jestjs/jest · error · Error

Child can only be used on a forked process

Error message

Child can only be used on a forked process

What it means

Error "Child can only be used on a forked process" thrown in jestjs/jest.

Source

Thrown at packages/jest-worker/src/workers/threadChild.ts:101

            reportInitializeError,
          );
        } else {
          reportSuccess(void 0);
        }
      }
      break;

    default:
      throw new TypeError(
        `Unexpected request from parent process: ${request[0]}`,
      );
  }
};
parentPort!.on('message', messageListener);

function reportMemoryUsage() {
  if (isMainThread) {
    throw new Error('Child can only be used on a forked process');
  }

  const msg: ParentMessageMemUsage = [
    PARENT_MESSAGE_MEM_USAGE,
    process.memoryUsage().heapUsed,
  ];

  parentPort!.postMessage(msg);
}

function reportSuccess(result: unknown) {
  if (isMainThread) {
    throw new Error('Child can only be used on a forked process');
  }

  try {
    parentPort!.postMessage([PARENT_MESSAGE_OK, result]);
  } catch (error) {

View on GitHub (pinned to f49721c78e)

Solutions

1. This setup code only runs in a worker thread spawned by jest-worker. It was executed outside a worker thread; run the code through Jest's worker farm instead of invoking the thread script directly.

When it happens

Trigger: Thrown at packages/jest-worker/src/workers/threadChild.ts:101 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of jestjs/jest@f49721c78e (2026-08-03). Data as JSON: /data/errors/83e27114427628aa.json. Report an issue: GitHub.