Fiddler Classic Upstream Proxy Chaining on a Corporate Network

A network can require its corporate proxy while you also want requests to pass through an existing Fiddler Classic installation. The intended path is:

application → Fiddler Classic → corporate proxy → website

For Classic’s documented system-proxy chaining workflow, close Fiddler first. Set the upstream proxy’s address and port in Windows Internet Options under Connections → LAN settings, then restart Fiddler. Its About dialog should list the upstream proxy it picked up.

Closing it before changing the original setting matters: the parent should be the corporate endpoint, not Fiddler’s own listener. A parent pointing back to the local listener creates a loop. Fiddler’s system-proxy registration while capturing is separate from the saved parent endpoint.

The application still needs to send its requests to Fiddler. Seeing a corporate address in About establishes the onward gateway configuration; it does not establish that an application with separate proxy settings entered Fiddler at all.

If the network’s parent configuration changes, revisit the captured gateway and the applicable startup settings. Keep both hop addresses with a session when comparing it with a request sent directly to the corporate proxy.