What is the biggest difference between 510 and 501?

510 communicates Not Extended, while 501 communicates Not Implemented. Choosing the right one keeps clients and intermediaries predictable.

Do 510 and 501 have SEO or caching impact?

Yes. Search engines and caches interpret status classes differently. Use each code according to its semantics to avoid accidental indexing, stale responses, or crawl inefficiency.

Can APIs safely return 510 instead of 501?

Only when it matches contract semantics. API clients often branch logic by exact code, so swapping them can break retries, auth handling, or user-facing errors.

510 vs 501: Not Extended vs Not Implemented

Both codes signal that the server cannot fulfill a request due to missing functionality, but they differ in why and what the client should do next.

Aspect	HTTP 501 — Not Implemented	HTTP 510 — Not Extended
RFC	RFC 9110, Section 15.6.2	RFC 2774 (HTTP Extension Framework)
Meaning	Server does not support the HTTP method used in the request	Server requires an HTTP extension declared in the request that was not provided
Caused by	Unsupported method (e.g., PATCH on a server that only handles GET/POST)	Missing required extension declaration in request headers
Client fix	Use a supported HTTP method	Add the required `Ext` headers declaring the extension
Cacheable	Yes	No
Common in practice	Yes — frequently seen on legacy APIs and form endpoints	Extremely rare — HTTP extensions are rarely used

501: The Server Does Not Know This Method

501 Not Implemented means the server does not support the HTTP method in the request. This is distinct from 405 Method Not Allowed (where the method is supported globally but not on this specific URL). 501 indicates the method is entirely unrecognized or unimplemented on the server.

In modern APIs, 501 appears when:

A client sends PATCH to a server that only handles GET and POST
A client sends a WebDAV method like PROPFIND to a non-WebDAV server
An API endpoint is planned but not yet built — some teams return 501 as a placeholder during development

PATCH /api/users/5 HTTP/1.1
Content-Type: application/merge-patch+json

{"name": "Updated Name"}

HTTP/1.1 501 Not Implemented
Content-Type: application/json

{"error": "not_implemented",
 "message": "PATCH method is not supported. Use PUT instead."}

501 is cacheable by default, which means if a server returns 501 for a given method, a cache can remember that and return the same 501 for subsequent requests with the same method to the same URL. This makes it appropriate for genuinely unimplemented methods.

510: A Required HTTP Extension Is Missing

510 Not Extended comes from RFC 2774, which defined the HTTP Extension Framework. This framework allowed clients to declare that a request uses optional or mandatory HTTP extensions in Opt and Man headers. If the server requires a mandatory extension that the request did not declare, it returns 510.

RFC 2774 was published in 2000 and is classified as Experimental. The HTTP Extension Framework never gained widespread adoption. 510 is nearly nonexistent in production systems. It appears in status code registries and HTTP client libraries as a matter of completeness.

# Hypothetical: server requires a mandatory extension
GET /api/secure-resource HTTP/1.1

HTTP/1.1 510 Not Extended

If you encounter a 510 in the wild, it is most likely a misconfigured server, a proxy with unusual behavior, or an application framework that returns it for an unrelated purpose.

Decision Rule

Use 501 when a server does not support the HTTP method in the request. Consider it the server-level “this method does not exist here” response. Do not implement or expect 510 in new applications — the HTTP Extension Framework is experimental and unused in modern deployments.

If you need to signal that a specific endpoint is not yet implemented, 501 is a reasonable placeholder, though some teams prefer 404 or a custom error until the endpoint is built.

FAQ

What is the difference between 501 and 405?

405 Method Not Allowed means the server supports the method generally but not on this URL. 501 Not Implemented means the server does not support the method at all. A REST API that handles GET and POST everywhere but receives a PATCH returns 501. An API that handles PUT on some URLs but not this one returns 405 with an Allow header listing valid methods.

Is 510 ever safe to return from an application?

Technically yes, but there is no practical reason to. It is associated with a framework (RFC 2774) that no modern application uses. If you want to signal missing functionality, use 501 for unsupported methods or 400/422 for unsupported request features.

Can 501 be used as a planned endpoint placeholder?

Yes. Some teams return 501 for endpoints that are in the roadmap but not yet implemented. This signals to developers that the endpoint will exist and gives them a more informative response than 404 (which implies the URL will never be valid). It is also more honest than 200 with a stub response.

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