Why does clearing cookies fix a 431 error?
The browser sends every cookie for the site in one Cookie header. When there are too many or they are too large, the header exceeds the server limit. Clearing them shrinks it.
HTTP 431 REQUEST HEADER FIELDS TOO LARGE
One header, or all of them together, are too big. Often oversized cookies. 431 belongs to the 4xx client error class and is defined in RFC 6585.
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About HTTP 431
What Does HTTP 431 Request Header Fields Too Large Mean?
One header, or all of them together, are too big. Often oversized cookies. The code is registered with IANA and defined in RFC 6585.
Client error: the request has a problem, so it cannot be fulfilled as sent. Every code from 400 to 499 shares that general meaning, so a client that does not recognise 431 must treat it like 400.
When Do You See a 431 Response?
431 Request Header Fields Too Large means one header, or all of them together, exceed the server's limit. By far the most common cause is cookies: a site sets many or very large cookies, often from analytics, A/B testing, or a session stored entirely in a cookie, until the Cookie header passes about 8 KB. Long Referer values and huge Authorization tokens can trigger it too.
How Do You Fix a 431?
As a user, clearing cookies for the site usually fixes it immediately. As a developer, inspect the request headers in DevTools or paste them into the 2http Header decoder to find the offender, then shrink or remove cookies and move large session data server-side. Servers can name the oversized field in the body. Raising header buffer limits is a short-term patch, not a cure.
What Does a 431 Response Look Like?
HTTP/1.1 431 Request Header Fields Too Large
Content-Type: text/html
Connection: close431 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 431?
- 400 Bad Request: The request is malformed: bad syntax, invalid framing, or an unreadable body.
- 413 Content Too Large: The body is bigger than the server accepts. Formerly "Payload Too Large".
- 414 URI Too Long: The URL is longer than the server will read.
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
Some servers return 400 instead of 431, why?
Older servers and some proxies predate RFC 6585 or simply use a generic 400 for oversized headers. The cause and fix are the same.
Is HTTP 431 an error?
Yes. 431 is a client error response: the problem is in the request, so repeating it unchanged will usually fail again.
Is a 431 response cacheable by default?
No. Caches store a 431 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, 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.