socketio/socket.io · error · Error

Acknowledgements are not supported

Error message

Acknowledgements are not supported

What it means

Thrown by the postgres-emitter's serverSideEmit() when the last argument is a function (an acknowledgement callback). The postgres emitter (and the multi-server emit pattern generally) is fire-and-forget across the cluster via pg_notify, which has no request/response channel to deliver acknowledgements, so ack callbacks are unsupported.

Source

Thrown at packages/socket.io-postgres-emitter/lib/index.ts:526

        close,
      },
    });
  }

  /**
   * Send a packet to the Socket.IO servers in the cluster
   *
   * @param ev - the event name
   * @param args - any number of serializable arguments
   */
  public serverSideEmit<Ev extends EventNames<ServerSideEvents>>(
    ev: Ev,
    ...args: EventParams<ServerSideEvents, Ev>
  ): void {
    const withAck = args.length && typeof args[args.length - 1] === "function";

    if (withAck) {
      throw new Error("Acknowledgements are not supported");
    }

    this.publish({
      type: EventType.SERVER_SIDE_EMIT,
      data: {
        packet: [ev, ...args],
      },
    });
  }
}

View on GitHub (pinned to ae7fb46e08)

Solutions

1. Remove the acknowledgement callback from the serverSideEmit call; use it purely for fire-and-forget messaging.
2. If you need responses from other servers, use the main Socket.IO server's serverSideEmit/serverSideEmitWithAck with the Postgres adapter attached to the server (not the standalone emitter).

Example fix

// before
emitter.serverSideEmit('ping', (res) => { /* unsupported */ });

// after
emitter.serverSideEmit('ping'); // fire-and-forget

function noAck(args){ return !(args.length && typeof args[args.length-1]==='function'); }

function isAckFree(args){ return args.length===0 || typeof args[args.length-1]!=='function'; }

When it happens

Trigger: Calling emitter.serverSideEmit('event', arg1, (response) => {...}) — i.e. passing a function as the final argument.

Common situations: Treating serverSideEmit like socket.emit (which supports acks) or like the main server's serverSideEmit (which DOES support acks via the cluster adapter); copy-pasting an ack-style call into the postgres emitter.

Related errors

• Acknowledgements are not supported
• "${String(ev)}" is a reserved event name
• "${String(ev)}" is a reserved event name
• "${String(ev)}" is a reserved event name

AI-assisted analysis of socketio/socket.io@ae7fb46e08 (2026-08-03). Data as JSON: /data/errors/a8d0dd6eeba15a45.json. Report an issue: GitHub.