[Bug]: Setting up Vitest does not account for configs that do not use defineConfig

[yannbf]

Describe the bug

While testing, I noticed that if a Vitest config file looks like this, Storybook initializes correctly with testing features:

import { mergeConfig, defineConfig, coverageConfigDefaults } from 'vitest/config'
import viteConfig from './vite.config'

export default mergeConfig(
  viteConfig,
  defineConfig({
    test: {
      name: 'node',
      environment: 'happy-dom',
    },
  })
)

But when it does not use defineConfig (or a function call) and rather uses just an object:

import { mergeConfig, coverageConfigDefaults } from 'vitest/config'
import viteConfig from './vite.config'

export default mergeConfig(
  viteConfig,
  {
    test: {
      name: 'node',
      environment: 'happy-dom',
    },
  }
)

Then the file will be deemed with a not supported format.

The bug occurs in this specific method:
https://github.com/storybookjs/storybook/blob/next/code/core/src/cli/AddonVitestService.ts#L370

Which is called as part of this method:
https://github.com/storybookjs/storybook/blob/next/code/core/src/cli/AddonVitestService.ts#L331

I tried forcing the function to just return true and the latter logic which modifies the file works as intended, so it's just a matter of fixing the detection.

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🔗 Similar Issues

Related Issues

• [Bug]: Storybook Vitest addon setup misconfigures vitest config files that contain coverage setting #32804
• [Bug]: vite.config is not automatically merged correctly in Storybook Tests (@storybook/addon-vitest) #31896
• [Bug]: Addon vitest setup adds unnecessary triple slashes to vitest config file #32806
• [Bug]: Storybook 9 - Storybook Test postinstall does not detect vitest.projects.ts file #31125
• [Bug]: addon-vitest doesn't recognize story files named stories.tsx without component prefix #31966

🔗 Related PRs

#32134 - CLI: Drop contextual dependencies in create-storybook [merged]
#32590 - A11y: Make search clear button keyboard accessible [merged]
#32753 - Addon Vitest: Support modifying mergeConfig on addon setup [merged]
#32844 - Addon Vitest: Fix incorrect file modifications during setup [merged]
#33086 - Build: Upgrade to Vitest 4 [merged]

👤 Suggested Assignees

• yannbf
• GustavoHenriqueP
• Suto-Michimasa
• 43081j
• ritoban23

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[dosubot]

The detection logic in AddonVitestService.ts previously only recognized Vitest configs that used defineConfig or mergeConfig with defineConfig, so plain object configs were flagged as unsupported. This was inconsistent with the update logic, which already handled plain object merges.

This issue has been addressed in recent Storybook releases. As of PR #32846, the detection logic now supports mergeConfig calls with plain object arguments, so configs like the one you described are recognized as valid and will initialize correctly with testing features. You can see the relevant changes and discussion here: PR #32846.

If you are still seeing this issue, please make sure you are using Storybook 10.0.0-rc.2 or later. If the problem persists on the latest version, please provide a minimal reproduction so it can be investigated further.

To reply, just mention @dosu.

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