HTTP 501 Not Implemented

HTTP 501 Not Implemented — The server doesn't support the functionality (usually the HTTP method) needed to fulfill the request.

Class

5xx Server Error

Defined in

RFC 9110 §15.6.2

Official

Yes (IANA registry)

Cacheable by default

Yes (heuristically, per RFC 9110)

Safe to retry

Sometimes — idempotent requests only

What does HTTP 501 mean?

501 Not Implemented means the server doesn't support the functionality required to fulfil the request — most often, it doesn't recognise the request method at all and can't support it for any resource.

It's a server-side status (5xx) because the limitation is in the server, and it's cacheable by default. Compare 405, where the method is known but not allowed for that particular URL.

When is 501 used?

• A server receives an unknown or custom method (e.g. PURGE, PROPFIND on a non-WebDAV server).
• An API stub or planned endpoint that isn't built yet.
• A proxy that doesn't support a feature such as a particular Transfer-Encoding.

Common causes of a 501

• The client uses a method the server software doesn't implement.
• The endpoint is a placeholder in a beta API.
• Unsupported Transfer-Encoding values in a request.

How to fix 501 Not Implemented

If you're visiting the site

1. There's nothing you can fix; the site doesn't support that action. Try again later or contact the site owner.

If you run the site or API (developers)

1. Use 405 (with an Allow header) if the method exists on your server but not on that route.
2. Return 501 for deliberately unimplemented features and document when they'll be available.
3. If a CDN purge (PURGE method) returns 501, the origin server — not the CDN — received the request.

Returning or handling 501: server examples

app.all("/api/v2/reports", (req, res) =>
  res.status(501).json({ error: "not_implemented", message: "Reports arrive in v2.1" }));

// app/api/example/route.ts (Next.js App Router)
export function GET() {
  return Response.json({ error: "not_implemented" }, { status: 501 });
}

# nginx answers unknown methods itself; e.g. FOO / HTTP/1.1 → 405 for static files,
# while unparseable methods get 400. Let the app decide for proxied routes:
location /api/ { proxy_pass http://app; }

Example 501 response

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

{"error":"not_implemented"}

Related status codes

• 405 Method Not AllowedThe URL exists but doesn't support the HTTP method used; the Allow header lists the ones it does.
• 500 Internal Server ErrorThe server hit an unexpected condition that prevented it from fulfilling the request.
• 505 HTTP Version Not SupportedThe server doesn't support the HTTP protocol version used in the request.

Related tools

• HTTP status codes — full list
• HTTP headers reference
• cURL converter
• Mock API — fake REST endpoints with custom status codes
• Webhook tester — inspect incoming requests