HTTP Status Codes

HTTP 400 — Bad Request

HTTP 400 (Bad Request) means the server rejected the request because it was malformed — bad syntax, invalid parameters or a corrupted request the server can't understand.

A 400 is a client-error (4xx) status: the problem is in the request, not the server. Causes include malformed URLs, oversized headers, invalid cookies or broken query strings. Users usually see it after a bad link or a browser/extension issue.

For SEO, isolated 400s on odd URLs are rarely harmful, but systematic 400s on real, linked pages block crawlers from reaching content and should be fixed. Check server logs and Search Console for patterns.

If a URL is genuinely gone, return a 404 or 410 instead; if it moved, use a 301 redirect. Reserve 400 for actually invalid requests.

This is a technical SEO topic I handle for clients: Technical SEO →

Related terms

405 Method Not AllowedHTTP 405 means the HTTP method used isn't allowed for that resource. 404 Not FoundHTTP 404 means the requested page could not be found on the server. HTTP status codesOverview of HTTP response codes and their SEO impact.

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