HTTP 100 – Continue
The initial part of the request has been received and the client should continue with the rest of it.
Common use case
Sent when a client includes an "Expect: 100-continue" header before uploading a large request body, so the server can reject it (e.g. with 401 or 413) before the client wastes bandwidth sending it.
Example
HTTP/1.1 100 Continue
History
Defined in the original HTTP/1.1 specification, RFC 2068 (1997), and carried forward into RFC 9110.
Did you know?
It is the only status code that is always followed by a second, final response for the same request - clients must keep reading after a 100.