Development of agent systems is naturally a complex task due to the fundamental characteristics of agents. In addition, agent internals and inter-agent behavior models inside Multi-agent Systems (MAS) may become even more difficult to implement when interactions of agents with web services on the Semantic Web are taken into account. Our approach consists of the utilization of a Domain-specific Modeling Language (DSML) during MAS development in order to cope with the abovementioned challenge. This paper describes how the formal semantics of this DSML can be defined by especially focusing on its viewpoint on agent-semantic service interactions and discusses the use of this semantics definition on MAS validation. Determined semantic rules are both defined and implemented by using Alloy specification language which has a strong description capability based on both relational and first-order logic.