jestjs/jest · error · Error

Both --runInBand and --maxWorkers were specified, only one i

Error message

Both --runInBand and --maxWorkers were specified, only one is allowed.

What it means

jest-cli's argument validator (packages/jest-cli/src/args.ts:12, the `check` function) rejects combining `--runInBand` (run serially in the main process) with `--maxWorkers` (explicit worker pool size), because they are mutually exclusive ways to control parallelism. `--runInBand` is effectively `--maxWorkers=1` constrained to the current process.

Source

Thrown at packages/jest-cli/src/args.ts:17

/**
 * Copyright (c) Meta Platforms, Inc. and affiliates.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 */

import type {Options} from 'yargs';
import type {Config} from '@jest/types';
import {constants, isJSONString} from 'jest-config';

export function check(argv: Config.Argv): true {
  if (
    argv.runInBand &&
    Object.prototype.hasOwnProperty.call(argv, 'maxWorkers')
  ) {
    throw new Error(
      'Both --runInBand and --maxWorkers were specified, only one is allowed.',
    );
  }

  for (const key of [
    'onlyChanged',
    'lastCommit',
    'changedFilesWithAncestor',
    'changedSince',
  ]) {
    if (argv[key] && argv.watchAll) {
      throw new Error(
        `Both --${key} and --watchAll were specified, but cannot be used ` +
          'together. Try the --watch option which reruns only tests ' +
          'related to changed files.',
      );
    }
  }

View on GitHub (pinned to f49721c78e)

Solutions

1. Pick one: use `--maxWorkers=1` if you want serial execution with control, or `--runInBand` if you want serial execution in the main process (also needed for some debugging features).
2. Audit your package.json scripts and CI YAML for both flags and remove the redundant one.
3. If flags come from env vars, make them mutually exclusive in your script logic (e.g. only set maxWorkers when runInBand is not set).

Example fix

// before (package.json)
"scripts": { "test": "jest --runInBand --maxWorkers=2" }

// after
"scripts": { "test:serial": "jest --runInBand", "test:ci": "jest --maxWorkers=2" }

// Validate argv before passing to jest-cli.
function assertNoFlagConflict(argv: { runInBand?: boolean; maxWorkers?: number | string }): void {
  if (argv.runInBand && argv.maxWorkers !== undefined) {
    throw new Error('Pass either --runInBand or --maxWorkers, not both');
  }
}

const hasOwn = (o: object, k: PropertyKey) => Object.prototype.hasOwnProperty.call(o, k);
const flagsConflict = (a: any) => Boolean(a.runInBand) && hasOwn(a, 'maxWorkers');

When it happens

Trigger: Running `jest --runInBand --maxWorkers=4`, or a package.json script / CI config that concatenates both flags (often via env-variable expansion like `--maxWorkers=$MW` while a base script already adds `--runInBand`).

Common situations: Inheriting a script that uses runInBand for debugging and then appending maxWorkers for CI throughput; wrapper tools (Nx, turbo) that merge flag sets.

Related errors

• Both --${key} and --watchAll were specified, but cannot be u
• Both --onlyFailures and --watchAll were specified, only one
• The --findRelatedTests option requires file paths to be spec
• The --maxWorkers (-w) option requires a number or string to
• The --selectProjects option requires the name of at least on

AI-assisted analysis of jestjs/jest@f49721c78e (2026-08-03). Data as JSON: /data/errors/e720dea4f22cbb3e.json. Report an issue: GitHub.