HTTP Error 431 (Request Header Fields Too Large) — Causes, Fixes & Best Practices

Picture this: you’re working on your website or browsing a page and suddenly you receive a message in your browser like: “HTTP Error 431 – Request Header Fields Too Large”. You think “What on earth does that mean?” You reload, maybe clear cookies, and it still happens. This kind of error is less common than the dreaded 404 or 500, but when it happens it can be puzzling.

Why should you care? Because an unexpected error like this interrupts the user’s flow, affects your website’s reliability, might confuse visitors, and from a developer’s side it signals something sub‐optimal with how requests are being made or handled. Fixing it helps your site run smoother, improves user experience, and avoids hidden issues.

In this article I’ll walk you through exactly what HTTP Error 431 means, why it happens, real situations I’ve seen where it pops up, how you can diagnose and fix it (both as a user and as a developer), and what you should do to keep it from happening again. If you’re managing a website or working with web apps or APIs, you’ll find practical value here.

2. What is HTTP Error 431?

In simple terms: HTTP Error 431 means that the web server is refusing to process a request because the request’s header fields are too large—either a single header field is oversized, or the combined size of header fields has become too big for what the server will accept.

If you’re familiar with the HTTP status code system, this belongs to the 4xx class: client error responses. In particular, 431 is defined in the standard as “Request Header Fields Too Large”. MDN Web Docs+1

Here’s another way to think about it: whenever your browser (or client) asks the server for something (for example a web page, an API call, an image) it sends a request with a set of “headers”. These headers carry metadata: things like cookies, authentication, accepted languages, referrer URLs, custom headers your app might add, etc. If those headers become too big—too many cookies combined, too long URLs in the referrer or query string, too many custom headers—the server might decide “this is too much” and return error 431 rather than keep trying.

According to the reference documentation:

The server refuses to process the request because the request’s HTTP header fields are too long. The request may be resubmitted after reducing the size of the request headers. MDN Web Docs

In short: 431 = “Your headers are too big — I won’t process your request.”

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3. How does it happen? (Causes)

Now we get to the more interesting part: what triggers it? I want to walk you through the common causes I’ve encountered, plus a few less obvious ones, so you can both diagnose and avoid them.

a) Oversized or too many cookies

One of the most frequent causes is simply: you’ve got too many cookies stored in the browser, or you’re sending cookies that contain a lot of data, so the “Cookie” header becomes large. Since cookies are included in every request to same‐domain, they add up quickly. If your site uses multiple tracking cookies, or you’re loading third‐party scripts setting many cookies, etc., the aggregate size of the Cookie header might push the request over the server’s limit.

For example: On browsers you might attempt to access a site and see the 431 error. You clear cookies for that domain and suddenly it works. That tells you the “Cookie header size” was likely too big.

b) Very long URL query strings or referrer headers

Another cause: You’re sending request headers that include a very long “Referer” URL, or query parameters in the URL that are long and hence include many characters in headers. Some servers consider the Referer header (or maybe “Origin” or others) as part of the header size budget. If the URL is wildly long (tracking parameters, encoded JSON in query string, etc.), the header size might exceed limits.

For example: A web app may append lots of tracking variables (UTM params, custom session data) and you click a deep‐linked URL. The browser sends a Referer header with the long URL plus the query string; the server rejects it with 431 saying “header fields too large”.

c) Excessive custom or redundant headers

Sometimes as a developer you might add many custom headers to your request — perhaps for tracing, analytics, or security tokens — and these headers add up. Additionally, if code duplicates headers or sends many middleware headers, you might run into this. On the server side, the configuration might have a maximum header size configured, and if exceeded you’ll get 431. For example, one user on the Microsoft Q&A forum noted that their error was due to “some additional headers being sent by the code SDK/POSTMAN which led to surpassing the maximum number of allowed headers.”

d) Server configuration limits

Even if your client’s headers are reasonable, the server might have strict limits on header size (either total or per‐field). If your request header size exceeds that configured limit, the server will respond with 431 (or just drop the connection). Some server technologies or proxies may assume default limits and reject large headers. For example, one mailing‐list discussion showed a resolution by editing server-config.xml to set requestHeaderSize to a larger value.

e) Accumulated header size through redirects, proxies, etc.

In more complex architectures you may hit 431 when a request goes through multiple proxies or layers, each adding headers (or cookies) and the eventual header size becomes large. Also if you’ve done lots of browsing on a site and it sets incremental cookies over time, the cumulative effect can push you over the limit. One community thread described exactly this: many cookies scoped to a domain eventually triggered the 431 error.

4. Real‐World Situations & Case Studies

To bring this closer to reality, I’ll share a few examples (from what I’ve seen or found) that illustrate how 431 shows up and how it was resolved. These help translate theory into practice.

Example 1: Browser access blocked by cookies
A user tried to visit a university site and got “HTTP Error 431 – Request Header Fields Too Large”. The fix: clear cookies for that domain (they had accumulated many over time). After clearing them, the site loaded normally. This is a non‐technical fix you can try if you’re just a user facing the issue.

