All HTTP status codes

HTTP 505 HTTP Version Not Supported

Server Error

The server doesn't support the HTTP protocol version used in the request.

Defined in RFC 9110

Common causes

  • Legacy clients speaking versions the server rejects
  • Malformed requests misread as wrong-version

How to fix it

  1. Use HTTP/1.1 or HTTP/2 from the client

  2. Terminate odd protocol versions at a modern proxy

Related status codes

426 Upgrade Required

The server refuses to serve the request over the current protocol; the Upgrade header says which protocol to switch to.

501 Not Implemented

The server doesn't support the functionality required — typically an HTTP method it doesn't implement at all (contrast 405: method known, not allowed here).

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