501 Not Implemented
The server doesn't know how to handle the requested HTTP method. Unlike 405 (method not allowed for this resource), 501 means the server doesn't implement this method at all. It's essentially the opposite of 200 — instead of "I handled this," it's "I have no idea how to handle this."
What can I do?
- This is a server-side error — not your fault.
- Wait a few minutes and try refreshing the page.
- If the error persists, contact the site owner or check their status page.
How to debug & fix
- Return 501 for genuinely unimplemented HTTP methods
- Use 405 for known methods not allowed on a specific resource
- Add a deprecation notice or documentation link in the response body
- Use 405 for known-but-unsupported methods on a specific route; reserve 501 for methods the server has never heard of.
Code Example
app.all('*', (req, res, next) => {
const supported = ['GET','POST','PUT','PATCH','DELETE','OPTIONS','HEAD'];
if (!supported.includes(req.method)) {
return res.status(501).json({ error: 'Not Implemented' });
}
next();
});Related Status Codes
How HTTP Status Codes Work
Every HTTP response carries a three-digit status code that tells the client — browser, API consumer, or search-engine crawler — exactly what happened. The first digit defines the class: 1xx informational (request in progress), 2xx success, 3xx redirection, 4xx client error (bad request, missing auth, not found), and 5xx server failure.
Status codes are standardised in RFC 9110 (HTTP Semantics, 2022). Extensions like WebDAV (RFC 4918) and rate-limit headers (RFC 6585) added codes beyond the core set. When a client receives an unrecognised code, the rule is to treat it as the generic x00 of its class.
Why the Right Code Matters
Semantically correct codes help search engines index accurately (301 passes link equity; 410 removes pages faster than 404), allow API clients to implement correct retry logic (429 + Retry-After, 503 + Retry-After), and let monitoring systems distinguish bugs (500) from load issues (503) from auth failures (401/403).
Looking up a different status code? The full reference covers all HTTP codes with causes, fix guides, and copyable code examples for Node.js and Python.
Browse the full HTTP Status Code reference →