The 431 Request Header Fields Too Large error is an HTTP status code that shows up when a request made to a web server has headers that are too big for the server to handle. This might be due to overly large cookies, too many cookies, or excessive header information like long user-agent strings or authentication tokens.

If you’re building a website and run into this error, it usually means the browser sent more information than the server is willing (or able) to process. The good news? It’s fixable.

Why does the 431 error happen?

Usually, it boils down to the following.

• Too many cookies. Cookies store user data, and when they pile up, the total size of the request headers grows.
• Large or duplicate request headers. Sometimes, the instructions your browser sends along with the page request (the headers) can get unnecessarily long or even have duplicate information. This can happen due to certain website plugins or scripts.
• Long URLs or query strings. If a web address is super lengthy, especially with added tracking parameters, it can also contribute to large headers.
• Client-side issues. Occasionally, your browser itself might be storing a lot of data across different browsing sessions, leading to larger headers over time.

How the 431 error affects websites

When you encounter a 431 request header fields too large error, the webpage you're trying to see won't load. Instead, you'll likely get a standard error message. This can: 

• Disrupt user experience
• Impact conversions if key pages aren’t loading
• Affect SEO if bots receive 431 responses too often

How do you fix a 431 error 

Here are some things to try.

• Clear cookies. If you're testing your own site and keep seeing this, try clearing your browser's cookies just for that website. It's often the quickest fix.
• Limit cookie use on your site. Take a look at any plugins or tools on your site that set cookies. Do you really need all of them?
• Avoid bloated request headers. Use your browser's developer tools (like in Chrome) to see what headers your site is sending. Are there any that look unusually long or repeated?
• Set sensible limits on the server side. If you manage your own server, you might be able to tell it to accept slightly larger headers. But be careful – making them too big can open up security risks.
• Redirect or reject oversized requests gracefully. Consider creating a custom error page for this situation that gives users some helpful advice.

FAQs about the 431 request header fields too large error

What causes a 431 error most often?

Usually, it’s because of too many or overly large cookies, especially for people who are logged in or have visited the site before.

Is the 431 error client-side or server-side?

It's a server-side error triggered by something your browser (the client) sent. The server is saying: “I can’t handle this much header data.”

How can I check for large headers in my requests?

Use your browser's developer tools (usually by pressing F12), go to the "Network" tab, and look at the "Headers" for any request.

Will this error impact all visitors to my website?

Not necessarily. It often affects specific requests, especially from returning users with lots of cookies. New visitors might not experience it at all.

Build a reliable, professional website with B12

Whether you're just starting out or managing a growing business online, understanding errors like "431 Request Header Fields Too Large" can help you avoid unexpected issues and keep your site running smoothly. With B12’s AI-powered website builder, you don’t have to worry about technical errors or misconfiguration. Your site is optimized for performance, reliability, and user experience from day one. Launch your website today!