1xx

101 Switching Protocols

The server is switching to a different protocol as requested by the client.

What is HTTP 101?

This response is sent when the server agrees to change the application protocol being used on this connection, as requested by the client via an Upgrade header. The server will switch to the protocol defined in the response's Upgrade header. This is most commonly seen when establishing WebSocket connections, where the connection upgrades from HTTP to the WebSocket protocol. After this response, both client and server communicate using the new protocol.

Defined in RFC 9110

Common Causes

  • WebSocket handshake upgrading from HTTP to the WebSocket protocol
  • Client requested an HTTP/2 upgrade via the Upgrade header
  • TLS negotiation where the connection upgrades to a secure protocol

How to Fix

  • No fix needed; this is expected behavior during protocol upgrades like WebSocket connections
  • If the upgrade fails, verify the server supports the requested protocol
  • Check that proxies and load balancers are configured to allow protocol upgrades