WebSockets

Vercel Functions can serve WebSocket connections, keeping a bidirectional connection open between a client and your server-side code. Use WebSockets for realtime features such as interactive AI streaming, chat, and collaborative apps.

A single WebSocket connection is pinned to one Vercel Function instance. Messages sent over that connection reach the same function instance for the lifetime of the connection, and Fluid compute allows a single function instance to handle multiple WebSocket connections.

View and deploy a starter template.

A WebSocket connection starts as an HTTP GET request with an Upgrade header. Before the connection is upgraded, the request goes through the same routing and security controls as other requests to Vercel Functions, including Routing Middleware, rewrites, Firewall rules, and rate limits. You can write Firewall rules that target the WebSocket request path, and rate limits apply to each upgrade request.

After the upgrade succeeds, messages sent over the WebSocket connection are delivered to the Vercel Function instance that accepted the connection.

WebSockets in Vercel Functions work exactly like any distributed WebSocket server, so you can use libraries like ws to upgrade connections with no additional configuration:

import http from 'http';
import { WebSocketServer } from 'ws';
 
const server = http.createServer();
const wss = new WebSocketServer({ server });
 
wss.on('connection', (ws) => {
  ws.on('message', (data) => {
    ws.send(data);
  });
});
 
export default server;

You can also use higher-level realtime libraries like Socket.IO with a client configured to use the WebSocket transport directly:

import http from 'http';
import { Server } from 'socket.io';
 
const server = http.createServer();
const io = new Server(server);
 
io.on('connection', (socket) => {
  socket.on('message', (data) => {
    socket.send(data);
  });
});
 
export default server;

import { io } from 'socket.io-client';
 
const socket = io('https://your-domain.com', {
  // Socket.IO appends /socket.io to the path by default,
  // so the full path becomes /api/socket-io/socket.io
  path: '/api/socket-io/socket.io',
  transports: ['websocket'], // required — Socket.IO defaults to HTTP long-polling
});

Python applications can use python-socketio with Asynchronous Server Gateway Interface (ASGI) or Web Server Gateway Interface (WSGI) applications. See the python-socketio server documentation for setup options. Flask applications can use Flask-SocketIO for a Flask-specific integration. See the Flask-SocketIO documentation for configuration details. These libraries implement the Socket.IO protocol, so clients must use a compatible Socket.IO client.

WebSocket connections close when a Vercel Function reaches its maximum duration.

Follow WebSocket client best practices by handling reconnects when a connection closes. Reconnect logic should recreate the connection, resubscribe to any channels or topics, and reload any state the client needs to continue.

let socket: WebSocket;
let reconnectDelay = 1000;
 
function connect() {
  socket = new WebSocket('wss://your-domain.com/api/ws');
 
  socket.addEventListener('open', () => {
    reconnectDelay = 1000;
  });
 
  socket.addEventListener('message', (event) => {
    console.log(event.data);
  });
 
  socket.addEventListener('close', () => {
    setTimeout(connect, reconnectDelay);
    reconnectDelay = Math.min(reconnectDelay * 2, 30000);
  });
}
 
connect();

New WebSocket connections are not guaranteed to reach the same Vercel Function instance. If a client reconnects, it may connect to a different instance. After a new deployment, new connections may reach the new deployment while existing connections remain on the previous deployment until they close.

Store durable state, presence, counters, rooms, and pub/sub coordination in an external data store instead of relying on in-memory variables. For example, you can use Redis from the Vercel Marketplace to share state across function instances and deployments.

Frameworks with native WebSocket support can serve WebSocket connections on Vercel Functions without additional Vercel-specific configuration. Choose the example that matches your framework.