Example 2: MERN stack app – Node.js API from React client
On a StackOverflow post, a developer reported:

“Failed to load resource: the server responded with a status of 431 (Request Header Fields Too Large)”
They were using a React front‐end and Node/Express back‐end. The API worked from Postman but from the browser it failed. The culprit turned out to be that the browser sent many cookies (because three projects used the same local port) and the server rejected the request. Stack Overflow

They solved it by cleaning up cookies and ensuring the header size was within acceptable limits. The learning: 431 may sneak in when your dev environment has many cookie contexts sharing a domain/port.

Example 3: Azure OpenAI Service – additional headers issue
In a Microsoft Q&A thread, someone was using the Azure OpenAI Service and got 431 errors when sending a request with many headers. They discovered that the SDK/POSTMAN was sending unnecessary headers which pushed them past the limit. They removed the extra headers and the issue resolved. This shows that even advanced APIs/services aren’t immune.

Example 4: Cloudflare Community – large user token
A Cloudflare community post noted that some clients got 431 errors due to very large user tokens (likely sent in headers) and these were rejected by the server.

Each of these examples highlights the diverse contexts where 431 can surface: simple browsing, single‐page apps, enterprise APIs, proxy services. Recognising the pattern helps with diagnosis.

5. How to Troubleshoot & Fix HTTP 431

Here’s a practical step‐by‐step guide to resolving HTTP Error 431, whether you’re a user or a developer responsible for a website/app. I’ll break it into client (browser) side and server/developer side.

Client (browser/user) side fixes

1. Clear cookies for the domain/site
   If the error occurs when accessing a particular site, clearing cookies for that site often resolves the issue. Many times the Cookie header has grown too large. For example: go to browser settings → site data → delete cookies for the site.

2. Try in incognito/private mode
   If the site works in an incognito window, then the problem is likely due to stored cookies/data or extensions interfering. Example: a Qlik user resolved the error by using incognito.

3. Try another browser or device
   If the error persists across browsers, maybe the issue is server‐side. If it only appears in one browser, focus on cookies/extensions there.

4. Shorten the URL if possible
   If you’re navigating via a very long URL (with many query parameters/tracking codes), try accessing a simpler URL (remove query parameters) and see if that helps. This may reduce the “Referer” header size.

5. Disable unnecessary browser extensions
   Some extensions add headers or cookies which might contribute to header bloat. Try disabling them temporarily to test.

Server / Developer side fixes

1. Log header sizes and which header field is oversized
   Configure your server (or use middleware) to log the size of request headers, and log instances where they exceed thresholds. This helps identify the culprit header (Cookie, Referer, Custom header).

2. Clean up cookie policy
   If your site sets many cookies (tracking, analytics, preferences), audit them. Remove redundant cookies, reduce size of cookie values, or shift some data to other storage (e.g., localStorage) if appropriate.

3. Limit URL query parameter length
   If you use long tracking URLs or embed big payloads in query strings, rethink this. Shorter, cleaner URLs reduce the “Referer” or “Origin” header size.

4. Review custom and middleware headers
   Check if your app is adding many headers (for debugging, tracing, feature flags). Remove unnecessary ones.

5. Adjust server configuration for header size limits
   If you control the server or proxy, you can raise the allowed header size threshold. Example: in one discussion, changing requestHeaderSize in server config resolved the issue. However, increasing the limit is not a substitute for reducing actual header size – large headers could signal inefficiencies or security risks (e.g., big cookies = memory/cpu cost).

6. Use compressed or simpler headers
   Where possible use simpler header values, reduce redundancy, avoid sending large tokens in headers unless necessary.

7. Monitor and alert
   Add monitoring on your endpoints for 431 errors. If you start seeing them spike, you know header bloat is growing.

Example Fix Flow (for a website owner)

• You notice some users report “HTTP Error 431” when logging in.

• You check server logs and see many 431 responses.

• You inspect request headers using DevTools and notice the “Cookie” header is huge (many small tracking cookies).

• You clean up the cookie logic: remove old cookies, reduce size of values, consolidate where possible.

• You also notice your login URL has many tracking query params; you refactor to shorter URLs or move tracking to POST body.

• You keep server limits same, but header size drops and 431 errors vanish.

Safety note

While increasing server header size limit may seem attractive, be cautious: large headers can be abused (denial-of‐service risk) and often indicate inefficiencies. Fixing the root cause is better than just raising limits.

6. Best Practices & Prevention

To avoid running into HTTP Error 431 in future, here are some best practices I’ve adopted and would recommend.

• Keep cookie sizes small: Set only essential cookies; avoid encoding big JSON objects in cookie values; consider using localStorage/sessionStorage for large client‐data instead of cookies.

• Limit number of cookies: Each cookie adds overhead. If you have many third‐party scripts setting cookies, audit them.

• Avoid huge query parameters: For tracking or analytics, if you’re using many UTM or custom parameters, compress the information or use server side storage instead of embedding huge payloads in the URL.

• Avoid sending large tokens/headers: If you’re sending authentication tokens or custom headers, ensure they are as compact as possible.

