# 505 HTTP Version Not Supported

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

Source: https://apiguide.dev/status-codes/505
Class: server-error
Defined in: RFC 9110, Section 15.6.6 <https://www.rfc-editor.org/rfc/rfc9110.html#section-15.6.6>
Updated: 2026-07-09

## 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.