HTTP 101 – Switching Protocols
The server is switching protocols as requested by the client, e.g. to WebSocket.
Common use case
The backbone of the WebSocket handshake: the client sends an "Upgrade: websocket" header and the server confirms with 101 before the connection becomes a raw bidirectional socket.
Example
HTTP/1.1 101 Switching Protocols
Upgrade: websocket
Connection: Upgrade
History
Introduced in RFC 2068 (1997) for generic protocol negotiation, later became essential to WebSocket (RFC 6455, 2011) and HTTP/2's h2c upgrade path.
Did you know?
After a 101 response, the connection is no longer HTTP at all - everything that follows is whatever protocol was switched to.