400 Bad Request — Causes & Fix
HTTP 400 (Bad Request) means the server could not understand the request because it was malformed — bad syntax, invalid framing, or deceptive routing.
- Applies to
- Any website or web server (HTTP standard)
- Time to fix
- 5–30 minutes
What it means
Causes include malformed JSON/parameters, an oversized or corrupt cookie, invalid headers, or a bad URL. HTTP status 400 is defined in RFC 9110 (HTTP Semantics). This is a client-side (4xx) error — the request itself had a problem, so the fix is usually on the requester's side or in how the server validates the request.
Symptoms
- The page or API returns "400 Bad Request"
- Requests fail regardless of login
How to fix it
- 1
Clear cookies / check the request
1. As a user: clear the site's cookies and cache (a corrupt cookie is a common cause) and retry. 2. Check the URL for typos.
- 2
Fix the payload (developers)
1. Validate the request body, headers, and parameters against the API spec. 2. Correct malformed JSON or encoding.
Frequently asked questions
Why do I get 400 on one site only?
Often a corrupt or oversized cookie for that site. Clearing that site's cookies usually fixes a user-side 400.
Last updated July 20, 2026
Guidance only — always consult a qualified professional or the official service manual before carrying out repairs.