Unbale to execute Power Apps desktop while using Fiddler Trace

[Victor]

Hi,

We are unable to execute any with Power apps  in Desktop application when Fiddler Trace is active, it keeps loading forever and after closing Fiddler it works fine.  Could you please advise?

Thanks in advance.

Victor Pontes

[Nick Iliev]

Hello Victor,

 

As noted in the official documentation of MS Power Apps, they do not automatically support proxy tooling.

https://learn.microsoft.com/en-us/power-apps/limits-and-config#proxies 

Power Apps doesn't support running with a proxy enabled. This can cause unpredictable behavior. If you encounter issues, disable the proxy and then try again.

Some proxies (such as Zscaler, Blue Coat) modify Power Apps requests by removing headers (CORS or authentication headers). Power Apps relies on these headers to load the app.
Some proxies (such as Microsoft Defender for Cloud Apps, McAfee) might intercept and change the URL of an app or embedded app. For example, if there's a Dynamics 365 app that is running under domain org.crm.dynamics.com or a canvas app that is running under domain apps.powerapps.com, the platform doesn't support a proxy that changes these domains to a custom domain such as mycustomdomain.com. This can cause unpredictable behavior when the platform tries to retrieve tokens that are necessary to run the app.

 

The above said, if it is not clear if you are using Fiddler Classic or Fiddler Everywhere, but in case you are using the new Fiddler Everywhere, then you can overcome the first limitation (modified CORS headers) by creating a rule to bypass CORS explicitly. An example of such a rule is present in the default rules presets in Fiddler Everywhere as shown here:

https://docs.telerik.com/fiddler-everywhere/rules-presets/tooling/bypass-cors 

The rule is available in Fiddler Everywhere default rule templates or can be explicitly downloaded from the following GitHub repository:

https://github.com/telerik/fiddler-everywhere/tree/master/rules/bypass-cors 

With the above rule, we could successfully load the https://make.powerapps.com/ endpoint. However, note that you can still observe some additional side effects due to the missing proxy support noted by Microsoft.

 

Regards,
Nick Iliev
Progress Telerik

[Victor]

Thanks for the explanation, is it possible to do it in Fiddler Classic? If so, is there any step-by-step explaining how to do it?

[Nick Iliev]

In Fiddler Classic, you can use FiddlerScript and modify the header values through  the techniques shown here

https://docs.telerik.com/fiddler/knowledge-base/fiddlerscript/modifyrequestorresponse 

[Victor]

Thanks again for all the help you provided so far, but I'm still facing some issues. After modifying the the FiddlerScript Power Apps desktop did not load as it should:

 

It should load like this:

 

Could you please help us? Please let me know if you need more information