What it means
The server does not support the HTTP version used in the request.
Common causes
- Malformed request line that looks like an unknown version
- Very old or very new protocol versions on minimal servers
How to fix it
As a client or visitor
Use HTTP/1.1, which all servers support, and check that the request line is well-formed.
As the site or API owner
Respond with the supported versions in the body.
Example response
HTTP/1.1
HTTP/1.1 505 HTTP Version Not Supported
Date: Tue, 07 Oct 2025 09:30:00 GMT
Server: nginx
Content-Length: 0Specification
FAQ
› Can a typo cause 505?
Yes. A space in the URL can split the request line so the version looks invalid.
› Does 505 happen with HTTP/2?
Rarely; HTTP/2 is negotiated during TLS, so an unsupported version simply falls back to HTTP/1.1.