101 Switching Protocols

HTTP 101 Switching Protocols is the server's acknowledgment of a client request to change the communication protocol on the same underlying TCP connection. The client signals this desire with an Upgrade header, commonly paired with Connection: Upgrade, naming the target protocol such as websocket or h2c. If the server is willing and able to speak that protocol, it responds 101, and from that point on both sides stop speaking HTTP and start exchanging frames or messages defined by the new protocol entirely.

The single most common use in production is the WebSocket handshake: a browser opens what looks like a normal GET request carrying Upgrade: websocket, Connection: Upgrade, and a Sec-WebSocket-Key, and the server replies 101 with a computed Sec-WebSocket-Accept value. Once that exchange completes, the connection is no longer HTTP at all, it is a raw bidirectional WebSocket stream, and any HTTP-aware middleware or logging in front of the server needs explicit support for the upgrade or it will break the handshake.

101 only ever appears over HTTP/1.1 connections, since HTTP/2 and HTTP/3 multiplex many logical streams over one connection and cannot hand the whole transport over to a different protocol; HTTP/2 instead defines h2c upgrade rules that are largely obsolete in practice, and WebSocket over HTTP/2 uses the separate CONNECT-based extended connect mechanism. Proxies, CDNs and load balancers must be configured to pass Upgrade and Connection headers through untouched, or the switch never completes and clients fall back to polling.