A 400 Bad Request error means the server cannot process your request because it is malformed. The request syntax is invalid, a required header is missing, the URL is too long, or the request body is corrupted. The server is telling your browser: “I cannot understand what you are asking.”
Fix 1: Check the URL
Malformed URLs with special characters, missing slashes, or encoded characters that decode incorrectly cause 400 errors. Retype the URL manually or use a search engine to find the correct page.
Fix 2: Clear Browser Cookies
Corrupted or oversized cookies are the most common cause of 400 errors. The browser sends cookies with every request. If a cookie is malformed, the server rejects the request. Clear cookies for the specific domain.
Fix 3: Clear Browser Cache
Cached data from a previous session may conflict with the current page. Ctrl+Shift+Delete, clear cache for “All time.”
Fix 4: Reduce File Upload Size
If uploading a file when you get 400, the file may exceed the server’s maximum upload size. Compress the file or check the site’s upload limits. Common limits: 2MB, 10MB, or 50MB depending on server configuration.
Fix 5: Disable Extensions
Extensions that modify request headers can create malformed requests. Test in Incognito. If it works, find the problematic extension.
Frequently Asked Questions
What is the difference between 400 and 404?
400 means the request itself is malformed and the server cannot parse it. 404 means the request is valid but the requested resource does not exist. 400 is your problem (bad request). 404 is the server’s information (resource not found).
Can a 400 error be the server’s fault?
Sometimes. If the server’s request parser is too strict or has a bug, valid requests may be rejected as malformed. But in most cases, 400 means the client actually sent something wrong.
Why do I get 400 only on one website?
Different servers have different request parsing strictness. A cookie or header that one server accepts may be rejected by another. Clear cookies for the affected site specifically.
