Human health coaching has been established as an effective intervention for improving clients’ health, but it is restricted in scale due to the availability of coaches and finances of the clients. We aim to build a scalable, automated system for physical activity coaching that is similarly grounded in behavior change theories. In this paper, we present our initial steps toward building a flexible system that is capable of carrying out a natural dialogue for goal setting as a health coach would while also offering additional support through just-in-time adaptive interventions. We outline our modular system design and approach to gathering and analyzing data to incrementally implement such a system.