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In part one of this Fiddler series, we focused the basic Composer functionality. Now it's time to focus on how it makes your life better.

Ever tried to test your API or a website with the UI? You click again and again, only to miss the breakpoint on the desired method or selected the wrong option. Fiddler makes this easier, allowing you to modify and execute existing request the same way your application would.

Modifying Existing Requests

Modifying an existing request and executing it again is pretty straightforward:

  1. Drag the session from the sessions list and drop it on the Composer tab
  2. Change the desired values
  3. Click the Execute button in order to execute the request


In the sessions list, you can find the newly executed request and the response from the server.


The Options tab exposes options that allow you to customize the behavior of the Composer.

  • Inspect Session - selects the new session and activates the Inspectors tab when the request is issued.
  • Fix Content-Length Header - adjusts the value of the Content-Length request header (if present) to match the size of the request body.
  • Follow Redirects - causes a HTTP/3xx redirect to trigger a new request, if possible. The Composer will follow up to fiddler.composer.followredirects.max default redirections.
  • Automatically Authenticate - causes Fiddler to automatically respond to HTTP/401 and HTTP/407 challenges that use NTLM or Negotiate protocols using the current user's Windows credentials.


The Composer tab in Telerik Fiddler can help you build your REST API with ease, while focusing on the responses rather than how to simulate it.

Don’t hesitate to drop a line or two below, If you have any questions or comments. Your feedback is highly appreciated.

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Kamen Velikov
About the Author

Kamen Velikov

Kamen is software developer and manager of the Fiddler team at Progress. He has more than 11 years of professional experience in software development. Programming has always been his passion and he feels lucky to work his hobby. In his free time, he enjoys traveling, motorcycling, freshly-roasted coffee, and trying new experiences. Geek by design. Writing is a new thing to him and he’ll appreciate your feedback and comments. 

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