What it means

A 100 Continue is an interim response. It tells the client that the initial part of the request has been received and the client should proceed with sending the rest of the request body. You will rarely see this in a browser context, but it shows up in API workflows where a client wants to confirm a server will accept a large upload before sending it.

Common causes

• The client sent an Expect: 100-continue header before uploading a large request body.
• An HTTP API is using interim responses for efficiency.

How to fix it

• No action needed. This is a normal part of the request lifecycle, not an error.