What HTTP 101 means
RFC 9110 §15.2.2. Most commonly seen during WebSocket upgrade handshakes. The Upgrade and Connection headers must be present in the request.
Typical causes
- WebSocket upgrade request
- HTTP/2 upgrade from HTTP/1.1
101 1xx RFC 9110 §15.2.2 Switching Protocols
Server is switching protocols as requested by the client (e.g., HTTP to WebSocket).