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Fiddler is the beloved network proxy for countless developers. We look under the covers to explore various flavors of Fiddler and how it continues to serve developer needs.

Developers should never be in doubt as to what happens in the network stack. This, however, is easier said than done today, with a plethora of app platforms/devices to build for. Developers need a consistent visual interface for productivity and demand visibility of network traffic while building wide variety of apps. There is only one way to achieve this—look under the covers.

Enter Progress Telerik Fiddler, the ultimate set of web debugging proxy tools for any browser, system or platform. Fiddler has been extremely popular in the developer community and has evolved a lot over the years. Let's take a close look at what Fiddler stands for today and how it can alleviate network stack pain for most developers.

Fiddler Beginnings

The need to inspect network traffic is nothing new and over the years, lots of tooling has been built for developers. The wonderfulness of Chrome DevTools is available to any developer using Chromium-based browsers. In addition to inspecting traffic, developers use multiple third-party tools for API testing, mocking and validating client/server behavior, collaboration, or even for hacking simulations. 

However, nothing beats the flexibility of complete network visibility—no matter what type of app developers are building. And for that, one has to go under the covers and use a network proxy. In the words of Gandalf the Grey: "You shall not pass" unless you go through me! That's the purpose of a network proxy and that's what gives developers full visibility.


Fiddler had humble beginnings with Eric Lawrence and is now maintained by exceptional engineers at Progress Software. Over the years, Fiddler has garnered a huge and loyal developer community, and is used today by hundreds of thousands of developers. A lot of folks are nostalgic about Fiddler—a tool they have used from early days to a mature career in software development.

Progress Telerik Fiddler logo

While several API/network sniffing tools are now around, nothing quite beats the full visibility and the extra power features offered by Fiddler. Simply put, the Fiddler proxy tools not only let you fiddle with network stack but a lot more—some key benefits include: 

  • Acting as a local proxy server
  • Support for any client-server platform
  • Capture, decrypt and inspect HTTP(S), WebSocket, gRPC traffic
  • Examining cookies, headers, cache, bodies, TLS certificates
  • Traffic recording & playback
  • Composing of any type of HTTP(S) request
  • Automated rules & Client/Server mocking
  • Manipulating web sessions
  • Load testing with compression
  • Overviewing HTTP(S) session's timings, sizes, and statistical data
  • Proxying for preconfigured browser and terminal instances
  • Emulating and testing different hacking scenarios

Fiddler's stance is simple - the web debugging proxy for any browser, system or platform. That is a lofty promise though. To do so, Fiddler offers the flexibility to be different things for different needs. This in fact, defines the ethos of modern day Fiddler—steady evolution to serve a variety of network proxy needs.


Flavors of Fiddler

So you've never tried Fiddler and now excited to get started? Great—but pause for just a bit to make sure you are using Fiddler exactly per your needs. There are some flavors to choose from ...

Fiddler Classic for Windows

Fiddler Classic is the original. What started out as a simple desktop application to act as a network proxy, evolved over years to be the most feature-rich Fiddler.


Fiddler on Windows got a much-adored new logo several years back and continues to rock. If you are on Windows OS, you can continue using the free Fiddler Classic app, but with an open mind towards trying out something new for future.


Fiddler Everywhere

So, you may ask, why should all of Fiddler goodness be restricted to developers on Windows OS? Touché. In fact, the demand for Fiddler outside of Windows continued to be most requested feature for quite some time. Thanks to crowdsourcing, the message was heard loud and clear. Yes, it took engineering efforts and investments, but Fiddler is finally ready to shed its old skin and step out onto whole new world.

Fiddler Everywhere is now available to download for macOS, Windows, and Linux distributions. Yes, you heard that right—we'll wait if you're hearing this for the first time and need to pinch yourself to believe it. You should try it today and provide feedback to help shape the new Fiddler. Fiddler Everywhere is clearly the future and will see the most innovations. 

The challenge, however, is to make the new Fiddler better than the original Fiddler Classic. This is not a small ask and demands the recreation of a decade's worth of prioritized features plus adding new ones. The efforts are already paying off with greatly improved UI and exciting new features like collaboration, extended Rules capabilities, HTTP/2 support, TLS 1.3 support, GRPC inspection, and many others in place. 

Curious how Fiddler Everywhere works everywhere? Hint, it is a cross-platform app that utilizes the man-in-the-middle technique as a local proxy with upgraded capabilities to fiddle with the HTTP(S) traffic per your requirements. Learn more here…

PS: You are on Windows and wondering if you should hop onto using the new Fiddler Everywhere? You should! Fiddler Classic will be maintained as usual, but Fiddler Everywhere already shapes the future of local proxy tooling.


Nerdy readers may also wonder: how are Fiddler features working consistently in every OS? The answer is abstraction - seperating Fiddler's network features from its UI. Most of Fiddler's core functionality has now been extracted out into a single .NET library and it is .NET Standard compliant. Welcome to FiddlerCore—the engine that powers Fiddler. The best part: you can now embed FiddlerCore in your .NET Framework or .NET Core apps and benefit from all of Fiddler's power just baked inside your app. Envision the use cases—dashboards, client support, monitoring and testing.

Here are some benefits FiddlerCore brings to the table, and it is ready for use right now.



Have you ever had bug reports from users that you simply cannot create on your end? Yup, constant black magic in software world, since you simply may not know all the variables while the user is running your app. Guess what would help—web traffic logs.  Telerik FiddlerCap is here to help.

FiddlerCap is a lightweight standalone web recorder app for Windows. Simply run this yourself or have the user run it for you. The simple UI allows for clearing cookies/caches before recording traffic and exporting the capture as a file. Load up the capture and you have a precise view of all the parameters that lead to the bug or crash as the user ran your app.



For years, Fiddler has been the beloved network proxy tool for countless developers. This is love and a responsibility not taken lightly by the engineers maintaining Fiddler. Fiddler continues to evolve and stick to the ethos of serving all developers across any platform. While Fiddler Classic continues to be a beloved free local proxy tool, Fiddler Everywhere already conquers exciting new horizons, and brings out the dream of being cross-platform yet feeling utterly native on Mac OS, Linux, and Window. FiddlerCore is another exciting development, providing developers a way to embed the full power of the Fiddler engine in their apps. 

The future of Fiddler is bright and you get to shape it with your feedback. Go ahead developers—you got this network visibility thing in your bag. Time to build some amazing apps!

About the Author

Sam Basu

Sam Basu is a technologist, author, speaker, Microsoft MVP, gadget-lover and Progress Developer Advocate for Telerik products. With a long developer background, he now spends much of his time advocating modern web/mobile/cloud development platforms on Microsoft/Telerik technology stacks. His spare times call for travel, fast cars, cricket and culinary adventures with the family. You can find him on the internet.

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