loot.tools
400

Bad Request

4xx Client Error

The server can't process the request due to a client error like malformed syntax.

400 Bad Request is a catch-all for 'the request itself is broken'. The server couldn't or wouldn't process it because something on the client side is malformed: invalid JSON, a missing required field, a bad query parameter, or a header the server can't parse. The fix is almost always in the request, not the server.

What 400 looks like on the wire

POST /api/users HTTP/1.1
Host: api.example.com
Content-Type: application/json

{"email": oops-missing-quotes}

HTTP/1.1 400 Bad Request
Content-Type: application/problem+json

{"title": "Request body is not valid JSON"}

About 4xx Client Error

The request has a problem the client needs to fix, like a bad URL, missing auth, or invalid input.

How to fix 400 Bad Request

  • Validate the request body - malformed JSON is the most common cause.
  • Check required fields and parameter types against the API docs.
  • Inspect headers like Content-Type for typos or wrong values.
  • Look at the response body - many APIs explain exactly what failed.

Tools that help with 400

JSON Formatter & Validator
Validate the request body first: a stray comma or an unquoted key is the usual cause of a rejected payload.
URL Encoder/Decoder
Check that spaces and special characters in your query parameters are percent-encoded the way the server expects.

Where it's defined

400 Bad Request is defined in RFC 9110 §15.5.1. MDN's 400 reference covers browser behavior and examples.

Looking for a different one? See the full HTTP status code reference.