In an ongoing project, we are building a debugger for distributed computations running on a heterogeneous set of machines. Such debuggers are commonly built by layering a sophisticated user interface on top of existing sequential debuggers. We have found that the implementation can be simplified by the precise specification of a client-server protocol to describe the interaction of the user interface and debugger core. In addition to facilitating heterogeneous debugging, this strategy also permits a high degree of client portability. This will likely encourage investment in application-domain specific debuggers. This paper describes some of the problems encountered in developing a portable debugger for heterogenous, distributed computing and how the architecture of our debugger avoids them. We also present detailed examples illustrating how the client and server interact to perform typical debugging operations.