How To Fix “Request Header Or Cookie Too Large” (400 Bad Request)

Request Header or Cookie Too Large error happens when a website receives more cookie or header data than a Web Hosting server can accept.  

This usually happens when: 

• Browser cookies have grown too big. 
• Too many cookies are stored for a site. 
• Browser extensions are modifying request headers. 

In most cases, the fastest fix for 400 Bad Request is to clear your browser cookies for that website. If that doesn’t work, we have included further suggestions in this guide to provide several possible causes and solutions for the 400 Bad Request error. 

KEY TAKEAWAYS 

• A 400 Bad Request error occurs when a browser sends too much cookie or header data in a request, causing the server to reject it. 
• In most cases, the fastest 400 Bad Request error fix is to clear the cookies for the affected site. 
• First start with cookie-related fixes, since oversized cookies are the major cause of this error, then try other fixes. 
• The request header too large error is preventable with some routine cookie and browser maintenance.  

What Does Request Header or Cookie Too Large Mean? 

When you visit a website, your browser sends a request to the server that hosts it. That request includes HTTP headers and cookies. 

HTTP headers are pieces of data that accompany every request and response between a browser and a web server. When your browser requests a page, it sends headers that describe the request and provide context.  

The data in headers usually includes: 

• Browser type and version. 

• Accepted content formats. 

• Authentication tokens. 

• Stored cookies for the site. 

• Referrer (the page you came from) information. 

This data helps the server understand how to process the request and what content to return. 

Cookies are small files that sites save in browsers to remember information such as login sessions, preferences, or shopping carts. Every time you visit, your browser sends those cookies back to the server in the request header, and over time, they can accumulate or expand.  

Servers have a limit in place on the amount of header data they will accept. When your browser sends a request that exceeds that limit, the server rejects it and displays a 400 Bad Request: Request Header or Cookie Too Large error. These limits are put in place for two main reasons: 

1. Security: Extremely large headers can be used in DoS (Denial of Service) attacks that overwhelm a server, making it inaccessible to legitimate traffic. 

2. Performance & Stability: Large headers consume more RAM and CPU resources. If a server accepted them, a single request could potentially slow down or crash a site. 

The “Request Header or Cookie Too Large” error is most common on sites you visit regularly, particularly WordPress, membership websites, and online stores. 

It usually happens when: 

• Too many cookies are stored for a site. 

• Individual cookies are very large 

• Browser extensions modify request headers. 

• Long or malformed URLs and parameters create oversized headers. 

We will cover those in more detail later.  

Quick Definition: A 400 Bad Request error means a web server received and rejected a request because the browser sent more cookie or header data than the server can process in a single request. 

Fastest Fixes for Request Header or Cookie Too Large 

The fastest Request Header Too Large error fix is relatively simple: the first in the list usually resolves; if not, when you read through the rest of the list (in order), virtually every other known cause is covered. 

1. Clear browser cookies for the website in question. 

2. Open the site in incognito/private mode to test it without the stored cookies 

3. Turn off browser extensions (ad blockers, VPNs, privacy tools) one by one. 

4. Flush your DNS cache to clear old or corrupted domain data. 

5. Check the URL for formatting or encoding issues and re-enter it. 

6. If the error appears during an upload, check and reduce the file size. 

In the following sections, we discuss each of these in more detail. 

How to Fix Request Header or Cookie Too Large

Clear Browser Cookies (Most Common Fix) 

As we’ve discussed, cookies are the most common cause of this error. Over time, sites store multiple cookies in your browser (session tokens, preferences, tracking data, etc.), increasing the header size request and pushing it over the server’s limit. 

Clearing cookies for the specific site removes the data (Session ID) the server uses to recognize you. When you reload the page, the server considers you a new visitor and lets the browser start a new session, which, in most cases, solves the problem.  

Here are the steps to clear them in Google Chrome, Mozilla Firefox and Microsoft Edge. 

Chrome: 

1. Click Tune (two horizontal lines with circles) on the left side of the address bar. 

2. Select Cookies and site data. 

3. Select Manage on-device site data. 

4. Click Trash next to the site name. 

5. Reload the page. 

Firefox: 

1. Click the Padlock icon (left side of the address bar). 

2. Select Clear cookies and site data. 

3. A confirmation box will appear; click Remove. 

4. Reload the page. 

Edge: 

1. Click the Padlock icon (left side of the address bar). 

2. Select Cookies and site data. 

3. Click Cookies (X cookies in use). 

4. Choose the site’s cookies and click Remove. 

5. Reload the page. 

Open The Website in Incognito Mode 

Incognito or private mode is great for quick troubleshooting because it isolates the webpage. Firstly, it opens a temporary session that doesn’t use your existing browsing data. This means stored cookies aren’t sent as part of the HTTP request to the server. 

Secondly, most browsers also disable extensions in incognito mode by default unless you have specifically allowed them to run there.  

As a result, the browser sends a smaller request, without stored cookies or header changes introduced by extensions. If the site loads as normal, that means the issue is caused by cookies or extensions, not the site itself. 

To open incognito mode: 

• Chrome: Ctrl+Shift+N (Windows) or Cmd+Shift+N (Mac). 

