Fiddler is an HTTP debugging proxy server application which captures HTTP and HTTPS traffic and logs it for the user to review (the latter by implementing man-in-the-middle interception using self-signed certificates). Fiddler can also be used to modify ("fiddle with") HTTP traffic for troubleshooting purposes as it is being sent or received.
You can use Fiddler for many purposes, but we will mention under this tutorial why a lot of developers in Adform are using auto-respond (official documentation) functionality and how to do that yourself.
Fiddler's AutoResponder tab allows you to return files from your local disk instead of transmitting the request to the server. It makes the process of developing or testing the banner's (especially those banner's which you can't test on your machine like local connection banner's) easier.
Let's make a use case there we will auto-respond a file (script.js in our case) from our HTML banner with Fiddler.
Load your banner on which you are working to the Adform platform or load your project to Adform Studio.
We can preview this banner by selecting an icon (eye symbol) in the top right corner. We will use this preview for developing our banner with the help of Fiddler from now.
Open Fiddler. Go to the AutoResponder tab. Select the "Enable automatic responses" option if it is not selected. Drag your script from the banner folder in the white space under the "if request matches" and press "save" The rule editor shows what it is looking for in a typical request (in our case for "script.js") and with what it is going to replace (in our case with another script.js from local folder).
Afterward, go an refresh your Studio preview link a couple of times and you should see something like this in Fiddler when the file on the server will be switched with your local copy of the file:
This grayish line means that Fiddler detected and switched the file with your copy. That is it.
This determines the tutorial on how you can use Fiddler to develop banner's for Adform by using Adform preview without uploading the banner's at all times, but just changing your code in your local copy of the file and refreshing Adform preview.
Note that Fiddler is a third-party tool developed and maintained by Telerik and not by Adform.