denoland/deno · error · Error

ERR_INSPECTOR_NOT_WORKER

ERR_INSPECTOR_NOT_WORKER

Error message

Current thread is not a worker

What it means

Session.connectToMainThread() attaches a worker thread's inspector session to the main thread's inspector front-end. It is only valid inside a worker: when worker_threads.isMainThread is true it throws ERR_INSPECTOR_NOT_WORKER before any connection attempt (via op_inspector_connect with toMainThread=true).

Source

Thrown at ext/node/polyfills/inspector.js:120

  #nextId = 1;
  #messageCallbacks = new SafeMap();
  #pendingMessages = [];
  #drainScheduled = false;
  #isDraining = false;

  connect() {
    if (this.#connection) {
      throw new ERR_INSPECTOR_ALREADY_CONNECTED("The inspector session");
    }
    this.#connection = op_inspector_connect(
      false,
      (m) => this.#enqueueMessage(m),
    );
  }

  connectToMainThread() {
    if (lazyWorkerThreads().isMainThread) {
      throw new ERR_INSPECTOR_NOT_WORKER();
    }
    if (this.#connection) {
      throw new ERR_INSPECTOR_ALREADY_CONNECTED("The inspector session");
    }
    this.#connection = op_inspector_connect(
      true,
      (m) => this.#enqueueMessage(m),
    );
  }

  #onMessage(message) {
    const parsed = JSONParse(message);
    try {
      if (parsed.id) {
        const callback = this.#messageCallbacks.get(parsed.id);
        this.#messageCallbacks.delete(parsed.id);
        if (callback) {
          if (parsed.error) {

View on GitHub (pinned to 9ad36f7a2c)

Solutions

1. Branch on worker context: if (isMainThread) session.connect(); else session.connectToMainThread();
2. Move worker-specific inspector setup into the worker entry file only.
3. On the main thread always use plain connect().

Example fix

// before
const session = new inspector.Session();
session.connectToMainThread(); // throws ERR_INSPECTOR_NOT_WORKER on main thread

// after
import { isMainThread } from 'node:worker_threads';
const session = new inspector.Session();
if (isMainThread) session.connect();
else session.connectToMainThread();

import { isMainThread } from 'node:worker_threads';
function connectSession(session) {
  if (isMainThread) session.connect();
  else session.connectToMainThread();
}

import { isMainThread } from 'node:worker_threads';
function canConnectToMainThread() {
  return !isMainThread;
}

When it happens

Trigger: Calling inspector.Session.connectToMainThread() at the top level of the main module, or in a shared debug-utility module that both the main thread and workers import.

Common situations: A debug helper imported by both main and worker code; worker bootstrap code copied into the main entrypoint; frameworks that reuse one setup routine in every thread.

Related errors

• Expected data to be a string, Buffer, Uint8Array, or ArrayBu
• ERR_INSPECTOR_ALREADY_CONNECTED
• ERR_INSPECTOR_NOT_CONNECTED
• ERR_INSPECTOR_ALREADY_ACTIVATED
• ERR_INSPECTOR_NOT_ACTIVE

AI-assisted analysis of denoland/deno@9ad36f7a2c (2026-08-20). Data as JSON: /api/errors/78b5082eb4b6b35e. Report an issue: GitHub.