Expect

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 feature that uses this header is 100-continue.

Writing requests (clients)

The Expect header is designed to indicate to the server that the client expects a specific behavior in order to function properly.

In the case of 100-continue, the client is signaling that it won't begin uploading the request-body until the server signals it is acceptable to do so. Servers that do not support this would send 417, and the client can retry the request without 100-continue.

Reading requests (servers)

If the header exists, and is a value not 100-continue, return 417 Expectation Failed.

Overview table

Name

Expect

Description

Lists behaviors expected from the server to properly make the request.

Direction

Request

Specification

RFC 9110: HTTP Semantics §10.1.1. Expect

Syntax

Expect = "100-continue"

Example with 100 Continue

The client begins by writing just the headers of the request:

PUT / HTTP/1.1
Host: localhost
Content-Type: text/plain
Content-Length: 10
Expect: 100-continue

The server verifies it can accept an upload of text/plain of 10 bytes by sending an intermediate status:

HTTP/1.1 100 Continue

The client finishes writing out the body part of the message:

Message!

The server resumes its response message. Note this can still include 1xx intermediate responses.

HTTP/1.1 200 OK
Content-Type: text/plain

Success!

Example with Expectation Failed

The client begins by writing just the headers of the request:

PUT / HTTP/1.1
Host: localhost
Content-Type: text/plain
Content-Length: 10
Expect: 100-continue

A server that does not support 100-continue will inform the client by writing an error as the final response:

HTTP/1.1 417 Expectation Failed

The client can then retry the upload, making the entire upload and waiting for the server to validate it.

History

RFC7231 removed the "expectation-extension" rule previously defined by RFC2616:

Expect =      #expectation
expectation = token [ "=" ( token / quoted-string ) parameters ]

token          = 1*tchar
tchar          = "!" / "#" / "$" / "%" / "&" / "'" / "*"
					/ "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
					/ DIGIT / ALPHA
					; any VCHAR, except delimiters

quoted-string  = DQUOTE *( qdtext / quoted-pair ) DQUOTE
qdtext         = HTAB / SP / %x21 / %x23-5B / %x5D-7E / obs-text

parameters      = *( OWS ";" OWS [ parameter ] )
parameter       = parameter-name "=" parameter-value
parameter-name  = token
parameter-value = ( token / quoted-string )

History

1. 1999-06: RFC 2616 §14.20. Expect
2. 2014-06: RFC 7231 §5.1.1. Expect. The only recognized value for this header is "100-continue" (case-insensitive), and responding with 417 (Expectation Failed) is now optional (due to lack of widespread support in servers).
3. 2022-06: RFC 9110 §10.1.1. Expect. The list of tokens form (similar to RFC 2616) is restored.