Toronto SEO
All HTTP status codes
5xx Server Error
RFC 9110

505 HTTP Version Not Supported

The HTTP version used in the request is not supported by the server.

What it means

HTTP 505 HTTP Version Not Supported indicates the server does not support the HTTP protocol version used in the request. Very rare in modern systems where HTTP/1.1, HTTP/2, and HTTP/3 are widely supported.

When it's used

Legacy servers refusing modern HTTP versions, or restrictive servers requiring a specific version.

Common causes
  • Client sent HTTP/2 to an HTTP/1.0-only server
  • Misconfigured load balancer dropping HTTP/2
  • Custom protocol experiments
How to fix it
  • Downgrade client to HTTP/1.1 if server requires it
  • Upgrade server to support modern HTTP versions
  • Check load balancer protocol support
Example response
HTTP/1.1 505 HTTP Version Not Supported
Connection: close
SEO impact

Should not occur — Googlebot uses modern HTTP versions universally supported.