Ok, so I assume that you’re already able to monitor your client/server traffic. What you want to do is set a breakpoint on the response then fiddle with it before sending it on to the client.
Here are a couple of different ways to do that:
- Rules > Automatic Breakpoints > After Responses
- In the quickexec box (the black box at the bottom) type “bpafter yourpage.svc”. Now Fiddler will stop at a breakpoint before all requests to any URL that contains “yourpage.svc”. Type “bpafter” with no parameters to clear the breakpoint.
- Programmatically tamper with the response using FiddlerScript. The best documentation for FiddlerScript is on the official site: http://www.fiddler2.com/Fiddler/dev/
Once you’ve got a response stopped at the breakpoint, just double click it to open it in the inspectors. You’ve got a couple of options now:
- Right next to the green Run to Completion button (which you click to send the response) there’s a dropdown that lets you choose some default response types.
- Or, on the Headers inspector, change the response code & message in the textbox at the top.
- Or, click the “Raw” inspector and mess with the raw response to do arbitrary things to it. Also a good way to see what your client does when it gets a malformed response, which you’ll probably test accidentally 🙂