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Version: 4.x

Troubleshooting connection issues


The Admin UI can give you additional insights about the status of your Socket.IO deployment.

Common/known issues:

Other common gotchas:

Problem: the socket is not able to connect

Possible explanations:

You are trying to reach a plain WebSocket server

As explained in the "What Socket.IO is not" section, the Socket.IO client is not a WebSocket implementation and thus will not be able to establish a connection with a WebSocket server, even with transports: ["websocket"]:

const socket = io("ws://", {
transports: ["websocket"]

The server is not reachable

Please make sure the Socket.IO server is actually reachable at the given URL. You can test it with:

curl "<the server URL>/"

which should return something like this:


If that's not the case, please check that the Socket.IO server is running, and that there is nothing in between that prevents the connection.

Note: v1/v2 servers (which implement the v3 of the protocol, hence the EIO=3) will return something like this:


The client is not compatible with the version of the server

Maintaining backward compatibility is a top priority for us, but in some particular cases we had to implement some breaking changes at the protocol level:

  • from v1.x to v2.0.0 (released in May 2017), to improve the compatibility with non-Javascript clients (see here)
  • from v2.x to v3.0.0 (released in November 2020), to fix some long-standing issues in the protocol once for all (see here)

v4.0.0 contains some breaking changes in the API of the JavaScript server. The Socket.IO protocol itself was not updated, so a v3 client will be able to reach a v4 server and vice-versa (see here).

For example, reaching a v3/v4 server with a v1/v2 client will result in the following response:

< HTTP/1.1 400 Bad Request
< Content-Type: application/json

{"code":5,"message":"Unsupported protocol version"}

Here is the compatibility table for the JS client:

JS Client versionSocket.IO server version

[1] Yes, with allowEIO3: true

Here is the compatibility table for the Java client:

Java Client versionSocket.IO server version

[1] Yes, with allowEIO3: true

Here is the compatibility table for the Swift client:

Swift Client versionSocket.IO server version

[1] Yes, with allowEIO3: true (server) and .connectParams(["EIO": "3"]) (client):

SocketManager(socketURL: URL(string:"http://localhost:8087/")!, config: [.connectParams(["EIO": "3"])])

[2] Yes, allowEIO3: true (server)

[3] Yes, with .version(.two) (client):

SocketManager(socketURL: URL(string:"http://localhost:8087/")!, config: [.version(.two)])

The server does not send the necessary CORS headers

If you see the following error in your console:

Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at ...

It probably means that:

Please see the documentation here.

You didn't enable sticky sessions (in a multi server setup)

When scaling to multiple Socket.IO servers, you need to make sure that all the requests of a given Socket.IO session reach the same Socket.IO server. The explanation can be found here.

Failure to do so will result in HTTP 400 responses with the code: {"code":1,"message":"Session ID unknown"}

Please see the documentation here.

The request path does not match on both sides

By default, the client sends — and the server expects — HTTP requests with the "/" request path.

This can be controlled with the path option:


import { Server } from "";

const io = new Server({
path: "/my-custom-path/"



import { io } from "";

const socket = io(SERVER_URL, {
path: "/my-custom-path/"

In that case, the HTTP requests will look like <SERVER_URL>/my-custom-path/?EIO=4&transport=polling[&...].

import { io } from "";

const socket = io("/my-custom-path/");

means the client will try to reach the namespace named "/my-custom-path/", but the request path will still be "/".

Problem: the socket gets disconnected

First and foremost, please note that disconnections are common and expected, even on a stable Internet connection:

  • anything between the user and the Socket.IO server may encounter a temporary failure or be restarted
  • the server itself may be killed as part of an autoscaling policy
  • the user may lose connection or switch from WiFi to 4G, in case of a mobile browser
  • the browser itself may freeze an inactive tab

That being said, the Socket.IO client will always try to reconnect, unless specifically told otherwise.

Possible explanations for a disconnection:

The browser tab was minimized and heartbeat has failed

When a browser tab is not in focus, some browsers (like Chrome) throttle JavaScript timers, which could lead to a disconnection by ping timeout in Socket.IO v2, as the heartbeat mechanism relied on setTimeout function on the client side.

As a workaround, you can increase the pingTimeout value on the server side:

const io = new Server({
pingTimeout: 60000

Please note that upgrading to Socket.IO v4 (at least, due to this) should prevent this kind of issues, as the heartbeat mechanism has been reversed (the server now sends PING packets).

The client is not compatible with the version of the server

Since the format of the packets sent over the WebSocket transport is similar in v2 and v3/v4, you might be able to connect with an incompatible client (see above), but the connection will eventually be closed after a given delay.

So if you are experiencing a regular disconnection after 30 seconds (which was the sum of the values of pingTimeout and pingInterval in Socket.IO v2), this is certainly due to a version incompatibility.

You are trying to send a huge payload

If you get disconnected while sending a huge payload, this may mean that you have reached the maxHttpBufferSize value, which defaults to 1 MB. Please adjust it according to your needs:

const io = require("")(httpServer, {
maxHttpBufferSize: 1e8

A huge payload taking more time to upload than the value of the pingTimeout option can also trigger a disconnection (since the heartbeat mechanism fails during the upload). Please adjust it according to your needs:

const io = require("")(httpServer, {
pingTimeout: 60000

Problem: the socket is stuck in HTTP long-polling

In most cases, you should see something like this:

Network monitor upon success

  1. the Engine.IO handshake (contains the session ID — here, zBjrh...AAAK — that is used in subsequent requests)
  2. the Socket.IO handshake request (contains the value of the auth option)
  3. the Socket.IO handshake response (contains the Socket#id)
  4. the WebSocket connection
  5. the first HTTP long-polling request, which is closed once the WebSocket connection is established

If you don't see a HTTP 101 Switching Protocols response for the 4th request, that means that something between the server and your browser is preventing the WebSocket connection.

Please note that this is not necessarily blocking since the connection is still established with HTTP long-polling, but it is less efficient.

You can get the name of the current transport with:


socket.on("connect", () => {
const transport =; // in most cases, "polling""upgrade", () => {
const upgradedTransport =; // in most cases, "websocket"


io.on("connection", (socket) => {
const transport =; // in most cases, "polling"

socket.conn.on("upgrade", () => {
const upgradedTransport =; // in most cases, "websocket"

Possible explanations:

A proxy in front of your servers does not accept the WebSocket connection

Please see the documentation here.

Other common gotchas

Delayed event handler registration


io.on("connection", async (socket) => {
await longRunningOperation();

// WARNING! Some packets might be received by the server but without handler
socket.on("hello", () => {
// ...


io.on("connection", async (socket) => {
socket.on("hello", () => {
// ...

await longRunningOperation();

Usage of the attribute

Please note that, unless connection state recovery is enabled, the id attribute is an ephemeral ID that is not meant to be used in your application (or only for debugging purposes) because:

  • this ID is regenerated after each reconnection (for example when the WebSocket connection is severed, or when the user refreshes the page)
  • two different browser tabs will have two different IDs
  • there is no message queue stored for a given ID on the server (i.e. if the client is disconnected, the messages sent from the server to this ID are lost)

Please use a regular session ID instead (either sent in a cookie, or stored in the localStorage and sent in the auth payload).

See also:

Deployment on a serverless platform

Since most serverless platforms (such as Vercel) bill by the duration of the request handler, maintaining a long-running connection with Socket.IO (or even plain WebSocket) is not recommended.