denoland/deno · error · Error

ERR_INSPECTOR_ALREADY_ACTIVATED

ERR_INSPECTOR_ALREADY_ACTIVATED

Error message

Inspector is already activated. Close it with inspector.close() before activating it again.

What it means

inspector.open([port[, host[, wait]]]) activates the inspector for the process. If an inspector is already active — the process was started with --inspect/--inspect-brk, or inspector.open() was called earlier and not closed — op_inspector_enabled() returns true and open() throws ERR_INSPECTOR_ALREADY_ACTIVATED, with the message telling you to close it via inspector.close() first.

Source

Thrown at ext/node/polyfills/inspector.js:228

    }
    op_inspector_disconnect(this.#connection);
    this.#connection = null;
    for (
      const { 1: callback } of new SafeMapIterator(this.#messageCallbacks)
    ) {
      lazyProcess().default.nextTick(callback, new ERR_INSPECTOR_CLOSED());
    }
    this.#messageCallbacks.clear();
    this.#nextId = 1;
    this.#pendingMessages.length = 0;
    this.#drainScheduled = false;
    this.#isDraining = false;
  }
}

function open(port, host, wait) {
  if (op_inspector_enabled()) {
    throw new ERR_INSPECTOR_ALREADY_ACTIVATED();
  }
  // inspectorOpen() currently does not typecheck its arguments and adding
  // such checks would be a potentially breaking change. However, the native
  // open() function requires the port to fit into a 16-bit unsigned integer,
  // causing an integer overflow otherwise, so we at least need to prevent that.
  if (isUint32(port)) {
    validateInt32(port, "port", 0, 65535);
  } else {
    // equiv of handling args[0]->IsUint32()
    port = undefined;
  }
  if (typeof host !== "string") {
    // equiv of handling args[1]->IsString()
    host = undefined;
  }

  if (host && !isLoopback(host)) {
    lazyProcess().default.emitWarning(

View on GitHub (pinned to 9ad36f7a2c)

Solutions

Guard the call: only open when inspector.url() === undefined (url() returns undefined when no inspector is active).
Call inspector.close() before inspector.open() when re-activating after a previous open.
Drop the --inspect/--inspect-brk flag from the launch command if the program opens the inspector itself.

Example fix

// before - launched with: deno run --inspect app.ts
inspector.open(9229, '127.0.0.1'); // ERR_INSPECTOR_ALREADY_ACTIVATED

// after
if (inspector.url() === undefined) {
  inspector.open(9229, '127.0.0.1');
}

Defensive patterns

Strategy: validation

Validate before calling

import inspector from 'node:inspector';
function openInspector(port, host) {
  if (inspector.url() !== undefined) return inspector.url(); // already active
  inspector.open(port, host);
  return inspector.url();
}

Type guard

import inspector from 'node:inspector';
function inspectorIsActive() {
  return inspector.url() !== undefined;
}

Try / catch

try {
  inspector.open(9229, '127.0.0.1');
} catch (e) {
  if (e.code === 'ERR_INSPECTOR_ALREADY_ACTIVATED') {
    // inspector already running (e.g. via --inspect); continue with inspector.url()
  } else throw e;
}

Prevention

Never combine --inspect in the launch command with inspector.open() in code.
Make programmatic open() idempotent: check inspector.url() first, or close() before open() in reload paths.

When it happens

Trigger: Running `deno run --inspect app.ts` (or equivalent flags) where app.ts itself calls inspector.open(); calling inspector.open() twice, e.g. at startup and again on config reload/SIGHUP.

Common situations: Dev scripts that open the inspector programmatically while the launch command already has --inspect; watch/reload loops re-running init code; porting Node debugging recipes that combine both.

Related errors

AI-assisted analysis of denoland/deno@9ad36f7a2c (2026-08-20). Data as JSON: /api/errors/56dd48510bc98803. Report an issue: GitHub.