The built-in utility implementations of the
StaticResourceCalloutMock are not declared as such in the OST. This results in errors in the IDE even though the code is valid and runs on-platform. e.g.
HttpCalloutMock myMock = new MultiStaticResourceCalloutMock();
Produces an error
Illegal assignment from System.MultiStaticResourceCalloutMock to System.HttpCalloutMock even though this code can be deployed, runs, and works as expected.