• Firefox: Ctrl+Shift+P (Windows) or Cmd+Shift+N (Mac). 

• Edge: Ctrl+Shift+N (Windows) or Cmd+Shift+N (Mac). 

If the site loads, return to your regular browser and follow the instructions above to clear browser cookies. If that doesn’t work, move to the step below to disable extensions. 

Disable Browser Extensions 

Browser extensions can sometimes interfere with how requests are sent, by modifying or adding data to headers before they reach a server.  

If these changes increase the header size enough, the request will be rejected, and the “Request Header or Cookie Too Large” error will display. The same goes for having multiple extensions active simultaneously.  

Certain extensions alter request headers to provide security and monitoring features. While useful, it can also increase the header request size.  

Ad blockers inspect, filter and modify requests to block advertising scripts and trackers. However, this can increase the amount of header data or introduce additional request parameters that cause the error. 

Privacy tools can modify headers to remove tracking information, change referrer data, or block certain cookies to prevent tracking and protect personal data while you are browsing. Although this improves privacy, it can also add extra header processing; this may trigger the error. 

To test if this is the cause: 

1. Open your browser’s extensions page. 

2. Disable one at a time. 

3. Reload the site. 

If the error disappears after disabling an extension, re-enable each one at a time to find the one causing the issue. If you still cannot access the site, the issue may be a permissions problem, in which case you will need to fix the 403 Forbidden error instead. 

Flush DNS Cache 

DNS caching helps browsers load websites faster by storing domain information for the sites you have visited.  

A corrupted or outdated DNS cache can sometimes cause a 400 Bad Request: Request Header or Cookie Too Large error; however, you are more likely to use this option to resolve ERR_CONNECTION_TIMED_OUT errors.  

DNS flushing forces your browser to fetch fresh, verified connection data.  This fix is particularly effective if you are having errors across different websites. Here are the steps to flush your DNS cache for Windows, macOS, and Linux:  

Windows: 

1. Open Command Prompt as administrator. 

2. Type the command ipconfig /flushdns. 

3. Press Enter. 

4. You should see a message confirming that the DNS cache was flushed. 

macOs: 

1. Open Terminal (found in Applications under Utilities). 

2. Type the command sudo dscacheutil -flushcache; sudo killall -HUP mDNSResponder. 

3. Press Enter. 

4. Type your admin password and click Enter. 

Linux: 

1. For most modern distributions (Ubuntu, Fedora, Arch), use resolvectl instead of systemd-resolve. 

2. Run the sudo resolvectl flush-caches command in your terminal. 

3. Verify the cache is empty using resolvectl statistics. 

Restart your browser and try accessing the site again to see if it loads. 

Fix Invalid URLs 

This one isn’t cookie-related. Malformed or incorrectly encoded (invalid) URLs sometimes cause 400 Bad Request errors because they are included in the HTTP request.  

An overly long URL character count from long tracking strings (UTM codes) directly increases the size of the request header. If the server has a strict limit, it will immediately reject the request. 

Servers are also very picky about syntax, and spaces and symbols in URLs should be percent-encoded. If a URL is incorrectly encoded, for example, it contains a space instead of %20, the server may see it as a protocol error and drop the connection. 

If the URL contains long strings of characters after a question mark (example.com/page?tracking=longstringdata…), try removing them and accessing the main page with example.com/page. This removes unnecessary parameters that may inflate the request header. 

If you see special characters like #, [, ], or spaces, use a URL encoder or retype the address. 

If you are accessing a URL with parameters (e.g., a filtered search or a shared link), try navigating to the site’s homepage and accessing the page from there instead.  

URLs copied from documents or emails also sometimes pick up hidden characters, causing the same issue. 

Check Upload File Size 

If the error displays during an upload, the issue might be because the total request size (file and headers) exceeds the server’s maximum request size. If the upload request includes large headers, cookies, or unnecessary metadata, the server may reject it.  

To fix this: 

• Compress files before uploading using tools like Squoosh for images or Smallpdf for PDFs; or upload them in .zip folders. 

• Check the site’s file upload limits (e.g. 35MB) in their FAQs or Help pages.  

• Split large uploads into smaller batches if the website allows it. 

On WordPress sites, exceeding server resource limits during an upload can sometimes trigger a 500 Internal Server Error as well. If this happens, you can increase the maximum file upload size in your WordPress admin dashboard.

How to Prevent the Error in the Future 

Once you have implemented the right Request Header or Cookie Too Large fix and the website you want is loading, a few simple habits can help prevent it from happening again. 

• Clear Cookies: Periodically clear cookies for sites you visit frequently and log into regularly. See the clear browser cookies fix above to do this.  

• Limit Active Extensions: Only run the extensions you use. Each active extension can cause potential header request bloat. 

• Keep Browser Updated: Browser updates often include improvements to how cookies and headers are managed. An outdated browser may handle these less efficiently. 

• Monitor Cookie Usage: If you run a WordPress site, review how many cookies your plugins and themes are setting. Unnecessary cookies are a common cause of this problem for visitors. 

• Use Cookie Management Tools: Tools like Cookie AutoDelete automatically remove cookies from sites you aren’t visiting, so they aren’t stored and don’t build up. 

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