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Expect

The Expect HTTP request header indicates expectations that need to be fulfilled by the server in order to properly handle the request.

The only expectation defined in the specification is Expect: 100-continue, to which the server shall respond with:

  • 100 if the information contained in the header is sufficient to cause an immediate success,
  • 417 (Expectation Failed) if it 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 no

Syntax

No other expectations except "100-continue" are specified currently.

Expect: 100-continue

Directives

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 a 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 now checks the request headers and may respond with a 100 (Continue) response to instruct the client to go ahead and send the message body, or it will send a 417 (Expectation Failed) status if any of the expectations cannot be met.

Specifications

Specification Title
RFC 7231, section 5.1.1: Expect Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content

Browser compatibility

No common browsers are known to send this header.

See also

  • 417 Expectation Failed
  • 100 Continue

Document Tags and Contributors

 Contributors to this page: nwhittaker, fscholz, zbjornson
 Last updated by: nwhittaker,