Instead of using RegisterAsSystemProxy method use the ChainToUpstreamGateway method. This will chain FiddlerCore to the System Settings set by Fiddler desktop. Let me provide more details and references below.
First, ensure Fiddler desktop is running with the default settings, like Port 8888 and Register As System Proxy.
Then use the following FiddlerCore configuration to capture traffic and forward it to the upstream proxy settings.
static void Main(string args)
FiddlerApplication.AfterSessionComplete += session => Console.WriteLine(session.fullUrl);
var settings = new FiddlerCoreStartupSettingsBuilder()
After navigating to the FiddlerCore port, localhost:8865, in the browser, the following output is displayed in Fiddler and FiddlerCore.
For additional reference, see the FiddlerCore Configuration documentation.
Please let me know if you need any additional information. Thank you for building with FiddlerCore.
Eric R | Senior Technical Support Engineer
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