Client error

400 400 Bad Request

The server could not understand the request at all — malformed syntax, an oversized header, or an invalid character in the URL.

Common causes

A corrupted or oversized cookie, which is the commonest cause when one user sees it and nobody else does.
A proxy or load balancer mangling headers.
Malformed JSON sent to an API.

How to fix it

Ask the affected user to clear cookies for the site.
Check header size limits on the web server and any CDN in front of it.
For APIs, log the raw body and validate it against the schema.

See what a site actually returns

Run a live check and read the real status code, headers and timings.

Check a website

Other status codes

200 OK 201 Created 204 No Content 206 Partial Content 301 Moved Permanently 302 Found 303 See Other 304 Not Modified 307 Temporary Redirect 308 Permanent Redirect 401 Unauthorized 403 Forbidden 404 Not Found 405 Method Not Allowed 408 Request Timeout 410 Gone 413 Payload Too Large 418 I'm a Teapot 429 Too Many Requests 451 Unavailable For Legal Reasons 500 Internal Server Error 501 Not Implemented 502 Bad Gateway 503 Service Unavailable 504 Gateway Timeout 507 Insufficient Storage 508 Loop Detected 521 Web Server Is Down 522 Connection Timed Out 523 Origin Is Unreachable 525 SSL Handshake Failed