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 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.
| Header type | Request header |
|---|---|
| Forbidden header name | yes |
Syntax
Expect: 100-continue
Directives
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.
Examples
Large message body
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.
Specifications
| Specification |
|---|
| HTTP Semantics # field.expect |