The paper presents the developed technology and software tool for design and implementation of knowledge-based multi-agent systems. The software tool comprises two components that are “Generic Agent” and “Multi-agent System Development Kit” (MAS DK). The former comprises reusable Visual C++ and Java classes and generic data and knowledge base structures, whereas the latter comprises several developer-friendly editors aimed at formal specification of the applied multi-agent system (MAS) under development and installation of the resulting application in particular computer network environment. The developed technology and MAS DK were used in the design and implementation of the MAS prototype for computer network assurance and intrusion detection and distributed attack simulator. Several other applications are currently under development.