HTTP Version Not Supported

Server perspective

Use 505 when the request's HTTP major version is unsupported or intentionally refused by the server.

When to use

• •Return 505 when the client speaks a protocol version the server will not service
• •Use it for version incompatibility, not for unsupported methods or optional protocol upgrades
• •Client uses unsupported HTTP version
• •HTTP/0.9 requests to modern servers
• •Future major HTTP versions the server refuses to process
• •A server or intermediary enforcing a minimum or maximum supported HTTP major version

How to respond

• •If practical, describe which HTTP versions the server supports so clients can recover
• •Validate that the failure is really version-related and not just a malformed request line better described by 400

Headers to consider

• •No status-specific header is required; include explanatory metadata in the body when recovery is possible

Response body

• •A body is recommended; describe why the version is unsupported and list the versions the server accepts (RFC 9110 SHOULD).

Server-side pitfalls

• •Do not use 505 when the server merely wants the client to switch protocols or upgrade features; compare with 426 Upgrade Required
• •Do not collapse general parsing bugs into 505 if the issue is ordinary malformed syntax
• •Protocol upgrade required (use 426 Upgrade Required)
• •Method not implemented (use 501 Not Implemented)
• •Bad request format (use 400 Bad Request)

Examples