HTTP 510 — Not Extended
HTTP 510 demands an extension: the request needs an HTTP extension the client has not declared, and the server will not proceed without it. The response may advertise what is missing.
When 510 shows up
- Experimental protocols layered on HTTP requiring declared extensions.
- Gateways enforcing mandatory extension headers.
Mocking a 510 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/experimental",
"status": 510,
"delay": 180,
"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 510 mean?
The request cannot be served until the client declares the required HTTP extension. The response body may describe what to add.
How can I test a 510 response without a real backend?
Define a mock rule with the method and path your client requests and set the status to 510. The example configuration above returns this response after the listed delay, so the client exercises its 510 handling exactly as it would against a live server.
Related status codes
Need the response in your own project? Spin up rules for any status with the ApiSimul mock server, or browse every status code page.