What it means
The server has received the request headers and the client may go ahead and send the body. It is an interim response sent before the final status.
Common causes
- The client sent Expect: 100-continue before a large upload
- curl adds Expect: 100-continue to POST bodies over 1 KB
- A proxy or server confirms it will accept the body
How to fix it
As a client or visitor
Nothing to fix: wait for 100, then stream the body. If a server never answers, send the body after a short timeout or drop the Expect header (curl -H 'Expect:').
As the site or API owner
Most servers send 100 automatically. Validate headers such as auth and Content-Length first, and answer with a final error (401, 413) instead of 100 when you would reject the body anyway.
Example response
HTTP/1.1 100 ContinueSpecification
FAQ
› Will I see 100 Continue in my browser's network tab?
Usually not. Browsers do not send Expect: 100-continue, and interim responses are hidden; you mostly meet 100 with curl, SDKs and upload clients.
› Is 100 Continue the final response?
No. It is informational. The server must still send a final status such as 200, 201 or 413 after the body arrives.