HTTP 100 — Continue

What 100 actually means

This is a handshake, not an error. A client with a large body to upload can send its headers first with an `Expect: 100-continue` header and wait, rather than pushing megabytes at a server that was going to reject the request anyway. A 100 is the server saying "headers look fine, go ahead". Browsers do not use this; it shows up in curl, in API clients, and in HTTP debugging traces.

Common causes

• The client sent `Expect: 100-continue` before a large request body
• curl adds `Expect: 100-continue` automatically for bodies over 1 KB
• A proxy in the path is negotiating the upload on the client's behalf

If you're just trying to view the page

1. Nothing — this never reaches you as a page, it is an intermediate step in a request
2. If you see it in a debugging tool, the request is proceeding normally

If it's your site

1. If a server hangs on uploads, it may be ignoring `Expect: 100-continue`; send `-H "Expect:"` with curl to disable it
2. Nginx handles this automatically; older backends behind a proxy sometimes do not
3. Check that intermediaries are not stripping the `Expect` header before it reaches the origin

Reference

Status code

100

Reason phrase

Continue

Category

1xx — Informational

Defined in

RFC 9110 §15.2.1