[Updated] vi.mock ignored when mocked module is pre-loaded transitively via setupFiles

Really sorry, updated the current one again. Thank god Claude could reproduce it without me making mistakes.

Describe the bug

I closed the previous topic (https://github.com/vitest-dev/vitest/issues/10089) because the description was misleading and there was no reproduction link.

vi.mock ignored when mocked module is pre-loaded transitively via setupFiles

Description

vi.mock is silently ignored when the module being mocked has already been loaded into the module cache by setupFiles — transitively through a barrel (index.ts).

vi.mock is hoisted and registered before the test runs, but the mock factory never intercepts the module because it was already executed and cached during the setupFiles phase.

Reproduction

Minimal repo: https://github.com/...

File structure:

src/
  setup.ts
  mocks/server.ts       ← imports '../ui' (forces barrel execution)
  ui/
    consts.ts           ← exports SORT
    Simple.tsx          ← imports @mui/material/Box
    index.ts            ← barrel: re-exports Simple + SORT
    Simple.test.tsx     ← vi.mock broken ❌
    Simple.doMock.test.tsx  ← workaround ✅

setup.ts:

import { beforeAll, afterEach, afterAll } from 'vitest';
import { server } from './mocks/server';

beforeAll(() => server.listen());
afterEach(() => server.resetHandlers());
afterAll(() => server.close());

mocks/server.ts:

import { setupServer } from 'msw/node';
import '../ui'; // forces barrel → Simple.tsx → @mui/material/Box into cache

export const server = setupServer();

ui/index.ts:

export { Simple } from './Simple';
export { SORT } from './consts';

ui/Simple.tsx:

import Box from '@mui/material/Box';

export function Simple() {
  return <Box><p>simple</p></Box>;
}

ui/Simple.test.tsx (broken):

import { render, screen } from '@testing-library/react';
import { describe, expect, it, vi } from 'vitest';
import { Simple } from './Simple';

vi.mock('@mui/material/Box', () => ({
  default: ({ children }: React.PropsWithChildren) => (
    <div data-testid='box'>{children}</div>
  ),
}));

describe('vi.mock (broken)', () => {
  it('fails — real MuiBox renders instead of mock', () => {
    render(<Simple />);
    expect(screen.getByTestId('box')).toBeInTheDocument(); // ❌
  });
});

Expected behavior

vi.mock('@mui/material/Box', ...) intercepts the module and the test passes.

Actual behavior

The real @mui/material/Box renders (class="MuiBox-root"). The mock is ignored because Box was already executed and cached during setupFiles before vi.mock could intercept it.

Workaround

// ui/Simple.doMock.test.tsx
vi.resetModules(); // clear the pre-loaded cache
vi.doMock('@mui/material/Box', () => ({
  default: ({ children }: React.PropsWithChildren) => (
    <div data-testid='box'>{children}</div>
  ),
}));

it('passes', async () => {
  const { Simple } = await import('./Simple'); // fresh load gets the mock
  render(<Simple />);
  expect(screen.getByTestId('box')).toBeInTheDocument(); // ✅
});

Vitest version

4.1.2

Additional context

The bug only manifests when the module being mocked is loaded transitively through a barrel during setupFiles. Direct imports in setupFiles from the mocked module itself exhibit the same issue, but this case is particularly surprising because the test file imports Simple directly (not via the barrel) and there is no visible connection between setupFiles and the mock target.

Reproduction

https://github.com/Ghostblad3/vitest-bug

System Info

System:
    OS: Linux 6.6 Ubuntu 24.04.3 LTS 24.04.3 LTS (Noble Numbat)
    CPU: (14) x64 Intel(R) Core(TM) Ultra 7 265U
    Memory: 12.64 GB / 15.35 GB
    Container: Yes
    Shell: 5.2.21 - /bin/bash
  Binaries:
    Node: 20.19.2 - /home/igris/.nvm/versions/node/v20.19.2/bin/node
    Yarn: 1.22.22 - /home/igris/.nvm/versions/node/v20.19.2/bin/yarn
    npm: 10.8.2 - /home/igris/.nvm/versions/node/v20.19.2/bin/npm
    pnpm: 10.29.2 - /mnt/c/nvm4w/nodejs/pnpm
    bun: 1.3.5 - /home/igris/.bun/bin/bun
  npmPackages:
    @vitejs/plugin-react: ~5.1.4 => 5.1.4 
    vite: ^8.0.3 => 8.0.3 
    vitest: ~4.1.2 => 4.1.2

Used Package Manager

pnpm

Validations

• Follow our Code of Conduct
• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.
• Check that this is a concrete bug. For Q&A open a GitHub Discussion or join our Discord Chat Server.
• The provided reproduction is a minimal reproducible example of the bug.

Start with the linked Ghostblad3/vitest-bug reproduction, especially setup.ts, mocks/server.ts, ui/index.ts, ui/Simple.tsx, and the broken and workaround test files. Run the reproduction and compare vi.mock with vi.resetModules plus vi.doMock. Done means the standard vi.mock test intercepts @mui/material/Box even when it was loaded transitively through setupFiles.