431 Request Header Fields Too Large: what it means and what to do
A 431 means the metadata attached to your request outgrew what the server accepts. Defined in RFC 6585, it states that the server is unwilling to process a request because its header fields are too large, either a single field or the whole set. In everyday browsing the culprit is almost always cookies: months of accumulated cookies for a domain can grow past a server or proxy limit, and every request to that site starts failing at once.
If you are just visiting a site
- Clear cookies for that site. This fixes the overwhelming majority of visitor-side 431s. Your browser has been sending an oversized pile of cookies with every request, and clearing them resets the pile to zero.
- Try a private window. If the page loads there, stored cookies or an extension in your normal window is confirmed as the cause.
- Disable extensions and retry. Some extensions inject their own headers or tracking cookies into requests. A 431 that follows you across sites often traces to one of these.
- Shorten absurdly long links. Tracking-heavy URLs with enormous parameter strings occasionally trip header limits at strict proxies. Opening the site's homepage and navigating manually sidesteps it.
If it is your site or your API
- Find which layer rejected it. Load balancers, CDNs, and app servers all cap header size, and the strictest one wins. The rejection usually appears in that layer's log, not the application's.
- Raise the limit deliberately if your app needs it. Token-heavy architectures, such as large JWTs in headers or many cookies, legitimately need more room. Raise the specific layer's cap rather than disabling checks.
- Audit your cookies. If your site sets dozens of cookies or stores large values in them, your users will keep hitting 431 at gateways you do not control. Move bulk data server-side.
- Watch for redirect loops of doom. A classic failure is a site that sets a cookie on every visit while its gateway rejects large cookie headers, so users break their own access just by browsing.
Visitors: clear the site's cookies and the 431 almost always disappears. Owners: if many users hit it, your cookie habits, not their browsers, are the root cause.
Common questions
Why did a site suddenly start giving me 431 on every page? Your cookies for that domain grew past a size limit. Because cookies are attached to every request automatically, the failure is total and sudden: the site worked until the request that crossed the line, and then nothing on the domain loads until the cookies are cleared.
What is the difference between 431 and 413? A 413 is about the size of the request body, the content being uploaded. A 431 is about the size of the headers, the metadata around the request, most often cookies. Different part of the request, different fix.
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