When using vi.mock() with In-Source Testing, mockReturnValue doesn't work

Describe the bug

project structure

My project structure is below:
(See also the repro link on stackblitz)

.
├── src
│   ├── a.ts
│   ├── main_a.ts
│   ├── main_b.ts
│   └── x.ts
└── vite.config.ts

graph LR;
    MA((main_a.ts))--import-->A((a.ts))
    MA((main_a.ts))--import-->MB((main_b.ts))
    A--import-->X((x.ts))
    MB--import-->X

The problem I'm experiencing

I'm using vi.mock() in In-Source Testing.
I want to use mockReturnValue() on a module that has been vi.mock()ed, but I can't mock the function implementation.

// main_a.ts
if (import.meta.vitest) {
  const { describe, it, expect, vi } = import.meta.vitest;

  vi.mock('./x.ts', () => ({
    getName: vi.fn(() => '[[mocked from a]]'),
  }));

  describe('run', () => {
    it('enable mock', () => {
      vi.mocked(getName).mockReturnValue('[[mocked in test]]');

      const res = run();

      // If mocking is enabled, `[[mocked in test]]` should be returned.
      // but it returns `[[mocked from a]]`.
      expect(res).toBe('hi, [[mocked in test]]');
    });
  });
}

 FAIL   node  src/main_a.ts > run > enable mock
AssertionError: expected 'hi, [[mocked from a]]' to be 'hi, [[mocked in test]]' // Object.is equality

Expected: "hi, [[mocked in test]]"
Received: "hi, [[mocked from a]]"

The above result shows that mockReturnValue() is not working.

As far as I've tried, this seems to occur under the following conditions:

• In-source testing
• Calling vi.mock twice (in both main_a.ts and main_b.ts)

One odd thing I've come across is that if I change the filename from a.ts to z.ts the test passes. Why does the test result change depending on the filename? Is this expected behavior?

Reproduction

https://stackblitz.com/edit/vitest-dev-vitest-g93bquev?file=src%2Fmain_a.ts

System Info

System:
    OS: macOS 15.5
    CPU: (8) arm64 Apple M1 Pro
    Memory: 110.41 MB / 16.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 24.1.0 - ~/.anyenv/envs/nodenv/versions/24.1.0/bin/node
    npm: 11.3.0 - ~/.anyenv/envs/nodenv/versions/24.1.0/bin/npm
    Deno: 2.3.6 - ~/.deno/bin/deno
  Browsers:
    Chrome: 140.0.7339.214
    Chrome Canary: 143.0.7445.0
    Safari: 18.5
  npmPackages:
    vite: ^7.0.5 => 7.0.5 
    vitest: ^4.0.0-beta.15 => 4.0.0-beta.15 


Note: Same behavior with vite@3 .

Used Package Manager

npm

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 by running the StackBlitz reproduction with src/main_a.ts, src/main_b.ts, a.ts, x.ts, and vite.config.ts, then compare the result after renaming a.ts to z.ts. Trace the two in-source vi.mock() calls and the mockReturnValue() assertion. Done means the test returns 'hi, [[mocked in test]]' and the filename-dependent behavior is covered by a regression test.