Getting to Know Fiddler: Part VII: Wrapping up our Fiddler World Tour

Posted on Sep 22, 2014

In the first six parts of our series, we examined some of the power that Fiddler has to offer us: we covered remote debugging, composing requests, breakpoints, autoresponders, plugins, and a bit of FiddlerScript. So what's next? There's still an incredible amount that Fiddler can do, but despite the “Land Before Time”-esque post names, this was never meant to be a comprehensive Fiddler feature tutorial. The series was just meant to cover some of Fiddler's most powerful yet underused features; specifically features that have gotten me out of a bind a time or two.

So, no post? Well, not quite. There are still two little things that need to be covered before we call it quits.


One of the things that I have to do almost everytime I launch Fiddler is to disable caching from the Rules menu. This has the effect of stripping several headers from every outgoing request, preventing us from receiving a 304 Not Modified from the server.

While 304s are great for web performance, since they contain no actual body, they can make debugging a bit more difficult. Even more so if you are planning to export Fiddler's sessions for someone else to view. If we edit our Fiddler script again, we can cause caching to be disabled by default. Find m_DisableCaching and change the default value.

var m_DisableCaching: boolean = true;


In our post on AutoResponders, our last example showed how we could rewrite outgoing requests in order to get around the Same-Origin policy while developing. There's actually another way we could perform that redirection.

At the bottom-left of Fiddler there is a small black box that has the ever so hard to read text “[QuickExec] ALT+Q > type HELP…". While this extremely low contrast area appears to be a status message, it is actually an input to invoke QuickExec.

We could perform our same redirection shenanigans of redirecting Google to Bing (or maybe a production server to a dev server) by clicking the QuickExec box and entering:


Later, we can disable replacements by running urlreplace with no arguments.

You can view the other 22 commands, or add your own by reading the OnExecAction method at the bottom of the default FiddlerScript. As a command-line addict, you can find yourself comfortably at home here.

Seven Posts Later…

And with that, we end our Fiddler series. I've definitely had fun, but I think it might be time to take a break from our favorite web debugger1. Tune in next week for a post that almost certainly won't be related to Fiddler… Unless it is.


  1. Sorry Charles, but after using Fiddler you just seem a bit… limited. ↩︎