WebSocket disconnects after hours of uptime, then enters reconnect loop with "Invalid frame header"

[Yago-Tavares]

I have a Socket.IO setup that works perfectly at first, but after several hours of uptime (sometimes ~3 hours, other times 1–2 days), the WebSocket connection suddenly drops. After that, the client tries to reconnect, but every attempt fails in a loop until I restart the server.

The browser console shows:

WebSocket connection to 'wss://test.com/socket.io/?userId=1&companyId=1&EIO=4&transport=websocket' failed: Invalid frame header

On the client, I log connection errors:

socket.io.on("reconnect_error", err => console.error("Error on trying to reconnect:", err));
socket.on("connect_error", err => console.error("Connect error:", err.message, err));
socket.on("disconnect", reason => console.warn("Socket disconnected:", reason));

But the only message I consistently get is:
Connect error: websocket error F: websocket error

After this point, the only way to recover is restarting the server.

socket.io: 4.8.1
socket.io-client: 4.8.1
@socket.io/redis-adapter: 8.3.0
redis: 5.8.2
Node (client): 16.20.2
Node (server): 20.12.2
Deployment: multiple pods behind Redis pub/sub

Server

io = new SocketIO(httpServer, {
  adapter: createAdapter(pubClient, subClient),
  cors: { origin: process.env.FRONTEND_URL },
  pingInterval: 30000,
  pingTimeout: 30000
});

Client

const socket = io(BACKEND_URL, {
  autoConnect: false,
  transports: ["websocket"],
  reconnection: true,
  query: { userId, companyId }
});

Expected behavior

Socket.IO should reconnect normally, not enter a permanent failure state until the entire server is restarted.

[darrachequesne]

Hi! Thanks for reporting the issue.

• does it fail for all clients? Or just one?
• how many clients are connected in parallel?
• is the server overloaded? Or out of memory?

Possibly related:

• WebSocket connection to failed: Invalid frame header #2557
• Invalid frame header #2690
• WebSocket connection to failed: Invalid frame header #4039

[Yago-Tavares]

• It fails for all clients.
• I initially tested with only one client, but I can connect multiple clients and the same error occurs for all of them.
• The server is running properly and is neither overloaded nor running out of memory.

Regarding the other related issues, I saw that the cause was often a mismatch between the client and server versions, but in my case both sides are using the same version: 4.8.1.

[Yago-Tavares]

Here are some new logs from the server side:

io.on("connection", async socket => {
      logger.info("Client Connected");
      logger.info("Handshake headers:", socket.request.headers);
      const { userId, companyId } = socket.handshake.query;

      if (userId && userId !== "undefined" && userId !== "null") {
        const user = await User.findByPk(`${userId}`);
        if (user) {
          user.online = true;
          await user.save();
        }
      }

      if (companyId) {
        socket.join(`notification_${companyId}`);
      }

      socket.on("disconnect", reason => {
        logger.warn(`Client disconnected. Reason: ${reason}`);
      });

      socket.on("connect_error", err => {
        logger.info(`Connect error: ${JSON.stringify(err)}`);
        console.log(`Connect error: ${err.message}`);
        console.log(err instanceof Error);
        console.log(err.message);
        console.log(err.data);
      });

      socket.on("error", err => {
        logger.error(`Socket error: ${err.message || err}`);
      });

      socket.on("reconnect_attempt", () => {
        logger.info("Client is attempting to reconnect...");
      });

      socket.on("joinChatBox", (ticketId: string) => {
        logger.info("A client joined a ticket channel");
        socket.join(ticketId);
      });

      socket.on("joinNotification", (companyId: string) => {
        logger.info(
          `A client joined notification channel for company ${companyId}`
        );
        socket.join(`notification_${companyId}`);
      });

      socket.on("joinNotificationPending", (companyId: string) => {
        logger.info(
          `A client joined notification pending channel for company ${companyId}`
        );
        socket.join(`notification_${companyId}`);
      });

      socket.on("joinTickets", (status: string) => {
        logger.info(`A client joined to ${status} tickets channel.`);
        socket.join(status);
      });

      socket.on("ping", callback => {
        callback();
      });
    });

    io.engine.on("connection_error", err => {
      console.log("connection_error");
      logger.error("connection_error")
      logger.error("Engine Socket.IO error:", {
        code: err.code,
        message: err.message,
        req: {
          headers: err.req.headers,
          url: err.req.url
        }
      });
    });

    io.engine.on("headers", (headers, req) => {
      logger.info(`Connection headers request: ${req.url}`);
    });

    setInterval(() => {
      if (!io || !io.sockets) {
        logger.error("Restart socket");
        initIO(httpServer);
        return;
      }

      logger.debug(
        `Healthcheck Socket.IO: ${io.sockets.sockets.size} clientes conectados`
      );
    }, 60000);```

When the error starts happening, I only receive the headers logs, like this:
`Connection headers request: /socket.io/?userId=1&companyId=1&EIO=4&transport=websocket`

But the client never connects — it keeps trying again and again.