• Configure your server/stack thoughtfully: Set realistic limits for header size, log header size usage, monitor errors like 431 or 414.

• Use clean URL design: Use friendly, shorter URLs rather than long encoded strings; reduce “Referer” header size when possible (especially when linking between pages).

• Educate your team: Developers, QA, DevOps should be aware of header size implications. Sometimes header bloat happens gradually (e.g., adding more tracking cookies, adding more custom headers) and nobody notices until the error appears.

• Monitor errors and user complaints: If you see any spike in 431 errors, act quickly. It’s easier to fix when the problem is fresh than when many users are affected.

• Test on multiple browsers/devices: Sometimes certain browsers handle cookies differently or impose additional limits. Confirm compatibility.

By following these practices you reduce the risk of anyone hitting a mysterious “Request Header Fields Too Large” error and make your site/app more robust.

7. Related errors & distinctions

It’s useful to know how HTTP Error 431 differs from similar status codes or related header issues so you don’t misdiagnose the problem.

• HTTP 414 (URI Too Long): This error happens when the request URI (path + query) is too long for the server. While 431 deals with header size (including cookies, headers, referrer), 414 is specifically about the URL being too long. They are distinct but sometimes appear together (e.g., extremely long URL may contribute to large headers).

• Other 4xx header errors: Some servers may reject requests because of invalid headers, too many headers, invalid characters, etc. 431 is specific to size/length of header fields, not necessarily invalid values.

• 5xx errors: Unlike 431 which is in the 4xx “client error” class (meaning the client sent something the server won’t accept), a 5xx is a server‐side error. So if you see 431, you should look at what the client is sending.

• Dropped connections (no status code): Some servers may not send 431 but simply drop the connection. So logging might show “client closed” or “connection reset” rather than explicit 431. In that case, you still suspect header size issues.

Understanding these distinctions helps you pick the right path: Is it the URL? The headers? The server? The client? This speeds up troubleshooting.

8. My personal take / lessons learned

I’ve encountered HTTP Error 431 a few times, especially while working on web apps with many third‐party integrations. One particular project had analytics scripts, multiple cookies, tracking parameters and custom headers for A/B testing. We began noticing occasional “Request Header Fields Too Large” errors in the logs. At first it felt weird — “how can a header be too large? We’re not uploading huge files!”

But the debugging process taught me a few valuable things:

• Even small increments matter: A few extra cookies, a few more tracking params, a slightly longer token—all of these added up.

• The client side (browser) is often overlooked: It’s easy to clear cookies locally and think “problem solved”, but unless the underlying cookie logic is fixed, the issue will recur for other users.

• Monitoring header sizes proactively is worth the effort: We started logging header sizes and built alerts for when totals exceeded certain thresholds. That gave early warning signs.

• Fixing the root cause is more sustainable than bumping server limits: At first we increased server max header size to avoid the errors, but later realised we were masking deeper inefficiencies (excess cookies, redundant headers). Cleaning them improved performance overall.

• User experience matters: Many users might simply reload and leave if they hit a 431 error. For them it’s just “the site failed”. For us it’s a signal something is off. I began treating 431 errors as “soft alarms” for header hygiene.

In summary, encountering 431 changed how I think about request headers. They are not unlimited resources. They matter.

9. Conclusion

HTTP Error 431 (“Request Header Fields Too Large”) may not be the most common error you’ll see, but it’s important. It signals that the request from the client exceeded what the server is willing to accept—either because of oversized headers, too many cookies, long URLs, or other header‐bloat issues.

If you face this error, start from the simpler fixes (clear cookies, try incognito, test a simpler URL) and then move into developer/server territory (audit cookies, limit tracking parameters, check server header‐size settings, log header sizes). If you manage a website or web app, applying best practices around cookie size, header size, URL length and server limits will help you avoid this issue altogether.

Think of headers like the “backpack” of every request: if you stuff too much into it, the server might say “Sorry, I can’t carry this.” Keeping your backpack light helps everyone—your users, your site performance, and your peace of mind.

10. FAQ

Q: Why am I seeing HTTP Error 431?
A: Because the request your browser (or client) sent had header fields that were too large—either one header was too big, or the sum of all headers exceeded the server’s allowed size.
Q: Will clearing cookies always fix it?
A: Often yes when the cookie header is the main culprit. But if the issue is long URL query strings or many custom headers, clearing cookies alone may not be enough.
Q: Can my server just allow larger headers (increase max header size) and be done with it?
A: Technically yes, but doing so without addressing root causes may lead to hidden inefficiencies, security risk, or performance problems. It’s better to reduce header size first.
Q: How can I tell which header is too large?
A: You can inspect the request using browser DevTools (Network tab) to see header sizes. On the server side you can log request header size, or check server logs for which header fields exceed limits. Some servers or proxies will indicate which header triggered the rejection.
Q: Will HTTP Error 431 affect SEO or Google crawling?
A: Indirectly yes—if search engine crawlers encounter 431 errors on your site they may not index pages correctly. But 431 is usually user/client side rather than something systematic across your site. Still, monitoring for it is wise.