Add Rules to Fiddler
Customize Rules
To add custom columns to the Fiddler UI, modify requests or responses, test application performance, and a variety of other custom tasks, add rules to Fiddler's JScript.NET CustomRules.js file in FiddlerScript.
Click Rules > Customize Rules....
Enter FiddlerScript code inside the appropriate function.
Save the file.
Fiddler will automatically reload the rules.
Use Additional .NET Assemblies
To use additional .NET assemblies in your script:
Click Tools > Fiddler Options.
Click the Extensions tab.
Edit the References list.
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Either:
Register the assembly in the GAC; or
Copy the assembly to the folder that contains Fiddler.exe.
To use the new assembly's functions without fully-qualifying them, update the #import clause at the top of the script.
Change the JScript Editor Launched from the Rules Menu
Click Tools > Fiddler Options.
Edit the Editor string.
Restore Default Rules
Delete the CustomRules.js file in ~/Documents/Fiddler2/Scripts.
Restart Fiddler.
Note: Fiddler's default rules are stored in ~/Program Files/Fiddler2/Scripts/SampleRules.js.