Multi-agent systems research community has developed solutions to model dynamic and complex systems. Those solutions have generated a growing interest in a variety of domains such as robotics or distributed controls. It is therefore important to build solutions to analyze the basic mechanisms of multi-agent systems such as interaction. In this paper, we formalize the epistemic interaction, we propose to use default theory to describe state transitions. The introduction of such semantics allows characterizing the verification of agent behavior when dealing with new information. Therefore, we introduce semantics to establish a basis for exploiting the properties of the agent behavior and its messages.We propose to formalize and verify the response of an agent when it receives a new information from another agent of its environment. Our approach extends the actions initiated in the mA * [5] language by applying the Reiter’s default rules [32]. A simple example is used to illustrate our approach.