
+          build-command: 'pnpm --filter=n8n-editor-ui^... build'
 
       - name: Install .github/scripts dependencies
         run: pnpm install --frozen-lockfile --dir ./.github/scripts --ignore-workspace

scripts/grind.mjs

@@ -98,12 +98,22 @@ const runnerArgs =
 		: ['jest', fileRelToPkg, '--colors=false'];
 
 let passed = 0;
+let firstFailureLogged = false;
 for (let i = 0; i < n; i++) {
 	const res = spawnSync('pnpm', runnerArgs, {
 		cwd: pkgRoot,
-		stdio: values.json ? ['ignore', 'ignore', 'ignore'] : ['ignore', 'inherit', 'inherit'],
+		stdio: values.json ? ['ignore', 'ignore', 'pipe'] : ['ignore', 'inherit', 'inherit'],
+		encoding: 'utf8',
 	});
-	if (res.status === 0) passed++;
+	if (res.status === 0) {
+		passed++;
+	} else if (values.json && !firstFailureLogged) {
+		// Without this, JSON-mode invocations (used in CI) swallow every
+		// vitest error message and leave authors with no diagnostic trail.
+		firstFailureLogged = true;
+		process.stderr.write(`\n[grind] first failing iteration for ${fileRelToPkg}:\n`);
+		if (res.stderr) process.stderr.write(res.stderr);
+	}
 	if (!values.json) process.stdout.write(res.status === 0 ? '.' : 'F');
 }