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[ REPAIR GUIDE ] WEBSITE ERRORS

How to fix 400 Bad Request (and Request Header or Cookie Too Large)

ERROR DETECTED >400 Bad Request

UPDATED NOVEMBER 20263 MIN READ8 FIXES

QUICK FIX

Delete the cookies for that one website and reload the page. If that does not work, check the address for typos or extra characters and open the site from its home page.

What it means

A 400 Bad Request means the website's server rejected what your browser sent because it looked invalid. On everyday sites this is most often caused by damaged or oversized cookies for that site, or a mistyped or broken link, so unlike most server errors it is often fixable on your side. Related messages such as "Request Header Or Cookie Too Large" and 431 Request Header Fields Too Large mean the cookies or headers your browser sent were bigger than the server allows.

Common causes

  • Too many, too large or corrupted cookies for that site
  • A mistyped or broken URL, or one with invalid characters
  • A very long link with lots of tracking text in it
  • Uploading a file bigger than the site allows
  • A browser extension changing requests
  • On the site's side, header size limits set too low or a bug in the app

How to fix it

Work down the list. Each fix is safe to try before moving on to the next.

FIX 01Delete cookies for that websiteEASY

  1. In Chrome, open the three-dot menu and choose Settings.
  2. Go to Privacy and security > Third-party cookies > See all site data and permissions.
  3. Type the website's name in the search box.
  4. Click the trash icon next to the site and confirm with Delete.
  5. In Safari on a Mac, go to Safari > Settings > Privacy > Manage Website Data, select the site and click Remove.
  6. Reload the page and sign in again.

Why it helps: Oversized or corrupted cookies are the most common cause of 400 and 431 errors, and removing them only affects that one site.

FIX 02Check the web addressEASY

  1. Look for typos, spaces, stray % signs or odd characters in the address bar.
  2. Delete everything after the main page path (for example a long ? section) and press Enter.
  3. If that fails, go to the site's home page and navigate to the page from there.

Why it helps: A malformed or overly long URL can be rejected before the site even tries to load the page.

FIX 03Try a private window or another browserEASY

  1. In Chrome press Ctrl + Shift + N on Windows or Command + Shift + N on a Mac to open an Incognito window.
  2. Open the same page there.
  3. If it works, the problem is cookies or an extension in your normal browser profile.

Why it helps: Private windows start with no cookies and usually no extensions, so they bypass both causes.

FIX 04Turn off extensions one at a timeEASY

  1. In Chrome, go to the three-dot menu, then Extensions > Manage Extensions.
  2. Switch off extensions such as ad blockers, privacy tools or header changers.
  3. Reload the page, and turn extensions back on one by one to find the culprit.

Why it helps: Some extensions add or modify request headers in ways a server rejects.

FIX 05Check the file you are uploadingEASY

  1. If the error appeared while uploading, check the site's stated size limit.
  2. Compress or resize the file and try again.

Why it helps: Some servers answer an oversized or malformed upload with a 400 error.

FIX 06Contact the websiteEASY

  1. If the error continues after clearing cookies and in other browsers, contact the site's support.
  2. Tell them the page address, the time and what you clicked.

Why it helps: If it fails everywhere, the site is rejecting valid requests and must fix it.

// RUN THIS SITE? FIXES FOR SITE OWNERS

FIX 07Reduce cookie size on your siteMEDIUMSITE OWNERS

  1. Open your site in a browser's developer tools and check how many cookies your domain sets and how large they are.
  2. Remove unused cookies from plugins, analytics and A/B testing tools.
  3. Store large data on the server and keep only a short session ID in the cookie.
  4. Scope cookies to the subdomain or path that needs them instead of the whole domain.

Why it helps: Every cookie for your domain is sent with every request, so large cookies push requests over the header limit.

FIX 08Check server logs and header limitsADVANCEDSITE OWNERS

  1. Check the web server error log at the time of the 400 for a reason such as an oversized header or invalid request line.
  2. On Nginx, the header buffer size is set by large_client_header_buffers; on Apache, by LimitRequestFieldSize. Raise them only moderately and only if your cookies are legitimately large.
  3. If you use a load balancer, CDN or firewall, check its header limits too, since any layer can reject the request.
  4. For APIs, validate the request body (for example invalid JSON) and return a clear error message describing what is wrong.

Why it helps: The log shows which layer rejected the request and why, so you fix the right setting.

Check it's fixed

  • The page loads in your normal browser window.
  • You can sign in and stay signed in without the error coming back.
  • The same link works on another device.

When to get help

If clearing that site's cookies and trying another browser do not help, the problem is likely on the site; contact its support. Site owners who cannot find the cause in their logs should ask their host or CDN provider.

Questions

What does Request Header Or Cookie Too Large mean?

Your browser sent more cookie or header data than the server accepts. Deleting that site's cookies almost always fixes it.

What is the difference between 400 and 431?

400 is a general "invalid request" error. 431 Request Header Fields Too Large is more specific: the headers, usually cookies, were too big.

Will deleting the site's cookies log me out?

Yes, for that site only. Your other sites and saved passwords are not affected.

Do I need to clear all my browsing data?

No. Deleting cookies for just the problem site is enough in most cases and keeps you signed in elsewhere.

Related fixes

How to fix a 401 Unauthorized errorSign out of the site, delete its cookies, then sign in again with the correct username and password. If you know your details are right and How to fix 405 Method Not AllowedAs a visitor, go back to the site's home page and repeat the action from there instead of refreshing or using the back button. As a sitHow to fix ERR_TOO_MANY_REDIRECTSDelete the cookies for that site and reload. If it still loops on every browser and device, the site has a redirect loop that the owner mustHow to fix ERR_CACHE_MISSReload the page and confirm that you want to send the form again, or go back to the form and submit it fresh. Do not resend payment or order

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