4xx Client Errors
HTTP 400 Bad Request
The server cannot or will not process the request due to a client error.
When is 400 used?
- Malformed JSON or XML body
- Invalid query parameters
- Request body too large
How to handle a 400 response
Inspect the request — URL, method, headers, credentials, and body. Retrying the same request unchanged will usually fail again; 429 is the exception, where you should wait for the Retry-After period.
Check the status code a URL returns from the command line:
curl -s -o /dev/null -w "%{http_code}\n" https://example.comHow to return 400 from your server
Node.js (Express)
res.status(400).json({ message: 'Bad Request' });Python (Flask)
return {"message": "Bad Request"}, 400Go (net/http)
w.WriteHeader(400)Frequently Asked Questions
What does HTTP 400 mean?
400 Bad Request: The server cannot or will not process the request due to a client error.
Is 400 an error?
Yes. 400 is in the 4xx Client Errors class. Client errors: the request is malformed, unauthorized, or targets something that does not exist.