Expectations that need to be fulfilled by the server in order to properly handle the request.

Fastly reads this header from requests and writes it into requests. It is defined by an external standard.

PROTECTED:  This header cannot be modified in VCL.

The Expect header field in a request indicates a certain set of behaviors (expectations) that need to be supported by the server in order to properly handle this request. The only such expectation defined by RFC 7231 is 100-continue.

The Expect field-value is case-insensitive.

A server that receives an Expect field-value other than 100-continue MAY respond with a 417 (Expectation Failed) status code to indicate that the unexpected expectation cannot be met.

A 100-continue 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 if the request-line and header fields are not sufficient to cause an immediate success, redirect, or error response. This allows the client to wait for an indication that it is worthwhile to send the message body before actually doing so, which can improve efficiency when the message body is huge or when the client anticipates that an error is likely (e.g., when sending a state-changing method, for the first time, without previously verified authentication credentials).