The Expect HTTP request header indicates expectations that need to be met by the server to handle the request successfully.

Upon Expect: 100-continue, the server responds with:

• 100 (Continue) if the information from the request header is insufficient to resolve the response and the client should proceed with sending the body.
• 417 (Expectation Failed) if the server cannot meet the expectation

or any other status otherwise (e.g. a 4xx status for a client error, or a 2xx status if the request can be resolved successfully without further processing).

For example, the server may reject a request if its Content-Length is too large.

No common browsers send the Expect header, but some other clients such as cURL do so by default.