[darrachequesne]

Thanks, that's really useful 👍

This means that the client actually reaches the server, but the WebSocket connection is denied, either at the Engine level (engine.io package) or at the WebSocket level underneath (ws package).

Could you please log the headers, when the error starts happening? Notably the Upgrade and Connection headers.

Also, in the "connection" event handler, please register the listeners before doing some async work (await User.findByPk(....)), else you might miss some events.

[Yago-Tavares]

I added a console log on the server side to display all headers when the error starts:

io.engine.on("headers", (headers, req) => {
      logger.info(`Tentativa de conexão recebida: ${req.url}`);
      logger.info(JSON.stringify(req.headers, null, 2));
    });

And here’s the output:

{
  "host": "dev-api.xama.pro",
  "user-agent": "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/141.0.0.0 Safari/537.36",
  "accept-encoding": "gzip, deflate, br, zstd",
  "accept-language": "pt-BR,pt;q=0.9,en-US;q=0.8,en;q=0.7",
  "cache-control": "no-cache",
  "connection": "Upgrade",
  "cookie": "jrt=eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJpZCI6MSwidG9rZW5WZXJzaW9uIjowLCJjb21wYW55SWQiOjEsImlhdCI6MTc2MTc1ODIwNSwiZXhwIjoxNzYyMzYzMDA1fQ.vaEvf4eVPf45ahd1JXfEdpqmrJFFqU3O65QjyHlqSH0",
  "origin": "https://dev.xama.pro",
  "pragma": "no-cache",
  "sec-websocket-extensions": "permessage-deflate; client_max_window_bits",
  "sec-websocket-key": "/Q+KmcotNL1LdNvZxNtC9g==",
  "sec-websocket-version": "13",
  "upgrade": "websocket",
  "x-forwarded-for": "177.84.220.66",
  "x-forwarded-host": "dev-api.xama.pro",
  "x-forwarded-port": "443",
  "x-forwarded-proto": "wss",
  "x-forwarded-server": "traefik-secondary-5f7896bbcd-xx565",
  "x-real-ip": "177.84.220.66"
}

On the client side, I added the following logs to provide more information:

socket.on("connect_error", err => {
      console.log("Socket ativo:", socket.active);
      console.error("Erro no connect:", err.message, err);
      console.log("Description error:", err.description);
      console.log("Context error:", err.context);
      console.log(JSON.stringify(err));
    });

And this is the result:

[Yago-Tavares]

I put a debugger on the server and noticed this:

When everything works fine, the socket shows these logs upon connection:

2025-10-30T14:53:31.052Z socket.io:server incoming connection with id KFxsDNyP88OYZESRAAAC
2025-10-30T14:53:31.097Z socket.io-parser decoded 0 as {"type":0,"nsp":"/"}
2025-10-30T14:53:31.097Z socket.io:client connecting to namespace /
2025-10-30T14:53:31.097Z socket.io:namespace adding socket to nsp /
2025-10-30T14:53:31.098Z socket.io:socket socket connected - writing packet
2025-10-30T14:53:31.098Z socket.io:socket join room y1xf-k5ciujrf7WUAAAD
2025-10-30T14:53:31.098Z socket.io-parser encoding packet {"type":0,"data":{"sid":"y1xf-k5ciujrf7WUAAAD"},"nsp":"/"}
2025-10-30T14:53:31.098Z socket.io-parser encoded {"type":0,"data":{"sid":"y1xf-k5ciujrf7WUAAAD"},"nsp":"/"} as 0{"sid":"y1xf-k5ciujrf7WUAAAD"}
2025-10-30 11:53:31.098 - �[32minfo�[39m: Client Connected

But when the error starts happening, these are the logs:

