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HTTP Status Code
505 HTTP Version Not Supported
Server ErrorThe server does not support the HTTP protocol version used in the request.
HTTP status code reference, response example, common causes, fixes, and related status codes.
What does HTTP 505 HTTP Version Not Supported mean?
HTTP 505 HTTP Version Not Supported is a status code sent by a server to indicate the result of an HTTP request.
Status codes help browsers, APIs, apps, and backend systems understand whether a request succeeded, failed, was redirected, or needs additional action.
In practice, HTTP 505 HTTP Version Not Supported usually appears when a server responds under specific request, validation, permission, or infrastructure conditions.
Response example
HTTP/1.1 505 HTTP Version Not Supported
HTTP example
HTTP/1.1 505 HTTP Version Not Supported
Common causes
- Client using an HTTP version the server does not support
- Very old or very new protocol version mismatch
- Misconfigured server or client protocol settings
How to fix it
- Ensure the client uses a supported HTTP version
- Check server configuration for supported protocol versions
- Update the server or client if a version mismatch is confirmed
Common mistakes
- Assuming the status code alone explains the full backend issue
- Ignoring related response headers that add important context
- Treating temporary errors as permanent failures
- Retrying too aggressively without checking the cause
- Debugging the frontend only when the problem is server-side
How browsers and APIs use it
Browsers, APIs, and backend services use HTTP status codes to understand the outcome of a request. Depending on the status code, an application may render content, retry a request, redirect the user, show an error, or trigger a different flow in the client or server.
Practical developer insight
505 is rare in practice because most HTTP clients and servers agree on a common version. It can appear in edge cases involving HTTP/0.9, very old proxy configurations, or misconfigured HTTP/2 and HTTP/3 setups.
Client-side example
if (response.status === 505) {
console.error("HTTP version not supported by this server.");
}