505
Server Error
505 HTTP Version Not Supported
The server does not support the HTTP protocol version used in the request.
Description
505 HTTP Version Not Supported indicates that the server does not support, or refuses to support, the HTTP version used in the request. The server should include a description of why that version is not supported and what other protocols it supports.
Rarely encountered in practice, as modern servers and clients negotiate protocol versions automatically. May appear when a very old client connects to a modern server requiring HTTP/2 or when a client misreports its HTTP version.
Common Causes
- Very old HTTP/0.9 or HTTP/1.0 client connecting to HTTP/1.1+ server
- Misconfigured client sending wrong protocol version string
- Server configured to only accept HTTP/2 with no HTTP/1.1 fallback
- Custom HTTP implementation with protocol version bugs
How to Fix
- Update your HTTP client to support modern HTTP versions (HTTP/1.1 minimum, HTTP/2 preferred)
- Check the response body for the server's supported versions
- Configure the server to support HTTP/1.1 as a fallback
- Review client HTTP version negotiation settings
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