Web developers can easily extend Fiddler using the .NET Framework. There are two way to go about it: create either Custom Rules or Inspectors.
Read more in the Documentation
Fiddler’s rich extensibility model enables developers to add powerful new features to Fiddler via easily installed add-ons. Among the most popular 3rd party extensions are the ones that enhance Fiddler’s ability to test the performance and security characteristics of web applications.
An alternative way to extending and customizing Fiddler according to your needs is by adding Inspectors which can be written in any .NET language. Inspectors can be read-only or read-write. If an Inspector is read-write, you can use it to modify the HTTP request or response before the server or the client receives it. The changes you make while the paused at a breakpoint are committed automatically by the Inspector.
A quick list of commonly used request/response Inspector types includes:
[RW] Headers—Shows request headers and status.
[RW] TextView—shows the request body in a text box.
[RO] XML—shows the request body as an XML DOM in a tree view.
[RW] Transformer—Removes GZIP, DEFLATE, and CHUNKED encodings for easier debugging.
[RW] Headers—Shows response headers and status.
[RO] ImageView—shows the response body as an Image. Supports all .NET image formats.
[RO] XML—shows the response body as an XML DOM in a tree view.
Fiddler supports a rich extensibility model which ranges from simple FiddlerScript to powerful Extensions which can be developed using any .NET language.
There are various rules that you can start using out of the box:
Alternatively, you can create your own custom rules by using FiddlerScript.