WCF has a very powerful extensibility mechanism around modifying HTTP headers. However, you will get tripped up if you try and modify the “Expect: 100-Continue” header on outgoing requests. This is because System.Net will (by default) add this header on outgoing requests.

As a workaround, you can change the default for your application by setting a System.Net static:

System.Net.ServicePointManager.Expect100Continue = false;