HTTP Version Not Supported

Overview

505 HTTP Version Not Supported means the server does not support the major HTTP version used in the request. It occurs when the client forms its request line with an HTTP version the server cannot handle.

This code is very rarely seen in practice. Normal browsers and clients negotiate a supported version with the server, so it mostly shows up from a broken low-level client or a request with a corrupted version string.

When it happens

• Requesting with an HTTP version the server does not support (e.g. a corrupted version string)
• Forcing a modern-only HTTP version against an older server
• A hand-rolled HTTP client that writes the wrong version in the request line

Request / Response example

GET / HTTP/3.5
Host: example.com

HTTP/1.1 505 HTTP Version Not Supported
Content-Type: text/plain

The server does not support HTTP version 3.5.

In code

# Pin to HTTP/1.1 (or --http2) if the server rejects the negotiated version
curl --http1.1 -i https://example.com/

# Offer HTTP/2 so clients can negotiate a supported version
server {
  listen 443 ssl;
  http2 on;
}

Notes

• This code is about a major version mismatch; minor version differences are usually absorbed by negotiation.
• If the situation is really that the request version is too low and an upgrade is needed, 426 Upgrade Required may be more appropriate than 505.

Related status codes

Related headers

Specification