The HTTPExpectrequest header indicates that there are expectations that need to be met by the server in order to handle the complete request successfully.

When a request has anExpect: 100-continue header, a server sends a100 Continue response to indicate that the server is ready or capable of receiving the rest of the request content.Waiting for a100 response can be helpful if a client anticipates that an error is likely, for example, when sending state-changing operations without previously verified authentication credentials.

A417 Expectation Failed response is returned if the server cannot meet the expectation, or any other status otherwise (e.g., a4XX status for a client error, or a2XX status if the request can be resolved successfully without further processing).

None of the more common browsers send theExpect header, but some clients (command-line tools) do so by default.

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 a100 Continue interim response.

Examples

Large message body

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

PUT /somewhere/fun HTTP/1.1Host: origin.example.comContent-Type: video/h264Content-Length: 1234567890987Expect: 100-continue

The server checks the headers and generates the response, where a100 Continue instructs the client to send the message body:

HTTP/1.1 100 Continue

The client completes the request by sending the actual data:

[Video data as content for PUT request]

Specifications

See also

• 417 Expectation Failed
• 100 Continue