Vercel Functions using the Bun runtime support the native Bun.serve() WebSocket API. Call server.upgrade() from the fetch handler, then define the connection handlers in the websocket option:

Bun.serve({
  fetch(request, server) {
    if (server.upgrade(request)) {
      return;
    }
 
    return new Response('Expected a WebSocket connection', { status: 400 });
  },
  websocket: {
    message(socket, message) {
      socket.send(message);
    },
  },
});

Alternatively, deploy the server from api/server.ts. Vercel serves it at /api/server, and route overrides in vercel.json must use the full path.

server.publish() and socket subscriptions share messages within one Vercel Function instance. Use an external data store to coordinate messages across instances.

The Bun WebSocket API on Vercel currently has these differences from a standalone Bun server:

• Headers passed to server.upgrade() are not applied to the upgrade response
• The drain handler is not invoked
• socket.send() does not return Bun's -1 backpressure status

Node.js server frameworks such as Express, Hono, and h3 can also serve WebSocket connections on Vercel:

import { createServer } from 'node:http';
import express from 'express';
import { WebSocketServer } from 'ws';
 
const app = express();
const server = createServer(app);
const wss = new WebSocketServer({ server });
 
wss.on('connection', (ws) => {
  ws.on('message', (data) => {
    ws.send(data);
  });
});
 
export default server;

Nitro has native WebSocket support powered by crossws. If your app uses Nitro directly or through a framework like Nuxt, enable WebSockets in your Nitro config:

import { defineConfig } from 'nitro';
 
export default defineConfig({
  features: {
    websocket: true,
  },
});

Then export a handler with defineWebSocketHandler() from a route file. The route path is the connection path, so routes/_ws.ts handles connections on /_ws:

import { defineWebSocketHandler } from 'nitro';
 
export default defineWebSocketHandler({
  message(peer, message) {
    peer.send(message.text());
  },
});

View the Nitro and Nuxt examples.

Next.js does not expose an API for handling WebSocket upgrades. As a workaround, you can use the experimental_upgradeWebSocket() API:

import {
  experimental_upgradeWebSocket,
  type WebSocketData,
} from '@vercel/functions';
 
export async function GET() {
  return experimental_upgradeWebSocket((ws) => {
    ws.on('message', (data: WebSocketData) => {
      ws.send(data);
    });
  });
}

Learn more about how to use this API with Next.js.

Python frameworks that use the Asynchronous Server Gateway Interface (ASGI) or Web Server Gateway Interface (WSGI) can serve WebSocket connections on Vercel Functions. You don't need a Vercel-specific upgrade API.

Add a WebSocket implementation (websockets, wsproto, or the dependencies included with uvicorn[standard]) to your project:

[project]
name = "my-python-websocket"
version = "0.1.0"
requires-python = ">=3.12"
dependencies = [
    "fastapi>=0.137",
    "uvicorn[standard]>=0.49",
]

import fastapi
 
app = fastapi.FastAPI()
 
@app.websocket("/api/ws")
async def websocket_endpoint(websocket: fastapi.WebSocket):
    await websocket.accept()
    try:
        while True:
            data = await websocket.receive_text()
            await websocket.send_text(data)
    except fastapi.WebSocketDisconnect:
        pass

Django applications can use Django Channels to add WebSocket consumers and routing to their ASGI entrypoint. Follow the Django Channels setup guide.

Flask applications can use flask-sock to accept WebSocket connections. Add flask and flask-sock to your project:

[project]
name = "my-python-websocket"
version = "0.1.0"
requires-python = ">=3.12"
dependencies = [
    "flask>=3.1",
    "flask-sock>=0.7",
]

from flask import Flask
from flask_sock import Sock
 
app = Flask(__name__)
sock = Sock(app)
 
@sock.route("/api/ws")
def websocket_endpoint(websocket):
    while True:
        data = websocket.receive()
        if data is None:
            break
        websocket.send(data)

WebSocket connections use Vercel Functions and follow the same limits and pricing model as other Function invocations. This includes Function usage while the connection is active, plus Fast Data Transfer and Fast Origin Transfer for data sent over the connection.

WebSockets require Fluid compute to be enabled. This is the default for new projects created on or after April 23, 2025.

• Vercel Functions
• Streaming with Vercel Functions
• @vercel/functions API Reference