Customizing fiddler

Start fiddler with composer window seperated (and at a fixed position)

Posted by Vinay on August 27, 2015

I use fiddler for all my api testing, development and debugging purposes. It’s simple enough to start with and comes with a lot of functionalities. It lets me capture incoming and outgoing traffic from any application and inspect headers and response in a range of formats. All in all, it’s a great tool in any developer’s arsenal and over the years I’ve come to love it.

One thing that always annoyed me, though, is that fiddler starts up like this:

![](/img/fiddler-old-startup.png)

Being primarily interested in testing out my REST API, every time I start up fiddler, I:

  • switch to Composer tab,then to Options and check the tear apart option, to seperate the composer window
  • Move and resize seperated window to appropriate position

This looks something like this:

![](/img/fiddler-composer-away.jpg)

This lets me easily test my api endpoints and inspect the response without a lot of switching tabs around. While the main window remembers it’s position and size, the composer window starts of fresh and I always have to manually fix it. Over the last 4 or so years, it’s become a bit of a chore to manually doing this and I while wondering if there was a way to better way to do this, I shot off an email via fiddler feedback (under help menu) describing my (trivial) problem.

I got a reply right away from the awesome Eric Lawrence, the creator of fiddler, detailing how to do just that. Turns out, fiddler supports customisations through scripting and to automate what I want, all I had to do was:

  • Click Rules > Customize Rules to open the FiddlerScript
  • Scroll to the OnBoot function and add the following lines inside it:
FiddlerApplication.UI.actQuickExec("!composer move 123,456,789,1000");
FiddlerApplication.UI.actQuickExec("!composer activate Scratchpad");

The first line get composer to detach and itself from main window and set it’s position on screen, with XPos, YPos, Width and Height for the composer’s window. The second line will open up the Scratchpad tab on composer window. You can of course change it to Parsed or any other tabs and that works well enough.

Things to note here though, initially, my whole onBoot method was commented out, and I had to uncomment it to get this working. Oh, the FiddlerScript is supported from version 4.4.7.1 and up.

Another cool tip about fiddler: if you usually don’t want to capture all the traffic going in and out from every application, but just want fiddler’s traffic, start fiddler with -noattach parameter and it will do just that. I’ve added shortcut to my taskbar with this param and use that when I’m working with endpoints, which is most of the time!