417 Expectation Failed
Meaning
The server cannot meet the requirements of the Expect request-header field. This usually relates to the Expect: 100-continue header.
When to Use
When the client’s request includes an Expect header that the server cannot or will not fulfill. Often, if a server does not support the Expectation indicated (like "100-continue" or other token).
Example
A client sends a request with Expect: 100-continue but the server does not support this expectation, it can respond with 417 Expectation Failed, indicating the expectation cannot be met.
When Not to Use
Do not use if there was no Expect header in the request. If the failure is due to something else (e.g., authentication, etc.), use the appropriate status code instead.
Source
RFC 9110