What does HTTP status 505 (Version Not Supported) mean?

Status / error code:		Category:

Constant name:
Source:		Operating System:
Description: HTTP 505 means the server cannot or will not complete the request using the HTTP major version sent by the client.

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HTTP_STATUS_GATEWAY_TIMEOUT	HTTP_STATUS_VARIANT_ALSO_NEGOTIATES

HTTP_STATUS_VERSION_NOT_SUPPORTED

The response concerns protocol version support, not the version of the resource or API. It is rare with modern clients but can appear with custom proxies, protocol tests, or incorrect transport negotiation.

Inspect the actual request line and ALPN or upgrade negotiation. Confirm that the front-end and upstream agree on supported HTTP versions.

This status concerns the HTTP major version used for the request, not an application API version in the URI. Verify protocol negotiation and the capabilities of every intermediary on the path. If a proxy translates versions, determine which hop actually generated 505 before changing the client's transport configuration.

RFC 9110: HTTP Semantics

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