HTTP Status 505 - HTTP Version Not Supported

GET /api/data HTTP/4.0
Host: api.example.com

HTTP/1.1 505 HTTP Version Not Supported
Content-Type: application/json

{
  "error": "http_version_not_supported",
  "message": "Server does not support HTTP/4.0",
  "requested_version": "HTTP/4.0",
  "supported_versions": ["HTTP/1.1", "HTTP/2", "HTTP/3"],
  "recommendation": "Use HTTP/2 or HTTP/1.1"
}

GET /index.html

HTTP/1.1 505 HTTP Version Not Supported
Content-Type: text/plain

Error: HTTP/0.9 is not supported by this server.
This server requires HTTP/1.1 or newer.

Please upgrade your HTTP client to a modern version that supports:
- HTTP/1.1 (RFC 9112)
- HTTP/2 (RFC 9113)
- HTTP/3 (RFC 9114)

POST /api/submit HTTP/1.1-custom-extension
Host: api.example.com
Content-Type: application/json

{"data": "test"}

HTTP/1.1 505 HTTP Version Not Supported
Content-Type: application/json

{
  "status": 505,
  "error": "unsupported_protocol_extension",
  "message": "Custom HTTP protocol extensions are not supported",
  "received": "HTTP/1.1-custom-extension",
  "supported": ["HTTP/1.1", "HTTP/2"],
  "details": "Server only supports standard HTTP versions"
}