Expectation Failed

Overview

417 Expectation Failed means the server cannot meet the expectation set in the request's Expect header — most commonly Expect: 100-continue. A client sends 100-continue to check whether the server will accept a large body before actually uploading it.

If the server or an intermediate proxy does not understand this mechanism, it returns 417 instead of 100 Continue. It is usually fixed by dropping the Expect header and just sending the body.

When it happens

• Sending Expect: 100-continue before a large upload to a server that does not support it
• An older server or proxy that cannot handle the 100-continue handshake
• Using a non-standard Expect value the server does not recognize

Request / Response example

POST /large-upload HTTP/1.1
Host: api.example.com
Expect: 100-continue
Content-Length: 10485760

HTTP/1.1 417 Expectation Failed
Content-Type: text/plain

Expectation given in Expect header could not be met.

In code

# Some proxies choke on 100-continue; send an empty Expect to skip it
curl -X POST --data-binary @big.bin \
  -H 'Expect:' \
  https://api.example.com/large-upload

> POST /large-upload HTTP/1.1
> Expect: 100-continue
>
< HTTP/1.1 100 Continue      (server OK -> client sends body)
< HTTP/1.1 417 Expectation Failed   (server refuses the expectation)

Notes

• Some HTTP clients add Expect: 100-continue automatically when the body exceeds ~1 KB; suspect this header if you see mysterious delays or a 417.
• Only 100-continue is standardized for the Expect header, so any other value usually results in 417.

Related status codes

Related headers

Specification