With the large and increasing variety of architectures for conversational agent design, there is a need to investigate the necessary elements of practical conversational agents in light of new technologies. To address this need, we introduce a new abstract framework of conversational agents design which we call Conv-BDI. The BDI model is a longestablished theory of decision-making in artificial agents, which the Conv-BDI model extends to describe the design of conversational agents from traditional symbolic logic-based models or statistical models to more recent LLM-based agents. Specifically, we extend the core BDI model with notions of Purpose and Behavioral Guidelines, while also elaborating on the role of system actions within this framework. The Conv-BDI model thus provides a framework of intentionality in conversational agents that can be applied to to design of contemporary conversational agents.