Configure Fiddler to use a 3rd party Diff tool

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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:

  1. Open Registry, go to:
    2. HKEY_CURRENT_USER\Software\Microsoft\Fiddler2
  2. Add a new String Value called CompareTool and set the value to the path of your favorite Diff tool:
    3. "C:\Program Files\Beyond Compare 3\BCompare.exe"
  3. Restart Fiddler

That’s it!