I am trying to monitor an application which is attempting to tunnel HTTP connections. In some cases it requests a malformed URL like http:///test . I would like to tamper with that request and fix it up but it seems Fiddler doesn't parse HTTP requests in the typical way when they are done via CONNECT. That may (or may not) be why in the case of a malformed URL OnBeforeRequest isn't called. Please refer to the forum thread for more information.