Hi there,
I think, you can implement monitoring by creating a script that will obtain the needed data from Fiddler. I've analyzed Fiddler's GUI using TestComplete and found out that it uses Windows Forms, so it is a white box for TestComplete. Thus, this script should:
1. Start your Browser along with Fiddler.
2. Do the needed actions along with clicking the toolbar button.
3. Select the needed request in Fiddler's list.
4. Select the needed tab in the right part of Fiddler's GUI (the Headers tab, the TextView tab, etc.)
5. Get the needed data from the selected tab.
6. Compare the data with your base-line data through a
checkpoint or a custom routine.
