Cannot re-use static `browser.instances` config across projects

Describe the bug

Reusing the same test.browser.instances objects between test.projects crashes Vitest:

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ Startup Error ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯
Error: Cannot define a nested project for a chromium browser. The project name "browser (chromium)" was already defined. If you have multiple instances for the same browser, make sure to define a custom "name". All projects should have unique names. Make sure your configuration is correct.

For example:

const instances: BrowserInstanceOption[] = [
  {
    browser: 'chromium',
    provider: playwright()
  },
  {
    browser: 'firefox',
    provider: playwright(),
    fileParallelism: false
  }
];

export default defineConfig({
  test: {
    projects: [
      {
        extends: true,
        test: {
          name: 'browser',
          include: ['browser/**/*.test.*'],
          browser: {
            enabled: true,
            instances
          }
        }
      },
      {
        extends: true,
        test: {
          name: 'visual',
          include: ['visual/*.test.*'],
          browser: {
            enabled: true,
            instances
          }
        }
      }
    ]
  }
});

Reproduction

https://stackblitz.com/edit/vitest-dev-vitest-tylyrr4p?file=vite.config.ts,package.json,tsconfig.json&initialPath=__vitest__/

Browser tests don't work in stackblitz... @vitest/browser-preview is suggested, but it doesn't work either?

Error: stackblitz environment does not support the undefined provider. Please, use "@vitest/browser-preview" instead.

System Info

System:
    OS: Windows 11 10.0.26200
    CPU: (32) x64 AMD Ryzen 9 9950X3D 16-Core Processor
    Memory: 31.37 GB / 63.58 GB
  Binaries:
    Node: 25.8.1 - C:\Program Files\nodejs\node.EXE
    npm: 11.10.1 - C:\Program Files\nodejs\npm.CMD
    pnpm: 10.30.3 - C:\Users\Stepi\AppData\Roaming\npm\pnpm.CMD
  Browsers:
    Chrome: 146.0.7680.80
    Edge: Chromium (146.0.3856.59)
    Firefox: 148.0.2 - C:\Program Files\Mozilla Firefox\firefox.exe
  npmPackages:
    @vitejs/plugin-react: ^6.0.1 => 6.0.1
    @vitest/browser-playwright: ^4.1.0 => 4.1.0
    @vitest/coverage-istanbul: ^4.1.0 => 4.1.0
    @vitest/eslint-plugin: ^1.6.12 => 1.6.12
    playwright: ~1.58.0 => 1.58.2
    vite: ^8.0.0 => 8.0.0
    vitest: ^4.1.0 => 4.1.0
    vitest-browser-react: ^2.1.0 => 2.1.0

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 vite.config.ts and the linked reproduction; compare how the shared instances are processed for the two projects and locate the duplicate nested-project name validation. Verify the configuration locally, then confirm that reusing the instances across both projects no longer crashes while project names remain unique.