HTTP 101 Switching Protocols — WebSocket Handshake Explained

When to Return 101

When the client sends an Upgrade: websocket (or Upgrade: h2c for HTTP/2 over cleartext) header and the server agrees to switch. The connection protocol changes immediately after this response.

HTTP Response Example

HTTP/1.1 101 Switching Protocols
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Accept: HSmrc0sMlYUkAGmm5OPpG2HaGWk=

Code Examples

import { WebSocketServer } from 'ws'
import express from 'express'
import http from 'http'

const app = express()
const server = http.createServer(app)
const wss = new WebSocketServer({ server })

wss.on('connection', (ws) => {
  // Connection upgraded — 101 was sent automatically
  ws.on('message', (data) => ws.send(`Echo: ${data}`))
})

// WebSocket servers run outside Next.js App Router.
// Use a custom server.js or a separate WebSocket service.
// The 101 response is handled by the ws/socket.io library.

Related Status Codes