vitest-dev/vitest · error · Error

Runner ${task.worker} is not supported. Test files: ${format

Error message

Runner ${task.worker} is not supported. Test files: ${formatFiles(task)}.

What it means

Thrown by Pool.getPoolRunner when task.worker does not match any builtin pool ('forks','vmForks','threads','vmThreads','typescript') and does not match a registered custom config.poolRunner with the same name. The worker string on a PoolTask must correspond to a pool Vitest knows how to instantiate; an unknown value means spec resolution or config produced an invalid pool assignment.

Source

Thrown at packages/vitest/src/node/pools/pool.ts:264

      case 'vmForks':
        return new PoolRunner(options, new VmForksPoolWorker(options))

      case 'threads':
        return new PoolRunner(options, new ThreadsPoolWorker(options))

      case 'vmThreads':
        return new PoolRunner(options, new VmThreadsPoolWorker(options))

      case 'typescript':
        return new PoolRunner(options, new TypecheckPoolWorker(options))
    }

    const customPool = task.project.config.poolRunner
    if (customPool != null && customPool.name === task.worker) {
      return new PoolRunner(options, customPool.createPoolWorker(options))
    }

    throw new Error(`Runner ${task.worker} is not supported. Test files: ${formatFiles(task)}.`)
  }

  private getConcurrencyId() {
    let concurrencyId: number | undefined

    this.workerIds.forEach((state, id) => {
      if (state && concurrencyId == null) {
        concurrencyId = id
        this.workerIds.set(id, false)
      }
    })

    if (concurrencyId == null) {
      throw new Error('Cannot set concurrency id because there are no valid free ids.')
    }

    return concurrencyId
  }

View on GitHub (pinned to d568f8ce37)

Solutions

Check the project's config.pool value — it must be one of the builtin pools or match a registered poolRunner.name.
If using a custom poolRunner, ensure its .name exactly matches the pool string assigned to specs.
Re-validate the workspace/project config for typos in pool or poolRunner registration.
Report as a bug if the value is a builtin and this still fires (likely a version regression).

Example fix

// before: pool name does not match the registered runner
export default defineConfig({
  test: {
    pool: 'mypool',
    poolRunner: { name: 'my-pool', createPoolWorker: () => ... },
  },
})

// after: names match exactly
export default defineConfig({
  test: {
    pool: 'my-pool',
    poolRunner: { name: 'my-pool', createPoolWorker: () => ... },
  },
})

Defensive patterns

Strategy: validation

Validate before calling

// Validate that the configured pool resolves to a builtin or a registered custom runner
const BUILTIN_POOLS = new Set(['forks','threads','browser','vmThreads','vmForks','typescript'])
function validatePoolName(pool, poolRunner) {
  if (BUILTIN_POOLS.has(pool)) return
  if (poolRunner && poolRunner.name === pool) return
  throw new Error(`Unknown pool '${pool}'. Use a builtin or register poolRunner with matching name.`)
}

Type guard

function isValidPoolName(pool, poolRunner) {
  const BUILTIN = new Set(['forks','threads','browser','vmThreads','vmForks','typescript'])
  return BUILTIN.has(pool) || (poolRunner != null && poolRunner.name === pool)
}

Prevention

Double-check pool string spelling in project/workspace config.
When registering a custom poolRunner, ensure its .name exactly matches the pool value.
Validate config in CI before running tests.

When it happens

Trigger: A TestSpecification has pool set to an unrecognized string that is neither builtin nor a key in project.config.poolRunner (packages/vitest/src/node/pools/pool.ts:242-264). Reachable via a custom pool config name typo, a stale spec after config changes, or an internal bug assigning the pool.

Common situations: Typo in a per-project pool config; a custom poolRunner whose name does not match the pool value used on specs; version mismatch where a pool was renamed.

Related errors

AI-assisted analysis of vitest-dev/vitest@d568f8ce37 (2026-08-03). Data as JSON: /data/errors/69075794c1bb6620.json. Report an issue: GitHub.