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HTTP 510 Not Extended

Updated Free mock REST & WebSocket APIs

HTTP 510 Not Extended — The request needs an HTTP extension the client didn't declare (obsolete HTTP Extension Framework).

Class
5xx Server Error
Defined in
RFC 2774 (historic)
Official
Registered, but deprecated/reserved
Cacheable by default
No (unless Cache-Control/Expires say so)
Safe to retry
Sometimes — idempotent requests only

Try it: get a real 510 response

Our test endpoint answers every request with 510 Not Extended.

curl -i https://mockfox.dev/api/status/510

Use it in tests, health-check monitors or to see how your client handles a 510. Add ?delay=2000 to slow it down.

What does HTTP 510 mean?

From the experimental HTTP Extension Framework, which was never adopted; RFC 2774 was marked Historic in 2022 and the code is obsolete. You can ignore it.

How to fix 510

  1. Visitors: This is a server-side problem — wait a moment and retry, or check the site's status page.
  2. Developers: Check application and proxy logs at the time of the error, verify upstream services are healthy, and alert on 5xx rates.

Example 510 response

HTTP/1.1 510 Not Extended
Content-Type: application/json

{"status":510,"error":"Not Extended"}

5xx Server Error: The server failed to fulfill a valid request — usually temporary, or a server-side bug. See the full list of HTTP status codes.

Related status codes

Related tools

Frequently asked questions

What does HTTP 510 Not Extended mean?

The request needs an HTTP extension the client didn't declare (obsolete HTTP Extension Framework).

Is 510 a client or a server error?

510 is a 5xx code: the server failed to handle a request that may be perfectly valid. It's often temporary.

Where is 510 defined?

In RFC 2774 (historic). It's listed in the IANA HTTP Status Code Registry, but marked deprecated or reserved.

How can I test how my app handles a 510?

Call https://mockfox.dev/api/status/510 — it responds with 510 to any method, with CORS enabled. Add ?delay=2000 to simulate a slow response.