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/HTTP

Expect

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 sufficient to cause immediate success
417 (Expectation Failed) if the server cannot meet the expectation

or any other 4xx status otherwise.

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.

Header type	Request header
Forbidden header name	yes

Expect: 100-continue

There is only one defined expectation:

100-continue: Informs recipients that the client is about to send a (presumably large) message body in this request and wishes to receive a 100 (Continue) interim response.

A client sends a request with Expect header and waits for the server to respond before sending the message body.

PUT /somewhere/fun HTTP/1.1
Host: origin.example.com
Content-Type: video/h264
Content-Length: 1234567890987
Expect: 100-continue

The server checks the headers and generates the response. The server sends 100 (Continue), which instructs the client to send the message body.

Specification
HTTP Semantics # field.expect

	Desktop						Mobile
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	WebView Android	Chrome Android	Firefox for Android	Opera Android	Safari on IOS	Samsung Internet
`Expect`	No	No	No	No	No	No	No	No	No	No	No	No