Continue

The server received the request headers and the client should send the request body.

100 Continue is half of a two-step upload. The client sends its headers with Expect: 100-continue, waits for the server's go-ahead, and only then sends the body. That avoids pushing a large upload the server was going to reject anyway. curl adds the Expect header automatically on big POSTs, which is why some uploads pause for a beat before the transfer starts.

What 100 looks like on the wire

POST /upload HTTP/1.1
Host: api.example.com
Content-Length: 52428800
Expect: 100-continue

HTTP/1.1 100 Continue

About 1xx Informational

The request was received and the process is continuing. These are interim responses you rarely handle directly.

How to fix 100 Continue

If uploads stall for a second before starting, the server is ignoring Expect: 100-continue and the client is waiting it out.
In curl, send an empty Expect header to skip the handshake entirely.
Make sure any proxy in front of the app passes 1xx responses through instead of swallowing them.

Tools that help with 100

cURL to Code Converter

Once your Expect: 100-continue test works in curl, convert the command into fetch, axios, or Python code without retyping the headers.

Where it's defined

100 Continue is defined in RFC 9110 §15.2.1. MDN's 100 reference covers browser behavior and examples.