Understanding & Fixing HTTP Error 431 (Request Header Fields Too Large)

Have you ever clicked a link, waited for a website to load, and instead got a message that simply said something like “Error 431” or “Request Header Fields Too Large”? It’s one of those odd little errors that most of us don’t see very often, but when we do it can feel like a puzzle. I remember the first time it happened to me: I was developing a small web app, everything seemed fine in my local tests, but when I deployed it a user reported that they were unable to access one particular page and got a “431” error. I was baffled.

In this article, I’ll walk you through exactly what the HTTP error 431 means, why it happens, how you can fix it (whether you’re just visiting a site or you’re running one), and how you can avoid it in future. I’ll share some of my own experiences and opinions along the way, so you won’t feel like you’re reading a dry manual.

By the end of this I hope you’ll feel comfortable spotting the error, understanding the root cause, and knowing what steps to take to resolve it.

What Is HTTP Error 431?

Before we dive into the specifics, let’s take a quick step back and talk about HTTP status codes. You probably already know them: codes like 200 mean “OK”, 404 means “Not Found”, 500 means “Server Error”. They’re how a web server tells your browser what happened with the request you made.

The code 431 falls into the 4xx range, which generally means “client error” — something about the request is wrong or the server refuses to process it. According to the official documentation, 431 stands for “Request Header Fields Too Large.” What that means in plain language is: the server looked at your request, specifically at the header fields of the request (things like cookies, referer URL, other metadata) and concluded, “This is too big. I cannot process it.”

So when you see error 431, it means your browser (or your app) sent a request whose headers are too large (either one header field is too large, or all of them combined exceed what the server is willing to accept). It’s not a guarantee that the fault is entirely on your side (the user) — sometimes the web application or server code caused it. But it generally has to do with the request header size.

It’s worth noting how it differs from similar errors. For example, HTTP 414 means “URI Too Long” (you sent a very long URL) while 431 refers to headers too large. So if you have a URL that’s enormous, you might get 414. If your cookies or header fields are big, you might see 431.

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The Mechanics Behind 431

Let’s unpack some of the moving parts here, so you really understand what is happening.

What are HTTP request headers?

Whenever your browser (or app) sends a request to a web server, it doesn’t just say “give me page X”. It includes a bunch of metadata in the request headers: things like “Host”, “User-Agent” (info about the browser), “Cookie” (what cookies you have for that domain), “Referer” (which page you came from), “Accept-Language” (what language you prefer), and possibly a lot more custom headers if you’re working with APIs (for example, “X-Auth-Token”, “X-Requested-With”, etc.). All of these count toward the header size.

What does “too large” mean?

In the spec it’s explained that a server may reject a request with 431 when either a single header field is too large or the total size of all request headers is too large. Each server will have configured limits (implicitly or explicitly) on how big the headers it accepts can be. If the request exceeds that limit, the server decides not to process it.

What can make the header size balloon?

Here are common things that increase header size:

• Many or very large cookies. Cookies are part of the “Cookie” header field. If you accumulate many cookies (especially for the same domain) your Cookie header can become huge.

• Very long referer or origin URLs included in header fields.

• Custom headers added by your application or frameworks that send a lot of data in each header.

• Long query parameters or tracking parameters that end up in headers (sometimes indirectly).

• Reverse proxies, CDN or API gateways that append header fields automatically and thus enlarge the header set.

So when you get a 431 error, it’s usually because one of these has gone beyond the threshold.

Common Causes of HTTP Error 431

Let’s now look at how and why this error shows up in real life. I’ll split it by user perspective (just visiting a site) and developer/site-owner perspective.

From a user (visitor) side

If you’re simply browsing a website and you hit error 431, you might consider:

• You have too many cookies saved for that website. Maybe you visited many sub-domains, or the site put many tracking cookies, and the browser still sends them all.

• You’ve opened the site via a weird long URL (with lots of query parameters) or via a bookmark that includes a long referer string.

• Browser extensions or user scripts have added many custom headers.

• The site you’re visiting changed its policy recently and now rejects certain header volumes.

For example, the University of Minnesota documented that if you accumulate many site cookies under that domain, you could run into error 431 and clearing cookies solved it.

From the developer/website-owner side

If your users are seeing error 431 when using your app or site, you should consider:

• Your app is setting too many cookies or cookies that are too large (for example, storing JSON objects in cookies).

