What is the difference between 501 and 405?
501 means the server does not support the method at all. 405 means it supports the method in general, but not on this particular resource.
HTTP 501 NOT IMPLEMENTED
The server does not support the method at all. 501 belongs to the 5xx server error class and is defined in RFC 9110.
Loading the workbench.
About HTTP 501
What Does HTTP 501 Not Implemented Mean?
The server does not support the method at all. The code is registered with IANA and defined in RFC 9110.
Server error: the server failed to fulfill a valid request. Every code from 500 to 599 shares that general meaning, so a client that does not recognise 501 must treat it like 500.
When Do You See a 501 Response?
501 Not Implemented means the server does not support the functionality required to fulfil the request, typically because it does not recognize the method at all. A server that only knows GET and HEAD answers 501 to a PROPFIND or an unknown custom method. It differs from 405, where the server knows the method but this particular resource does not allow it. Some APIs also use 501 for planned features that are not built yet.
How Do You Fix a 501?
Clients should not retry the same request; it will fail the same way. Use a method the server supports, or a different endpoint. Servers should reserve 501 for methods they do not implement anywhere and return 405 with an Allow header when a known method is not allowed on a specific resource. A 501 is cacheable by default, so add Cache-Control if it may change.
What Does a 501 Response Look Like?
HTTP/1.1 501 Not Implemented
Content-Type: text/plain
Content-Length: 0501 is cacheable by default under RFC 9110: a browser or CDN may store it and reuse it even without explicit freshness headers, using a heuristic lifetime. Send Cache-Control to say exactly how long.
To inspect a real one without sending it anywhere:
- Copy the response headers from your browser Network tab, or from
curl -i. - Paste them into the Header decoder to see every header explained.
- Check caching in the Cache lane.
Which Status Codes Are Related to 501?
- 405 Method Not Allowed: The URL exists but not for this method. Must send an Allow header.
- 500 Internal Server Error: A generic server failure, usually an unhandled exception.
- 505 HTTP Version Not Supported: The server refuses this major HTTP version.
Browse every code on the HTTP status codes page.
What Other Lanes Are in the Reference Section?
- HTTP status codes: Every IANA-registered status code, with a page per code.
- MIME types: File extensions to Content-Type, searchable both ways, plus a Content-Type parser.
- HTTP methods: GET, POST, PUT, PATCH, DELETE and the rest: safe, idempotent, cacheable.
Elsewhere in 2http: Header decoder, Cookie decoder, Cache-Control decoder, CORS checker, User-agent parser, and all eleven lanes on the 2http home page. Part of the 2suite.io privacy-first ecosystem.
Frequently Asked Questions
Can I retry a request that got 501?
Retrying unchanged will not help. The server lacks the feature, so change the method or endpoint, or wait until the service implements it.
Is HTTP 501 an error?
Yes. 501 is a server error response: the server failed to handle a valid request, so retrying later may work.
Is a 501 response cacheable by default?
Yes. RFC 9110 lists 501 as heuristically cacheable, so caches may store it without explicit headers. Use Cache-Control to control how long.
How Can You Verify That Nothing Leaves This Browser?
You do not have to take our word that your headers stay here. Developer tools show every request a page makes, so you can watch this page work and confirm that nothing is sent. The check takes under a minute.
- Open your browser developer tools with
F12, orCmd+Option+Ion a Mac, and select the Network tab. - Reload the page and let it finish. You will see the page's own files, its web fonts, and one anonymous page-view request. Nothing has been typed yet, so none of them can contain your headers.
- Click the clear icon in the Network tab so that the log is empty.
- Paste real headers into any lane and read the result.
- Look at the Network tab again. It is still empty. For extra proof, turn on offline mode after the page loads and every lane keeps working.
Only two things can add a line to that log, and neither carries your headers. Submitting the feedback form sends the vote and comment you typed. Typing characters in a script the page has not displayed yet, such as Cyrillic or Greek, can make the browser fetch the matching font file, and that request names only the font.
2http never sends a request on your behalf: the request builder writes code for you to run, and the meta inspector reads source you paste instead of fetching a URL. The production site also sends a Content-Security-Policy header that tells your browser to refuse connections to anywhere except this site and the anonymous analytics endpoint.