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


Expect: 100-continue


There is only one defined expectation:


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.


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.


HTTP Semantics
# field.expect

Browser compatibility

BCD tables only load in the browser

See also