perf: leave Vitest workers uncapped in CI

Author: thymikee

scripts/lib/vitest-concurrency.ts

@@ -4,3 +4,7 @@
  * leaves roughly 3 cores for runners, subprocesses, simulators, and the OS.
  */
 export const DEFAULT_VITEST_MAX_WORKERS = 2;
+
+export function resolveVitestMaxWorkers(env: NodeJS.ProcessEnv = process.env): number | undefined {
+  return env.CI === 'true' ? undefined : DEFAULT_VITEST_MAX_WORKERS;
+}

src/__tests__/hermetic-env-setup.test.ts

@@ -2,7 +2,10 @@ import os from 'node:os';
 import path from 'node:path';
 import { afterEach, test, vi } from 'vitest';
 import assert from 'node:assert/strict';
-import { DEFAULT_VITEST_MAX_WORKERS } from '../../scripts/lib/vitest-concurrency.ts';
+import {
+  DEFAULT_VITEST_MAX_WORKERS,
+  resolveVitestMaxWorkers,
+} from '../../scripts/lib/vitest-concurrency.ts';
 import vitestConfig from '../../vitest.config.ts';
 
 const HERMETIC_ENV_SETUP = 'src/__tests__/hermetic-env-setup.ts';
@@ -15,7 +18,9 @@ const VITEST_CLAIMS_DIR = path.join(os.tmpdir(), `agent-device-vitest-claims-${p
 type ProjectShape = { test?: { name?: string; setupFiles?: readonly string[] } };
 
 test('vitest caps aggregate worker concurrency for parallel worktrees', () => {
-  assert.equal(vitestConfig.test?.maxWorkers, DEFAULT_VITEST_MAX_WORKERS);
+  assert.equal(vitestConfig.test?.maxWorkers, resolveVitestMaxWorkers());
+  assert.equal(resolveVitestMaxWorkers({}), DEFAULT_VITEST_MAX_WORKERS);
+  assert.equal(resolveVitestMaxWorkers({ CI: 'true' }), undefined);
 });
 
 // Wiring: the scrub only helps if every project loads it as a setup file. CI runs with the

vitest.config.ts

@@ -4,7 +4,7 @@ import contentionRetryReporter, {
   FAILURE_FILE_ENV,
 } from './scripts/lib/contention-retry-reporter.ts';
 import { SUBPROCESS_STUB_TESTS } from './scripts/lib/contention-retry.ts';
-import { DEFAULT_VITEST_MAX_WORKERS } from './scripts/lib/vitest-concurrency.ts';
+import { resolveVitestMaxWorkers } from './scripts/lib/vitest-concurrency.ts';
 import slowTestGateReporter from './scripts/vitest-slow-test-reporter.ts';
 
 // Tests that stub a real binary (adb/xcrun/npx) by mutating process.env.PATH and
@@ -52,8 +52,10 @@ export default defineConfig({
     // Vitest otherwise derives 11 workers from this 12-core host. Three
     // concurrent Codex worktrees can then request 33 workers and starve the
     // subprocess/test-server paths behind exact timeout budgets. Two workers
-    // per invocation preserves useful parallelism while leaving host headroom.
-    maxWorkers: DEFAULT_VITEST_MAX_WORKERS,
+    // per local invocation preserves useful parallelism while leaving host
+    // headroom. CI stays uncapped so Vitest derives the runner-appropriate
+    // worker count from the isolated machine's available CPU pool.
+    maxWorkers: resolveVitestMaxWorkers(),
     reporters: reporters(),
     projects: [
       {