Addon-Vitest: Append Storybook project to existing test.projects array without double nesting

[valentinpalkovic]

Closes #33706

What I did

When running the postinstall script for @storybook/addon-vitest, the migration logic could incorrectly wrap an existing Vitest config that already used a projects array inside a new project object. That produced an invalid double-nested structure (test.projects[0].test.projects) and broke Vitest.

Result: A config like:

test: {
  expect: { requireAssertions: true },
  projects: [
    { extends: "./vite.config.ts", test: { name: "client" } },
    { extends: "./vite.config.ts", test: { name: "server" } },
  ],
},

is now updated to add the storybook project as a third item in projects, instead of wrapping client/server in an extra project and breaking the structure.

Checklist for Contributors

Testing

The changes in this PR are covered in the following automated tests:

• stories
• unit tests
• integration tests
• end-to-end tests

Manual testing

1. In a project that already has vitest.config.ts with test.projects (e.g. client + server), add/init @storybook/addon-vitest.
2. Confirm vitest.config.ts is updated so the storybook project is a new entry in the existing projects array, not a wrapper around the previous config.
3. Run npx vitest --project=storybook and ensure tests run.

Documentation

• Add or update documentation reflecting your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

Checklist for Maintainers

• When this PR is ready for testing, make sure to add ci:normal, ci:merged or ci:daily GH label to it to run a specific set of sandboxes. The particular set of sandboxes can be found in code/lib/cli-storybook/src/sandbox-templates.ts

• Make sure this PR contains one of the labels below:

  Available labels

  • bug: Internal changes that fixes incorrect behavior.
  • maintenance: User-facing maintenance tasks.
  • dependencies: Upgrading (sometimes downgrading) dependencies.
  • build: Internal-facing build tooling & test updates. Will not show up in release changelog.
  • cleanup: Minor cleanup style change. Will not show up in release changelog.
  • documentation: Documentation only changes. Will not show up in release changelog.
  • feature request: Introducing a new feature.
  • BREAKING CHANGE: Changes that break compatibility in some way with current major version.
  • other: Changes that don't fit in the above categories.

🦋 Canary release

This pull request has been released as version 0.0.0-pr-33708-sha-df1d2018. Try it out in a new sandbox by running npx storybook@0.0.0-pr-33708-sha-df1d2018 sandbox or in an existing project with npx storybook@0.0.0-pr-33708-sha-df1d2018 upgrade.

More information

Published version	0.0.0-pr-33708-sha-df1d2018
Triggered by	@valentinpalkovic
Repository	storybookjs/storybook
Branch	valentin/addon-vitest-prevent-double-nesting
Commit	df1d2018
Datetime	Thu Jan 29 10:44:53 UTC 2026 (1769683493)
Workflow run	21475119273

To request a new release of this pull request, mention the @storybookjs/core team.

core team members can create a new canary release here or locally with gh workflow run --repo storybookjs/storybook publish.yml --field pr=33708

Summary by CodeRabbit

• Bug Fixes

  • Fixed Vitest configuration merging to prevent double nesting when appending Storybook projects to existing multi-project arrays.
  • Enhanced multi-project configuration handling to properly preserve existing test settings while adding new projects.

• Tests

  • Added test coverage for Storybook project configuration appending to existing multi-project arrays.

_{✏️ Tip: You can customize this high-level summary in your review settings.}

[coderabbitai]

📝 Walkthrough

Walkthrough

Introduces multi-project handling for Vitest configuration merging. A new helper detects existing test.projects arrays in target configs. When present, template project configurations are appended to the array. Additionally, a test case validates that storybook projects are appended to existing projects without duplication or nesting issues.

Changes

Cohort / File(s)	Summary
Vitest Multi-Project Configuration code/addons/vitest/src/updateVitestFile.ts, code/addons/vitest/src/updateVitestFile.test.ts	Implementation adds hasProjectsProp helper and conditional logic to append template projects to existing test.projects arrays, or restructure configs to single-project format when needed. Preserves coverage configuration at appropriate levels. Test case validates appending storybook projects to existing projects array without nesting.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~22 minutes

Possibly related PRs

• storybookjs/storybook#32753 — Also modifies updateVitestFile to handle defineConfig/mergeConfig with projects/workspace array restructuring.
• storybookjs/storybook#33080 — Adds logic to merge/append test.projects entries in Vitest configuration.
• storybookjs/storybook#32844 — Directly related changes to updateVitestFile.ts for test.projects merging and coverage preservation.

✨ Finishing touches

• 📝 Generate docstrings

---

_{Comment @coderabbitai help to get the list of available commands and usage tips.}

[nx-cloud]

View your CI Pipeline Execution ↗ for commit df1d201

Command	Status	Duration	Result
nx run-many -t compile,check,knip,test,pretty-d...	❌ Failed	9m 30s	View ↗

---

☁️ Nx Cloud last updated this comment at 2026-01-29 16:19:48 UTC