I was asked by a colleague as to why I bothered writing a Fiddler IFiddlerWebTestPlugin when I could just as well write a simple VSTS WebTest based WebRequestPlugin, so I thought I might start answering the question... In essence the difference between the two is: (One happens before code generation, the other at runtime) With the Fiddler plug-in you get the opportunity to insert/generate code at the source level when the .webtest xml file gets saved, which gives you plenty of opportunities...