EC
417
HTTPERRORNotable4xx Client ErrorHIGH confidence

Expectation Failed

Production Risk

Low. This is part of the standard flow control of HTTP. A compliant client should be able to handle this rejection by sending the request again without the 'Expect' header.

What this means

The expectation given in the 'Expect' request header field could not be met by the server. This is most commonly seen with the '100-continue' expectation.

Why it happens
  1. 1A client sends 'Expect: 100-continue' but the server is not configured to handle it or has already decided to reject the request.
  2. 2A proxy or intermediary server does not understand the 'Expect' header and cannot forward it properly.
  3. 3The server cannot fulfill the expectation for other reasons, such as authentication failure.
How to reproduce

A client sends an 'Expect: 100-continue' header before uploading a large file, but the server immediately rejects the request based on the headers (e.g., due to invalid authentication).

trigger — this will error
trigger — this will error
POST /upload HTTP/1.1
Host: example.com
Authorization: Bearer invalid-token
Expect: 100-continue

expected output

HTTP/1.1 417 Expectation Failed

Fix

Remove the 'Expect' Header

WHEN The server does not support it.

Remove the 'Expect' Header
The client should simply send the request with the body directly, without the 'Expect' header.

Why this works

Client-Side Correction

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