Issue Review: cloudflare/workers-sdk#7679

Summary

Vitest's __mocks__ directory feature doesn't work when using @cloudflare/vitest-pool-workers.

Findings

• Created: 2025-01-06
• Updated: 2025-10-30
• Version: vitest 2.1.8, @cloudflare/vitest-pool-workers 0.5.40 -> latest 0.12.6
• Component: vitest-pool-workers
• Labels: bug, vitest
• Comments: 0

Key Evidence

• Reproduction repo provided: https://github.com/saitolume/workers-sdk-bug-report
• Issue: __mocks__ directory modules not automatically applied as mocks
• Expected: Standard vitest behavior where __mocks__/uuid.ts would mock the uuid module
• Actual: Mock not applied, test fails because v4() returns actual UUID instead of mock value
• No comments or maintainer response in over a year
• 5 emoji reactions indicating user interest
• This is a feature parity issue - vitest-pool-workers doesn't support __mocks__ directory convention
• No PRs found addressing this

Recommendation

Status: KEEP OPEN

Reasoning: This is a valid bug affecting vitest feature parity. The __mocks__ directory convention is a standard vitest/jest feature that users expect to work. The issue has a clear reproduction and 5 emoji reactions showing user demand. However, this may be an inherent limitation of how vitest-pool-workers runs tests in isolated workerd runtimes.

Action: Keep open. May require documentation if this is an intentional limitation. Users should use explicit vi.mock() calls as a workaround.

Suggested Comment

Thanks for the detailed reproduction. This appears to be a limitation of how vitest-pool-workers runs tests in isolated workerd runtimes - the __mocks__ directory convention may not be supported due to the module resolution differences.

Workaround: Use explicit vi.mock() calls instead:

import { vi } from 'vitest';
vi.mock('uuid', () => ({
  v4: () => 'expected'
}));

Could you confirm if explicit vi.mock() works in your case? If so, we should document this as a known limitation of vitest-pool-workers.

Are you still experiencing this with the latest @cloudflare/vitest-pool-workers version (0.12.6)?