import.meta.env.BASE_URL vite built-in constant breaks with vitest 4.x

Describe the bug

With vitest 4.x the vite built-in import.meta.env.BASE_URL no longer has the expected value set (base from the vite config as per docs).

For example if I have a component that displays the value of import.meta.env.BASE_URL doing a regular vite the value is displayed as expected.

If I now write a test with vitest 4.x that asserts for this value, the test breaks because the value is just / regardless of what is set in the base field of the vite config.

Reproduction

Small example project here

If you run vitest run with vitest version 4.0.6 for example, the one test asserting for the BASE_URL value will fail.
If you switch the vitest version to a 3.x version 3.2.4 for example, all tests succeed.

System Info

System:
    OS: Windows 11 10.0.22631
    CPU: (24) x64 12th Gen Intel(R) Core(TM) i7-12850HX
    Memory: 28.41 GB / 63.61 GB
  Binaries:
    Node: 22.14.0 
    npm: 10.9.2 
    pnpm: 10.8.1 
  Browsers:
    Chrome: 141.0.7390.123
    Edge: Chromium (140.0.3485.54)
    Internet Explorer: 11.0.22621.3527
  npmPackages:
    @vitejs/plugin-vue: ^6.0.1 => 6.0.1 
    vite: ^7.1.7 => 7.1.12 
    vitest: ^4.0.6 => 4.0.6

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 and run vitest run using Vitest 4.0.6, then compare its result with Vitest 3.2.4. Trace how the test environment supplies import.meta.env.BASE_URL relative to the Vite base configuration. Done means the assertion receives the configured base value under Vitest 4.x.