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Note: Prefer to write all of your code in .NET?  Use the IAutoTamper interface instead!


Extending FiddlerScript using .NET

Using .NET Assemblies in FiddlerScript, step-by-step

For instance, here's an example of a managed C# addon that tweaks the user-agent string.

1. Close Fiddler.

2. Save the following as UASimulator.cs

using System;

using System.Windows.Forms;

using Fiddler;

namespace FiddlerUtility{

      public class UASimulator {

            string m_sUAString;

            public UASimulator(string s_UAString){

                  m_sUAString = s_UAString;


            public bool OverwriteUA(Session oSession){

                  oSession.oRequest["User-Agent"] = m_sUAString;

                  return true;




3. Go to a VS command prompt in the folder where the .CS file is found.

csc /target:library /out:c:\UASim.dll UASimulator.cs /reference:"C:\program files\fiddler2\fiddler.exe"

4. Inside Fiddler, add C:\UASim.dll to the ScriptReferences list in Tools | Fiddler Options.

5. Boot Fiddler.

6. Use Rules | Custom Rules to update your script.

import System;

import System.Windows.Forms;

import Fiddler;

import FiddlerUtility;

class Handlers{

      static var UASim = new UASimulator("Mozilla/12.0");

      static function OnBeforeRequest(oSession:Fiddler.Session){



   static function Main(){

       var today: Date = new Date();      

       FiddlerObject.StatusText = " CustomRules.js was loaded at: " + today;




Learn more about Building assemblies to run in both Fiddler v2 and v4