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400 Bad Request
HTTP 400 Bad Request. The server cannot process the request because it is malformed — bad syntax, invalid framing, or otherwise something the client got wrong. When to use it: Return when the request itself is invalid at a structural level and the client must change it before retrying. It is a client-error (4xx) status, which means the problem is with the request and the caller must change something before retrying. (Reference: RFC 9110 §15.5.1.)
The server cannot process the request because it is malformed — bad syntax, invalid framing, or otherwise something the client got wrong.
- When to use: Return when the request itself is invalid at a structural level and the client must change it before retrying.
- How to handle: Check request syntax, headers, and payload structure; fix the malformed part and resend.
- Reference: RFC 9110 §15.5.1
- Category: 4xx Client Error
Frequently asked questions
What does HTTP 400 mean?
HTTP 400 Bad Request: The server cannot process the request because it is malformed — bad syntax, invalid framing, or otherwise something the client got wrong. It is a client-error (4xx) status, which means the problem is with the request and the caller must change something before retrying. It is defined in RFC 9110 §15.5.1.
What is the difference between 400 and 422?
400 Bad Request means the request is malformed at a structural level — bad syntax or a body that will not even parse. 422 Unprocessable Content means the request parsed fine but is semantically invalid, such as a well-formed JSON body that fails field or business validation. Many REST/JSON APIs return 422 for validation errors and reserve 400 for genuinely unparseable input.