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HTTP/5xx (Server Error)

501 Not Implemented

The server does not support the functionality required to fulfill the request.

❓ Why does this happen?

Unsupported HTTP method (e.g. PATCH or PROPFIND) on a server or route that never implemented it.

✅ How to fix it

Use a supported method, or implement the handler (in Next.js, export the missing PATCH/PUT function from route.ts).

Return or handle 501 in your stack

// app/api/resource/route.ts
export function GET() {
  return Response.json(
    { error: "Not Implemented", status: 501 },
    {
      status: 501
    }
  );
}

Live 501 mock endpoint

Point your app, tests, or retry logic at this free public URL to reproduce a real 501 response (CORS enabled, optional ?delay=ms up to 5000).

https://castov.com/api/mock/501

Frequently Asked Questions

What does HTTP Status 501 mean?
The server does not support the functionality required to fulfill the request.
Why am I getting a 501 status code?
Unsupported HTTP method (e.g. PATCH or PROPFIND) on a server or route that never implemented it.
How do I fix a 501 Not Implemented error?
Use a supported method, or implement the handler (in Next.js, export the missing PATCH/PUT function from route.ts).
How can I test how my app handles a 501 response?
Send a request to https://castov.com/api/mock/501 — it returns a real 501 Not Implemented response with CORS enabled. Add ?delay=2000 to simulate a slow server.

Other 5xx status codes

500501502503504505507508511520521522523524525526

Browse the full HTTP status codes directory or troubleshoot terminal errors in the Error Fix Encyclopedia.