HTTP status codes / 4xx — Client error
400 Bad Request
What it means
The server refuses to process the request because it is malformed — broken JSON, invalid parameters, bad headers, oversized cookies.
What causes it
Client bugs: malformed request body, wrong content-type, invalid query encoding, corrupted cookies, headers too large.
How to fix it
Compare your request against the API docs field by field; validate JSON before sending. As a user: clearing site cookies fixes the surprisingly common cookie-overflow variant.