100 1xx RFC 9110 §15.2.1

Continue

Server has received the request headers and the client should proceed to send the request body.

What HTTP 100 means

RFC 9110 §15.2.1. Used with the Expect: 100-continue header. Modern HTTP/2 clients rarely use this; it remains relevant in HTTP/1.1 with large request bodies where the client wants to confirm acceptance before transmitting the body.

Typical causes

Client sent Expect: 100-continue header
Server signaling readiness for request body