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505 HTTP Version Not Supported

The server does not support the major HTTP version used in the request.

Status

HTTP/1.1 505 HTTP Version Not Supported

Details

  • Category: 5xx
  • Status: Standard. Registered with IANA and defined by a current standards-track RFC.
  • Specification: RFC 9110 §15.6.6
  • Cacheable: Only with explicit cache headers
  • Response body: Allowed
  • Retry: Retry only after changing the request

505 rejects the protocol version on the request line. It is rare, because version negotiation normally happens before this point: HTTP/2 is selected during the TLS handshake via ALPN, and HTTP/1.1 is universally supported.

In practice it shows up with hand-written HTTP clients that send a malformed version token, and occasionally with servers configured to refuse HTTP/1.0.

Common causes

  • A malformed or unsupported version string on the request line.
  • A server configured to refuse HTTP/1.0 requests.
  • A raw socket client constructing requests by hand with the wrong syntax.

How to fix it

As the client

  • Send HTTP/1.1, which every modern server supports.
  • Let the TLS handshake negotiate HTTP/2 through ALPN rather than trying to request it in the request line.

As the server

  • Include the supported versions in the response body so the client can correct itself.
  • Consider accepting HTTP/1.0 for legacy monitoring tools rather than failing them outright.

Examples

An unsupported HTTP version

HTTP request

GET / HTTP/3.5
Host: www.example.com

HTTP response

HTTP/1.1 505 HTTP Version Not Supported

Specifications

See also