The HTTP Status Code 417 means that the server cannot meet the requirements of the Expect request-header field.
Frequently Asked Questions
What does HTTP 417 mean?
HTTP 417 Expectation Failed is a client error response that means the server cannot meet the requirements specified in the request's Expect header field. It tells the client that the expectation it declared before sending a full message body was rejected by the server.
When does a server return a 417 status code?
A server returns 417 when a client includes an Expect header in its request — most commonly Expect: 100-continue — and the server is unwilling or unable to fulfill that expectation. This typically occurs when a client wants to verify the server will accept a large payload before transmitting the full request body.
How do I fix an HTTP 417 error?
The most direct fix is to remove or modify the Expect header from the request and send the full request body directly without waiting for a preliminary server acknowledgment. In .NET environments, you can disable the Expect: 100-continue behavior globally by setting ServicePointManager.Expect100Continue = false, or per request via HttpRequestMessage headers or HttpClientHandler configuration.
Is HTTP 417 a client-side or server-side error?
HTTP 417 is a client-side error in the 4xx range, meaning the problem originates with how the client constructed its request. The client sent an Expect header that the server does not support, so resolving it requires a change on the client side rather than on the server.
What is the difference between HTTP 417 and HTTP 100?
HTTP 100 Continue is the positive response a server sends when it is willing to accept the request body described in an Expect: 100-continue header. HTTP 417 Expectation Failed is the negative counterpart — the server's way of telling the client it cannot or will not honor that expectation, so the client should not proceed with sending the body.
How common is the HTTP 417 status code?
HTTP 417 is exceptionally rare in practice. Many web servers and application frameworks never fully implemented support for the Expect header handshake, so the scenario that triggers a 417 is uncommon in modern HTTP traffic. Developers are most likely to encounter it when working with HTTP clients — such as certain .NET libraries — that automatically add the Expect: 100-continue header to POST requests.
