HTTP 100 Continue

In short: The server received the start of the request and the client may send the rest (the body).

In more detail: Used when a client wants to send a large body and first asks with the header Expect: 100-continue whether the server will accept it at all. This saves transfer if the server would reject anyway (for example because of a missing login).

In Depth

When it occurs

• Large uploads where the client sends the headers first and waits for approval.
• Tools such as curl send Expect: 100-continue automatically for larger POST bodies.

What you can do

1. As a visitor you do not need to do anything, the code is not shown.
2. Operators: servers and proxies should support Expect, otherwise the client waits a few seconds before sending the body anyway.

See also: HTTP Status Codes Overview, HTTP