Telerik: this is a very specific case in which loading a Custom Exception from code file in Test Studio is not working, because the external assemblies are loaded only in the Runner process application domain during execution. It is not possible to deserialize them in the TestStudio process because the assemblies are not loaded in its application domain. We have implemented a fix now, so that the assemblies are loaded during test loading in Project explorer. The workaround however requires to put the Custom Exception code with all required serialization attributes in a separate assembly, build it and add it as reference in Test Studio Project Settings->Script options (http://docs.telerik.com/teststudio/features/coded-steps/add-assembly-reference). Therefore this will not work if you put the Custom Exception code in Test Studio independent code file directly. The build with the fix will be available in our next product release planned to be out by end of July 2016.