This paper is aimed at formalizing the interplay among a person to be assisted, an assistive agent-based software, and a caregiver. We propose principles that agents should follow in such interplay, this principles may have impact in different agent-based assistive technology. We propose a mechanism to integrate individual’s information into the final decision-making process of an agent. Moreover, we endow agents with mechanisms for evaluating the distance between independent and supported activity execution, the so called zone of proximal development (ZPD) in four scenarios: I) independent activity execution by a person; II) ZPDH activity performance of a person supported by another person (e.g. a therapist); III) the ZPDS representing a potential activities when a person is supported by a software agent; and IV) the ZPDH+S when a person is supported by a caregiver and a software agent. Formal argumentation theory is used as foundation. Our formal models were tested using a prototype using augmented reality as assistive software. A pilot study with older adults and health-care personnel was performed and formal and empirical results are presented.