Configure Fiddler to use a 3rd party Diff tool

I use the awesome Fiddler2 to debug HTTP, and I needed to compare sessions. Fiddler has a support for this, but when I clicked Tools – Compare Sessions, it prompted me to install Windiff.exe.

I haven’t found a way to set the preferred Diff tool in the UI (via Options), so after looking at the click handler for the menu in Reflector, I found out that the value can be set via Registry key:

  • Open Registry, go to:
  • Add a new String Value called CompareTool and set the value to the path of your favorite Diff tool, e.g.:
"C:\Program Files (x86)\Beyond Compare 3\BCompare.exe"

  • Restart Fiddler

That’s it!