100 Continue
The server has received the request headers and the client may go ahead and send the body. It answers a request that carried Expect: 100-continue.
1xx · Informational
Typical causes
Clients such as curl send Expect: 100-continue before uploading a large body, so they learn early whether the server will accept it instead of wasting the upload.
Defined in RFC 9110, §15.2.1