• You are sending custom headers that include heavy data.

• You are using frameworks or APIs that append headers or tracking data automatically.

• Your server/hosting environment has aggressive limits on header size and needs adjustment.

• You have a proxy/CDN (for example Cloudflare) that adds headers or forwards headers, and the forwarded set becomes too large. For example, Cloudflare users reported 431 when user tokens or many headers grew large.

A StackOverflow developer working on a MERN stack app found that cookies from the browser caused a 431 error whereas the same API request from Postman (which doesn’t send browser cookies) worked fine. That’s a very good signal: if it works without cookies but fails in a browser, likely the cookies are the culprit.

How to Identify & Troubleshoot 431 from a User Perspective

If you are simply visiting a website and you encounter error 431, here are the practical steps I recommend. I’ve used these myself many times.

1. Try incognito/private mode
   This disables your existing cookies and extensions (in many cases). If the site works in incognito but not in normal mode, then likely one of your cookies or extensions is the culprit.

2. Clear cookies for that specific site
   Rather than clearing all cookies (which logs you out everywhere) you can clear cookies just for the problematic domain. For example, in Chrome go to the padlock icon → Cookies and site data → Manage → Delete cookies for that site. Many sites still load fine after you do this. The University of Minnesota instructions follow this approach.

3. Check browser extensions
   Disable any extensions that might add headers (ad-blockers, analytics helpers, etc.). Then test again.

4. Try another browser/device
   If you still get 431 on a different device it may indicate a server issue rather than your browser. If it disappears, it’s likely something in your browser’s header footprint.

5. Contact the website’s support
   If you’re still seeing the error and you believe you’ve done everything you can, send details (browser version, what URL you hit, maybe screenshot) to the website’s help desk. They may have logs.

By following these steps you often find that cookies or a long referer are the root cause.

How to Fix HTTP Error 431 as a Website Owner or Developer

If you’re running the website or API and you notice users hitting 431, here are the steps you should take (in my opinion, based on what I’ve seen).

Step 1: Monitor and log header sizes

You should add logging (in dev environment) so that when a request is rejected you record the header size (or header fields) so you can see what is being sent. Many servers/ frameworks allow you to inspect header size or count. Without knowing what is going wrong, you’ll be guessing.

Step 2: Reduce cookie size and count

Cookies grow in two dimensions: number of cookies and size of each cookie. If your site sets many cookies (especially large ones that store objects) you should refactor so that:

• Store only essential data in cookies (such as session ID).

• Move larger data into server-side session storage or local storage (if appropriate).

• Use domain-scoped cookies properly (avoid storing separate cookies for each sub-domain if possible).

• Expire cookies promptly when they’re no longer needed.

Many times I found that a site I inherited had 10+ cookies for login, tracking, analytics, etc. just from one domain. Cleaning that up often solved the 431.

Step 3: Shorten URL query parameters and referers

If your site uses link parameters (such as UTM tracking codes, long referral strings) you might end up with a very long referer header when the user navigates from another site. Consider:

• Shortening query parameter length.

• Using link redirection (shortened link page) so the referer header isn’t absurdly long.

• Avoid passing huge payloads via URL when you can use POST or other approaches.

Step 4: Limit custom headers

If you have APIs or apps that send custom headers (for example auth tokens, tracking headers, locale, etc.) review them and see if you can minimize their size. Sometimes developers attach big JSON blobs in headers when they really could be in the body.

For example, someone working with the Azure OpenAI API found that additional headers caused 431. The fix was to stop sending unnecessary headers.

Step 5: Consider increasing server header size limit (with caution)

Some servers allow configuration of maximum header size. For example, one user changed a setting: <Set name="requestHeaderSize">32768000</Set> and solved their issue.

However this is a workaround rather than a solution — increasing the limit may hide the root cause, and very large headers may still degrade performance, security, and caching. Use this only if you’ve audited your header footprint and still need additional headroom.

Step 6: Test and verify

After making changes, test your site using multiple browsers, devices, networks. Use tools like Browser DevTools (Network tab) to inspect the request headers and confirm that they are reasonably sized.

In my experience, once I did the above cleanup, 431 errors dropped dramatically.

