FMAS (Flexible Multi-Agent System) is an open-source Python library for building, running, and visualizing multi-agent simulations. You can define agents with custom decision logic, configure an environment model, set up messaging channels for communication, and execute scalable simulation runs. FMAS provides hooks for monitoring agent state, debugging interactions, and exporting results. Its modular architecture supports plugins for visualization, metrics collection, and integration with external data sources, making it ideal for research, education, and real-world prototypes of autonomous systems.