HTTP 501 — Not Implemented

Server error responses (5xx) · Updated 2026-10-18

HTTP 501 says the server does not have the capability the request needs — an HTTP method it has never heard of, or an extension it does not support. Unlike 500, it is a statement about features, not health.

When 501 shows up

Exotic methods (PROPFIND, TRACE) hitting a server that skips them.
Feature-gated endpoints stubbed out on a deployment tier.

Mocking a 501 response

A response with this status can be faked end to end with a local mock rule — no backend changes needed. The configuration below mirrors the rule fields the ApiSimul editor uses (method, path, status, delay in milliseconds, response body):

{
  "method": "GET",
  "path": "/api/search",
  "status": 501,
  "delay": 200,
  "enabled": true,
  "body": null
}

Dynamic placeholders such as {{random.name}}, {{string.uuid}} or {{number.int 1 100}} can appear inside the body to vary each response, which keeps client-side list rendering and retry logic honest during development.

Frequently asked questions

What does HTTP 501 mean?
The server does not implement the functionality required to fulfil the request. Retrying cannot help; the server software must change.

How can I test a 501 response without a real backend?
Define a mock rule with the method and path your client requests and set the status to 501. The example configuration above returns this response after the listed delay, so the client exercises its 501 handling exactly as it would against a live server.

Related status codes

500 Internal Server Error 502 Bad Gateway 503 Service Unavailable 504 Gateway Timeout 505 HTTP Version Not Supported 506 Variant Also Negotiates 507 Insufficient Storage 508 Loop Detected 510 Not Extended 511 Network Authentication Required

Need the response in your own project? Spin up rules for any status with the ApiSimul mock server, or browse every status code page.