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HTTP 505 HTTP Version Not Supported
Server ErrorThe 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
Use HTTP/1.1 or HTTP/2 from the client
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 ImplementedThe 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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