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Is your app prepared to recover smoothly when facing a remote server error? Learn how Fiddler Everywhere can help you test and understand your app's resilience.

Many of us have been using Fiddler Classic  for years to inspect and debug HTTP/S traffic between our computer and the rest of the world. It trivializes the overly complex problems we experience when inspecting network activity both to and from both web and desktop apps. Oh, and we can also set up "auto-responding" rules and fiddle with the request and response. See what I did there? 🧐

Fiddler Everywhere is the next generation of Fiddler tooling. Fiddler Everywhere brings forward most of what you love about Fiddler to a new cross-platform tool with a revamped UX that performs identically across macOS, Linux, and Windows. 

We are kicking-off a blog series in which we'll take a quick look at scenarios most of us have encountered. From remote APIs failing, to diagnosing errors in production, to finding a pesky 404 - Fiddler Everywhere is our trusted co-pilot when debugging these (and plenty of other) issues:

Today we're going to examine how we can use Fiddler Everywhere to mock a remote server error. What happens when that third party API goes down or the network connection is dropped entirely? Is your app set up to gracefully recover from these types of errors?

Our Scenario: Bulletproofing an App by Mocking Issues

As a developer, my web and desktop apps rely heavily on remote backend services. Some of them I manage myself, while others are third party APIs that I have no control over. What happens when any one of them goes down? Is my app set up to resolve these errors and notify the user in a graceful manner?

Or will I end up taking crazy pills when an API fails on me?

i feel like i'm taking crazy pills gif

Fiddler Everywhere's Solution

The AutoResponder feature has been updated and renamed to Rule Builder as of July 2021.

Fiddler Everywhere includes a feature called Auto Responder.

Auto Responder can be used in virtually any situation where you want to redirect a request, alter a status code, delay a response, and so on.

For example, you may want to:

  • Replace a remotely-loaded library/image/file with a local version;
  • Redirect a request from one URL to a completely different one;
  • (In our case) see what happens when a remote connection is dropped!

Let's try out this scenario:

  1. Open Fiddler Everywhere and toggle the Live Traffic option to Capturing:

    fiddler everywhere capturing toggle

  2. Open the web app (or desktop app) and navigate to whatever functionality or feature will call the remote service for which you'd like to create a mock response.

  3. Back in Fiddler Everywhere, toggle the Live Traffic option to Paused so as to limit new sessions coming into our UI.

  4. Look for the specific request you want to mock. Note that you can use the filtering options to quickly discover matching sessions. For instance, you can filter by hostname, URL, status code, and so on.

    filter sessions

  5. When you've found the appropriate session, right-click it and choose Add New Rule. This will populate the Auto Responder tab with a new rule.

    add new auto responder rule

  6. Navigate to the Auto Responder tab and edit the newly-created rule. In the Action dropdown list, choose *drop, as this will mock what happens when a request to this URL is dropped.

    drop connection action

    NOTE: There are numerous other pre-defined actions you can utilize like *reset and *delay:100. Take a look at some explanations for these actions in the Fiddler Everywhere docs.

  7. With your rule activated (make sure the Enable Auto Responses box is checked!), reload your app. Fingers-crossed 🤞 now, hopefully your app handles this connection drop issue just fine. If not? This is an opportunity to alter your app and re-run these tests until your app performs as expected.


In this post we took a quick look at how Fiddler Everywhere's Auto Responder feature can allow you to quickly mock up a remote API failure, allowing you to adjust and improve your app before a problem occurs in production.

Start your journey with Fiddler Everywhere by downloading a trial today on macOS, Linux, or Windows.

Rob Lauer is a Product Manager for Telerik
About the Author

Rob Lauer

A maker at heart and a supporter of the open web, Rob is Developer Relations Lead at Blues Wireless. You can find Rob rambling incoherently on Twitter @RobLauer.

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