Test sequence causes tests to consistently fail or pass

Describe the bug

I am experiencing flaky tests. I've tracked it down to it something having to do with mocks/spies in combination with two specific tests being executed in a specific order by 1 worker/thread/fork/thing.

We (ideally) don't use test isolation, but from what I understand that's only if you start messing with global stuff (eg fetch, process.env) anyway. I've also tried various options of clearMocks, mockReset and restoreMocks, and it seems to trigger in all three unless I enable test isolation.

I understand that disabling test isolation might cause tests to have side effects that leak over to other tests, but I've narrowed it down to a reproducable example where it's just two very simple test files which if ordered alphabetically consistently fail. And I don't understand why. It also feels like this behaviour has gotten more common since vitest 4.

it's possible that this isn't technically a bug, or maybe our setup is just wrong, but I can't find anything about this in the documentation nor existing issues. But explicitly mocking a file and it still somehow calling the real thing feels suspicious, regardless of the test order.

Reproduction

https://github.com/barbeque-squared/vitest-mockbug

npm ci
npm run test # will fail
npm run test:pass # will pass, the only difference is the test order

System Info

System:
    OS: Linux 6.18 Arch Linux
    CPU: (12) x64 13th Gen Intel(R) Core(TM) i5-1335U
    Memory: 5.59 GB / 15.28 GB
    Container: Yes
    Shell: 5.3.9 - /usr/bin/bash
  Binaries:
    Node: 25.3.0 - /usr/bin/node
    npm: 11.7.0 - /usr/bin/npm
    pnpm: 10.28.0 - /usr/bin/pnpm
  Browsers:
    Chromium: 144.0.7559.59
    Firefox: 147.0.1
    Firefox Developer Edition: 147.0.1
  npmPackages:
    vitest: ^4.0.17 => 4.0.17

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 with the linked minimal reproduction: run npm ci, then npm run test and npm run test:pass to compare the two test orders. Trace why the explicitly mocked module can call the real implementation without test isolation, and consider the issue done when the order-dependent behavior is explained and a regression test or fix is identified.