The Semantic Web Sbrvices Initiative Architecture (SWSA) describes the overall process of semantic service execution in three phases: discovery, engagement and enactment. To accomplish the specified requirements of these phases, it defines a conceptual model which is based on semantic service agents that provide and consume semantic web services and includes architectural and protocol abstractions. In this paper, an MAS infrastructure is defined which fulfills fundamental requirements of SWSA's conceptual model including all its sub-processes. Based on this infrastructure, requirements of a planner module is identified and has been implemented. The developed planner has the capability of executing plans consisting of special tasks for semantic service agents in a way that is described in SWSA. These special tasks are predefined to accomplish the requirements of SWSA's sub-processes and they can be reused in real plans of semantic service agents both as is and as specialized according to domain requirements.