What does HTTP status code 505 mean?
The 505 HTTP Version Not Supported status code indicates that the server does not support the major HTTP version (like HTTP/3) used by the client.
When to use it in APIs
Enforced by load balancers or application hosts when receiving unsupported HTTP versions.
How it compares to other codes
- 426 Upgrade Required: Rejects a request because the connection must first be upgraded to a different protocol version (e.g. HTTP/1.1 to HTTP/2, or to a WebSocket), whereas 505 rejects a request because the version the client already sent isn’t supported at all.
