There are some problems when using Fiddler  with the Visual Studio built-in web server. By default Fiddler is only changing the proxy settings for http and https. The problem is that Visual Studio will start a web server on a random port address which will not be captured using Fiddler. To fix this you start Fiddler and modify the proxy settings in Internet Explorer (or any other web browser): set proxy to 127.0.0.1 and port 8888 for all protocols, not only http and https. This will allow you to use the one and only debugging proxy.