Describe the bug

When defining 2 or more environments (using Environment API) in the Vite config, mutations made by a plugin's configEnvironment hook in one environment can leak into other environments.

This happens because resolveConfig constructs a single defaultNonClientEnvironmentOptions object and reuses it for all non-client environments in the merge loop. Since mergeConfig does a shallow spread, nested objects like resolve retain their shared reference. When a plugin then mutates config.resolve for one environment, all other non-client environments see the same change.

In practice, this surfaces with Vitest when using 2+ environments (from Environment API) and custom environment (from Vitest) - Vitest's configEnvironment hook sets resolve.noExternal = true per environment, which propagates to all non-client environments, affecting this condition for the next configEnvironment call, causing CJS dependencies (like jsdom) to crash with ReferenceError: require is not defined. Take this with a grain of salt as this is only my theory and I don't have full understanding of Vite/Vitest.

If you name an environment client, it is unaffected because it gets a separately constructed defaultClientEnvironmentOptions. A single non-client environment also works fine since there's nothing to leak to.

Reproduction

https://github.com/Filipoliko/vitest-environment-bug

Steps to reproduce

Run npm ci followed by npm test in the reproduction repository. Check out the README file in the project for more info.

Check out also PR #22250 for a unit test in this repository that also showcases this issue.

System Info

System:
    OS: macOS 26.4.1
    CPU: (10) arm64 Apple M1 Pro
    Memory: 147.92 MB / 16.00 GB
    Shell: 5.2.37 - /opt/homebrew/bin/bash
  Binaries:
    Node: 24.14.1 - /Users/filip.satek/.nvm/versions/node/v24.14.1/bin/node
    npm: 11.11.0 - /Users/filip.satek/.nvm/versions/node/v24.14.1/bin/npm
    pnpm: 10.33.0 - /Users/filip.satek/.nvm/versions/node/v24.14.1/bin/pnpm
    bun: 1.2.12 - /opt/homebrew/bin/bun
  Browsers:
    Chrome: 147.0.7727.57
    Edge: 113.0.1774.57
    Firefox: 149.0.2
    Safari: 26.4

Used Package Manager

npm

Logs

$ npm test

> vitest-environment@1.0.0 test
> vitest run


 RUN  v4.1.4 /Users/filip.satek/git/vitest-environment

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ Unhandled Errors ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯

Vitest caught 1 unhandled error during the test run.
This might cause false positive tests. Resolve unhandled errors to make sure your tests are not affected.

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ Unhandled Error ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯
Error: [vitest-pool]: Failed to start forks worker for test files /Users/filip.satek/git/vitest-environment/test.spec.ts.
 ❯ node_modules/vitest/dist/chunks/cli-api.lDy4N9kC.js:3448:94
 ❯ processTicksAndRejections node:internal/process/task_queues:104:5
 ❯ Pool.schedule node_modules/vitest/dist/chunks/cli-api.lDy4N9kC.js:3448:5

Caused by: ReferenceError: require is not defined
 ❯ eval node_modules/vite/dist/node/module-runner.js:992:9
 ❯ ESModulesEvaluator.runInlinedModule node_modules/vite/dist/node/module-runner.js:992:161
 ❯ ModuleRunner.directRequest node_modules/vite/dist/node/module-runner.js:1247:80
 ❯ processTicksAndRejections node:internal/process/task_queues:104:5
 ❯ ModuleRunner.cachedRequest node_modules/vite/dist/node/module-runner.js:1154:73

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯


 Test Files  no tests
      Tests  no tests
     Errors  1 error
   Start at  12:31:44
   Duration  238ms (transform 117ms, setup 0ms, import 0ms, tests 0ms, environment 0ms)

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.
• Make sure this is a Vite issue and not a framework-specific issue. For example, if it's a Vue SFC related bug, it should likely be reported to vuejs/core instead.
• 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.