HTTP 510 NOT EXTENDED (OBSOLETED)

Obsoleted. The extension framework it belonged to was moved to historic status. 510 belongs to the 5xx server error class and is defined in RFC 2774.

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About HTTP 510

What Does HTTP 510 Not Extended (Obsoleted) Mean?

Obsoleted. The extension framework it belonged to was moved to historic status. The code is registered with IANA and defined in RFC 2774.

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 510 must treat it like 500.

When Do You See a 510 Response?

510 Not Extended came from the HTTP Extension Framework (RFC 2774), which let a server demand that a request declare certain mandatory extensions before it would be processed. The framework was never widely adopted and was moved to historic status, so IANA lists 510 as obsoleted. You are very unlikely to meet it except from a server deliberately implementing that old experiment or from custom software misusing the code.

How Do You Fix a 510?

Clients receiving a 510 should read the body, which was meant to describe the required extension, and adjust if possible; otherwise treat it as a generic server error. Servers should not send 510 in new software. Use a more appropriate standard code instead, such as 400 for missing request information or 426 when a protocol upgrade is required.

What Does a 510 Response Look Like?

HTTP/1.1 510 Not Extended
Content-Type: text/plain
(obsoleted: new servers should not send this)

510 is not cacheable by default: a browser or CDN stores it only when Cache-Control or Expires explicitly allows it.

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 510?

426 Upgrade Required: The client must switch protocols, named in the Upgrade header.
501 Not Implemented: The server does not support the method at all.
400 Bad Request: The request is malformed: bad syntax, invalid framing, or an unreadable body.

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

Is 510 Not Extended still used?

Practically no. The extension framework that defined it was moved to historic status, and IANA marks the code as obsoleted.

What should I use instead of 510?

Use a standard code that fits the problem, for example 400 for a missing required header or 426 when the client must switch protocols.

Is HTTP 510 an error?

Yes. 510 is a server error response: the server failed to handle a valid request, so retrying later may work.

Is a 510 response cacheable by default?

No. Caches store a 510 only when Cache-Control or Expires explicitly allows it.

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, or Cmd+Option+I on 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.

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