vitest-dev/vitest · critical · Error

Data was not properly initialized. This is a bug in Vitest.

Error message

Data was not properly initialized. This is a bug in Vitest. Please, open a new issue with reproduction.

What it means

Thrown by `executeTests` at tester.ts:228-229 when `preparedData` is undefined — i.e. `executeTests('run'|'collect', specs)` was called before `prepareTestEnvironment` resolved and assigned `preparedData`. The browser tester expects preparation (runner/state assembly) to complete before any test execution message arrives; reaching this branch means the lifecycle ordering is broken.

Source

Thrown at packages/browser/src/client/tester/tester.ts:229

    })
  }

  state.durations.prepare = performance.now() - options.startTime

  return {
    runner,
    config,
    state,
  }
}

let preparedData:
  | Awaited<ReturnType<typeof prepareTestEnvironment>>
  | undefined

async function executeTests(method: 'run' | 'collect', specifications: FileSpecification[]) {
  if (!preparedData) {
    throw new Error(`Data was not properly initialized. This is a bug in Vitest. Please, open a new issue with reproduction.`)
  }

  debug?.('runner resolved successfully')

  const { runner, state } = preparedData

  state.ctx.files = specifications
  runner.setMethod(method)

  const version = url.searchParams.get('browserv') || ''
  specifications.forEach(({ filepath }) => {
    const currentVersion = browserHashMap.get(filepath)
    if (!currentVersion || currentVersion[1] !== version) {
      browserHashMap.set(filepath, version)
    }
  })

  for (const file of specifications) {

View on GitHub (pinned to d568f8ce37)

Solutions

1. Ensure you're running matching versions of `vitest` and `@vitest/browser` across the workspace (run `pnpm why vitest`).
2. Restart the browser pool / Vitest process — transient races after reload often clear.
3. If reproducible, open a Vitest issue with the reproduction as the message requests.

Example fix

// no user code fix — this is an internal lifecycle violation
// verify version alignment and restart; report if persistent

try {
  // normal test run
} catch (err) {
  if (err instanceof Error && err.message.includes('Data was not properly initialized')) {
    // restart the browser runner / vitest process; this is an internal lifecycle race
    throw err
  }
  throw err
}

When it happens

Trigger: A race where the runner receives a `collect`/`run` message before the prepare step finishes. HMR or a reload that resets `preparedData` (it's a module-level `let`) while a queued execution message fires. Manual invocation of the tester entry that skips preparation.

Common situations: This is flagged as a Vitest bug in the message itself. Real-world causes are usually version mismatches between `@vitest/browser` client and server, a broken iframe reload, or a custom orchestrator that drives the tester entry directly.

Related errors

• Worker state is not found. This is an issue with Vitest. Ple
• Not called in the browser
• Cannot find iframe element. This is a bug in Vitest. Please,
• Page "${sessionId}" not found in ${this.browserName} browser
• [vitest] The provider was closed.

AI-assisted analysis of vitest-dev/vitest@d568f8ce37 (2026-08-03). Data as JSON: /data/errors/3c7dbbcdeac650bc.json. Report an issue: GitHub.