What it means
The error is on the client side: the request is malformed, too large or has invalid parameters. The server is reachable and working. A common browser example is “400 Bad Request: Request Header Or Cookie Too Large”.
Most common causes
- Invalid or incorrectly encoded characters in the URL (e.g. a % without a valid sequence).
- An oversized or corrupted cookie for the domain.
- Invalid JSON or a missing required field in an API request.
- A file or header that is too large for the server to accept.
What you can do as a visitor
- Check the URL and remove unusual characters or long parameters.
- Clear cookies for that domain only and reload the page.
- Try a private window. If it works there, cookies or an extension are the cause.
What the site owner or developer should do
- Check the logs for the exact rejection reason (WAF, proxy or application).
- Return API validation errors with a clear message and a 400 or 422.
- Check header size limits on the proxy (e.g. large_client_header_buffers in Nginx).
- Don't use 400 for server errors; those are 5xx.
Impact on SEO and Google
If Googlebot gets a 400 on a URL, the page won't be indexed. Often the cause is a broken internal link with bad parameters, so fix it at the source.
Frequently asked questions
Is a 400 caused by my internet connection?
Almost never. The problem is the request itself: the URL, cookies or data.
What does “Request Header Or Cookie Too Large” mean?
The cookies for the domain are too large. Clear that site's cookies.