vitest-dev/vitest · error · Error

[@vitest/mocker] `createMockInstance` is not defined. This…

Error message

[@vitest/mocker] `createMockInstance` is not defined. This is a Vitest error. Please open a new issue with reproduction.

What it means

`mockObject` needs a `createMockInstance` procedure supplied in its options to fabricate mock instances of class-like exports. If the caller did not provide one, the automocker cannot generate a mock and throws a plain `Error` asking for a bug report — this path is internal and should always be wired by Vitest.

Solutions

Update Vitest to the latest patch — this is flagged as a Vitest internal error.
If calling `mockObject` directly, pass `createMockInstance` in options.
Switch from `vi.mock` automock to an explicit factory to avoid the class-mocking path.
Open an issue with a minimal reproduction as the message requests.

Example fix

// before
vi.mock('some-module-with-class') // triggers automocker path

// after
vi.mock('some-module-with-class', () => ({
  MyClass: class { method() {} },
}))

Defensive patterns

Strategy: try-catch

Validate before calling

if (typeof options.createMockInstance !== 'function') {
  throw new Error('createMockInstance is required for class automocking')
}

Type guard

const hasCreateMockInstance = (
  o: { createMockInstance?: unknown },
): o is { createMockInstance: (...args: any[]) => any } =>
  typeof o.createMockInstance === 'function'

Try / catch

try {
  vi.mock('./module-with-class')
} catch (err) {
  // fall back to an explicit factory to avoid the automock class path
  vi.mock('./module-with-class', () => ({ MyClass: class { method() {} } }))
}

Prevention

Keep Vitest on the latest patch release.
Provide an explicit mock factory for modules exporting classes.
Do not call mockObject directly without supplying createMockInstance.

When it happens

Trigger: An automock/spy of a module that contains a class export where Vitest failed to inject `createMockInstance`; calling `mockObject` directly with incomplete options; a version mismatch where the runner does not pass the procedure.

Common situations: Upgrading Vitest and hitting an internal wiring regression; using the `@vitest/mocker` package standalone without providing `createMockInstance`; mocking a module whose export is a class in an environment Vitest does not fully support.

Related errors

AI-assisted analysis of vitest-dev/vitest@1fa9837ec2 (2026-08-11). Data as JSON: /api/errors/6bb546d1a4acd63a. Report an issue: GitHub.

Appendix: source

Thrown at packages/mocker/src/automocker.ts:36

  object: Record<Key, any>,
  mockExports: Record<Key, any> = {},
): Record<Key, any> {
  const finalizers = new Array<() => void>()
  const refs = new RefTracker()

  const define = (container: Record<Key, any>, key: Key, value: any) => {
    try {
      container[key] = value
      return true
    }
    catch {
      return false
    }
  }

  const createMock = (currentValue: (...args: any[]) => any) => {
    if (!options.createMockInstance) {
      throw new Error(
        '[@vitest/mocker] `createMockInstance` is not defined. This is a Vitest error. Please open a new issue with reproduction.',
      )
    }
    const createMockInstance = options.createMockInstance
    const prototypeMembers = currentValue.prototype
      ? collectFunctionProperties(currentValue.prototype)
      : []
    return createMockInstance({
      name: currentValue.name,
      prototypeMembers,
      originalImplementation: options.type === 'autospy' ? currentValue : undefined,
      keepMembersImplementation: options.type === 'autospy',
    })
  }

  const mockPropertiesOf = (
    container: Record<Key, any>,
    newContainer: Record<Key, any>,

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