socketio/socket.io · error · Error

Acknowledgements are not supported

Error message

Acknowledgements are not supported

What it means

Thrown by the redis-streams-emitter's serverSideEmit() when the last argument is a function (acknowledgement callback). The standalone redis-streams emitter publishes fire-and-forget messages to a Redis stream with no reply channel, so it cannot collect or return acknowledgements from other servers.

Source

Thrown at packages/socket.io-redis-streams-emitter/lib/index.ts:388

        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: MessageType.SERVER_SIDE_EMIT,
      data: {
        packet: [ev, ...args],
      },
    });
  }
}

function flattenPayload(message: ClusterMessage) {
  const rawMessage = {
    uid: message.uid,
    nsp: message.nsp,
    type: message.type.toString(),
    data: undefined as string | undefined,
  };

View on GitHub (pinned to ae7fb46e08)

Solutions

1. Drop the acknowledgement callback; use serverSideEmit for fire-and-forget only.
2. If acks are required, attach the RedisStreams adapter to the main Socket.IO server and use io.serverSideEmit()/serverSideEmitWithAck() instead of the standalone emitter.

Example fix

// before
emitter.serverSideEmit('sync', (ack) => { /* never resolves */ });

// after
emitter.serverSideEmit('sync');

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', arg, (response) => {...}) on the redis-streams emitter — passing a function as the final argument.

Common situations: Confusing the standalone emitter's serverSideEmit (no ack) with the main Socket.IO server's serverSideEmit (supports ack via the cluster adapter); copy-pasting an ack-style call.

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/ebb3025089142e8f.json. Report an issue: GitHub.