Addon-Vitest: Update Vitest plugin configuration to disable requireAssertions for expect

[valentinpalkovic]

Closes #33692

What I did

SvelteKit's default Vitest configuration sets expect.requireAssertions: true. When using addon-vitest, stories without a play function are transformed into smoke tests that contain no assertions. This causes the test suite to fail with: Error: expected any number of assertion, but got none.

I modified the @storybook/addon-vitest plugin to internally override the test.expect.requireAssertions setting. This ensures that generated smoke tests pass regardless of the user's global Vitest strictness settings, while keeping the rest of the user's configuration intact via the config hook.

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. Initialize a SvelteKit project: npm create svelte@latest my-app (Select "Library project", "TypeScript", and "Vitest").
2. Verify vite.config.ts: Ensure the config contains:

test: {
    expect: { requireAssertions: true },
    // ... projects config
}

3. Add Storybook: npx storybook@latest init
4. npm run test (or the equivalent vitest command).
5. Story tests should pass!

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 PR does not have a canary release associated. You can request a canary release of this pull request by mentioning the @storybookjs/core team here.

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

[nx-cloud]

View your CI Pipeline Execution ↗ for commit d33b9c3

Command	Status	Duration	Result
nx run-many -t compile,check,knip,test,pretty-d...	✅ Succeeded	9m 56s	View ↗

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☁️ Nx Cloud last updated this comment at 2026-01-28 13:32:38 UTC

[coderabbitai]

📝 Walkthrough

Walkthrough

A Vitest configuration property is added to the plugin, setting test.expect.requireAssertions to false within the base Vite configuration. This modifies the test runner's assertion validation behavior.

Changes

Cohort / File(s)	Summary
Vitest Configuration code/addons/vitest/src/vitest-plugin/index.ts	Adds test.expect.requireAssertions: false to the Vitest configuration under the base Vite config, disabling assertion requirement validation.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

✨ Finishing touches

• 📝 Generate docstrings

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