Second Brain is an agentic framework that functions as an AI operating system on the local machine. It indexes documents, code, PDFs, and other files; maintains durable context through retrieval; runs tools and schedules automated workflows; and communicates across multiple messaging platforms. The system routes conversations through a robust state machine where participants take actions, turns move between actors, and multi-step flows can suspend and resume. New commands, frontends, tools, and agents can be added as plugins while the system is running.