2025-10-29T20:16:29.496Z socket.io:server incoming connection with id DqYgIhxqYp_L5UAeAA2j
2025-10-29T20:16:29.496Z socket.io:client connecting to namespace /
2025-10-29T20:16:29.496Z socket.io:namespace adding socket to nsp /
2025-10-29T20:16:29.497Z socket.io:socket socket connected - writing packet
2025-10-29T20:16:29.497Z socket.io:socket joining room DqYgIhxqYp_L5UAeAA2j
2025-10-29T20:16:29.497Z socket.io:socket packet already sent in initial handshake
2025-10-29T20:16:29.497Z socket.io:client writing packet {
  "type": 2,
  "data": [
    "esm_start",
    [
      {
        "interval": 1,
        "retention": 60,
        "os": [...]],
  "nsp": "/"
}
2025-10-29T20:16:29.498Z socket.io-parser encoding packet
2025-10-29T20:16:29.497Z socket.io:client writing packet {
  "type": 2,
  "data": [
    "esm_start",
    [
      {
        "interval": 1,
        "retention": 60,
        "os": [...]],
  "nsp": "/"
}
2025-10-29T20:16:29.499Z socket.io-parser encoded 
2025-10-29T20:16:29.497Z socket.io:client writing packet {
  "type": 2,
  "data": [
    "esm_start",
    [
      {
        "interval": 1,
        "retention": 60,
        "os": [...]],
  "nsp": "/"
}
2025-10-29T20:16:29.498Z socket.io-parser encoding packet
2025-10-29T20:16:29.497Z socket.io:client writing packet {
  "type": 2,
  "data": [
    "esm_start",
    [
      {
        "interval": 1,
        "retention": 60,
        "os": [...]],
  "nsp": "/"
}
2025-10-29T20:16:17.441Z socket.io:socket joined room [ 'nSQomWgd0TEzDzSZAA2h' ]
2025-10-29T20:16:17.441Z socket.io:server incoming connection with id 349C0Hu7KH_go8NNAA2g
2025-10-29T20:16:17.497Z socket.io:client client close with reason transport error
2025-10-29T20:16:17.497Z socket.io:socket closing socket - reason transport error
2025-10-29T20:16:17.497Z socket.io:client client close with reason transport error
2025-10-29T20:16:17.578Z socket.io-parser encoding packet {"type":2,"data":["esm_stats",{"os":{"cpu":2,"memory":484.6171875,"ctime":812630,"elapsed":27989980,"timestamp":1761768976578,"pid":8,"ppid":7,"load":[3.43,3.75,3.92],"heap":{"total_heap_size":385527808,"total_heap_size_executable":9854976,"total_physical_size":379699200,"total_available_size":7208840464,"used_heap_size":350427840,"heap_size_limit":7566524416,"malloced_memory":2130032,"peak_malloced_memory":43691176,"does_zap_garbage":0,"number_of_native_contexts":1,"number_of_detached_contexts":0,"total_global_handles_size":778240,"used_global_handles_size":601952,"external_memory":46379082},"loop":{"min":1,"max":2,"num":231,"sum":237}},"responses":{"2":0,"3":0,"4":1,"5":0,"count":1,"mean":0.93963,"timestamp":1761761371446},"interval":1,"retention":60}],"nsp":"/"}
2025-10-29T20:16:17.578Z socket.io-parser encoded {"type":2,"data":["esm_stats",{"os":{"cpu":2,"memory":484.6171875,"ctime":812630,"elapsed":27989980,"timestamp":1761768976578,"pid":8,"ppid":7,"load":[3.43,3.75,3.92],"heap":{"total_heap_size":385527808,"total_heap_size_executable":9854976,"total_physical_size":379699200,"total_available_size":7208840464,"used_heap_size":350427840,"heap_size_limit":7566524416,"malloced_memory":2130032,"peak_malloced_memory":43691176,"does_zap_garbage":0,"number_of_native_contexts":1,"number_of_detached_contexts":0,"total_global_handles_size":778240,"used_global_handles_size":601952,"external_memory":46379082},"loop":{"min":1,"max":2,"num":231,"sum":237}},"responses":{"2":0,"3":0,"4":1,"5":0,"count":1,"mean":0.93963,"timestamp":1761761371446},"interval":1,"retention":60}],"nsp":"/"} as 2["esm_stats",{"os":{"cpu":2,"memory":484.6171875,"ctime":812630,"elapsed":27989980,"timestamp":1761768976578,"pid":8,"ppid":7,"load":[3.43,3.75,3.92],"heap":{"total_heap_size":385527808,"total_heap_size_executable":9854976,"total_physical_size":379699200,"total_available_size":7208840464,"used_heap_size":350427840,"heap_size_limit":7566524416,"malloced_memory":2130032,"peak_malloced_memory":43691176,"does_zap_garbage":0,"number_of_native_contexts":1,"number_of_detached_contexts":0,"total_global_handles_size":778240,"used_global_handles_size":601952,"external_memory":46379082},"loop":{"min":1,"max":2,"num":231,"sum":237}},"responses":{"2":0,"3":0,"4":1,"5":0,"count":1,"mean":0.93963,"timestamp":1761761371446},"interval":1,"retention":60}]

And it keeps repeating the same logs. This is the cycle of one connection attempt.
The esm_start logs are very large, so I only included a short excerpt here.
Does anyone know what these esm_start logs are?
I’m not emitting this event anywhere in my code, so it seems like it’s coming internally from the engine.io or @socket.io/component-emitter layer.

[darrachequesne]

The packet already sent in initial handshake log is quite surprising because it was removed in Socket.IO v3 (3289f7e, see the onconnect() method in the socket.ts file).

Would it be possible that you have an old version of the Socket.IO server running somewhere?

Also, the esm_start event is not emitted by the library. Maybe it's coming from a dependency?

[Yago-Tavares]

I founded the error. The logs esm_staI found the error. The esm_start logs were from the express-status-monitor library. I uninstalled it and the socket started working fine again.rt was of lib express-status-monitor. I uninstalled it and the socket returned to works fine