Despite the development of many modeling formalisms and model solution methods, most tool implementations support only a single formalism. Furthermore, models expressed in the chosen formalism cannot be combined with models expressed in other formalisms. This monolithic approach both limits the usefulness of such tools to practitioners, and hampers modeling research, since it is difficult to compare new and existing formalisms and solvers. This paper describes the method that a new modeling tool, called Möbius, uses to eliminate these limitations. Möbius provides an infrastructure to support multiple interacting formalisms and solvers, and is extensible in that new formalisms and solvers can be added to the tool without changing those already implemented. Möbius provides this capability through the use of an abstract functional interface, which provides a formalism-independent interface to models. This allows models expressed in multiple formalisms to interact with each other, and with multiple solvers.