Prevention & Best Practices

Now that you (hopefully) understand how to fix it, let’s talk about how to avoid it going forward.

• Keep cookies lean. Every cookie adds bytes. Audit your cookie list regularly.

• Avoid sending large custom headers. Use header space for essential metadata only.

• Shorten tracking URLs. Especially for marketing links that may embed heavy data.

• Monitor header sizes. Logging header sizes as part of your analytic or error-reporting stack helps detect creeping growth.

• Educate your team. Developers often add headers blindly; make sure they know header footprint matters.

• Understand server limits. Know the header size threshold on your stack (e.g., Nginx, Apache, Node, etc.) so you can design within them.

• User experience & SEO focus. While 431 is not directly an SEO issue, a frustrated user who hits a 431 error may bounce immediately. A site that consistently delivers errors will degrade trust and performance.

My Personal Story

I want to take a moment to share a little personal anecdote because I think it illustrates how sneaky a 431 error can be.

A couple of years ago, I built a prototype web-app in React + Node. I was excited to launch it to a few friends. A user reported that one page wouldn’t load; it kept returning a generic error message. I checked everything: the backend worked in Postman, my logs seemed fine, I couldn’t reproduce it locally. Then I ran the app in incognito mode on their machine and for some reason it worked. I realised: the user’s browser had hundreds of cookies for that domain from past tests, tracking scripts, different user accounts, etc. The Cookie header alone was huge.

Once I cleared those cookies, the page loaded. Then I went through our code and removed old cookies we were setting, collapsed multiple tracking cookies into one, and added logic to expire stale cookies. I also made sure our API headers were minimal.

From that day on I learned: even in 2025 when we have fancy tech, browser header size still matters. Keeping the headers small prevents weird errors like 431.

So if you’re facing a 431, don’t think it’s some unsolvable bug—it’s often just a size-limit broken by growth of headers.

FAQ

Q1. What’s the difference between HTTP 431 and HTTP 414?
A1. HTTP 414 means “URI Too Long” (the URL itself is too long). HTTP 431 means “Request Header Fields Too Large” (the header fields in the request are too big). Sometimes a long URL will cause 414. But if the headers (cookies etc.) are the issue, you’ll get 431.

Q2. Does clearing cookies always fix the issue?
A2. Not always, but often when the cause is many/large cookies it will help. If the root cause is custom headers, server setting or proxy configuration, clearing cookies may not fix it.

Q3. Is HTTP 431 a client error or server error?
A3. It’s classified as a 4xx client error, meaning the request from the client was improper for some reason (too large headers). But sometimes the server configuration can contribute (for example low header size limit). So as a developer you need to check both sides.

Q4. Does error 431 impact SEO?
A4. Indirectly yes. If users keep hitting the error and bounce, site usage can drop. Search engines prefer sites with good performance and minimal errors. While 431 is less common than 404 or 500, avoiding any error improves user trust and site health.

Q5. Should I just raise the server’s allowed header size limit to avoid this?
A5. You can, but you should be cautious. Increasing the header size limit treats the symptom rather than the root cause. Large headers can slow things, increase memory footprint, introduce security risks. Better to clean the headers first and only increase limits if absolutely necessary.

Q6. How can I check header size in browser?
A6. Open Developer Tools → Network tab → select the request → look at “Headers” section. The total size isn’t always shown, but you can spot large Cookie headers or many custom headers. On the server side you might log req.headers and compute size.

Q7. What platforms are more prone to this error?
A7. Any site with many cookies, heavy tracking, lots of custom headers, or which uses proxy/CDN layers that add headers. For example, developers on MERN stacks, or sites using Cloudflare, or Shopify store owners have reported it.

Conclusion

So there you have it—HTTP error 431 explained in plain language. At its core it’s just the server saying: “Sorry, your request headers are too big for me to handle.” But the causes can vary, and the fix may be simple (clear cookies) or require more work (refactor header logic, adjust server config).

If you’re a visitor: try clearing cookies, testing incognito mode, switching browser. If you’re a site owner or developer: monitor your headers, clean up cookies and custom headers, and consider server limits. From personal experience I can say that once you pay attention to header size, you’ll avoid ever seeing that weird “431” message again.

If you run into one tomorrow, don’t panic—you’ve got this. Treat it like “there’s too much baggage in the request”, lighten the load, and move on.