EC
101
HTTPINFOCommon1xx InformationalHIGH confidence

Switching Protocols

Production Risk

Low. This is the standard mechanism for protocol upgrades and is fundamental for technologies like WebSockets to function correctly.

What this means

This code is sent in response to an 'Upgrade' request header from the client, and indicates the protocol the server is switching to. The server will switch protocols to the one specified in the 'Upgrade' header.

Why it happens
  1. 1The client requests to switch from HTTP to a different protocol, such as WebSocket.
  2. 2The server supports the requested protocol switch and agrees to it.
  3. 3This is commonly used to initiate a WebSocket connection for real-time, bidirectional communication.
How to reproduce

A web browser initiates a WebSocket connection to a server to enable a live chat feature.

trigger — this will error
trigger — this will error
GET /chat HTTP/1.1
Host: example.com
Upgrade: websocket
Connection: Upgrade

expected output

HTTP/1.1 101 Switching Protocols

Fix

Handle the protocol switch on the client side

Handle the protocol switch on the client side
// Browser WebSocket — the 101 upgrade is handled automatically
const socket = new WebSocket('wss://example.com/chat');

socket.addEventListener('open', () => {
  console.log('Protocol switched to WebSocket (101 received)');
  socket.send(JSON.stringify({ type: 'hello' }));
});

socket.addEventListener('message', (event) => {
  const msg = JSON.parse(event.data);
  console.log('Received:', msg);
});

Why this works

After the server sends '101 Switching Protocols', the TCP connection is no longer HTTP — both sides must speak the negotiated protocol (e.g., WebSocket). The browser's WebSocket API and most HTTP client libraries handle the handshake automatically. The 'open' event fires once the 101 exchange completes and the connection is ready for full-duplex messaging.

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