Common causes
- Typos or illegal characters in the URL, such as unencoded spaces or a stray % sign
- Corrupted or oversized cookies stored in the browser
- Request headers that are too large
- Uploading a file larger than the server allows
- A malformed request sent by an app, form or API
How to fix it
As a visitor: check the URL for mistakes, clear the site’s cookies and cache, or try another browser.
As a site owner: find the failing URLs in your server logs or the Pages report in Google Search Console, fix broken links and redirects that produce bad URLs, and check your server’s limits for headers and uploads.
400 errors and SEO
A page that returns a 400 error can’t be indexed. A few 400s from odd bot requests are harmless, but if real pages or internal links return 400, fix them so that visitors and search engines can reach the content.