AutoGen is a multi-agent AI framework originally from Microsoft Research, now in maintenance mode and succeeded by Microsoft Agent Framework. The codebase is a large monorepo containing Python packages (autogen-core, autogen-agentchat, autogen-ext, autogen-studio, agbench) and a .NET implementation (Microsoft.AutoGen.*), with Python as the primary language. The architecture is layered: Core provides event-driven agent runtime, message passing, and distributed infrastructure via gRPC; AgentChat offers higher-level multi-agent teams and patterns; Extensions integrate external services (OpenAI, Azure, MCP, GraphRAG, etc.); Studio is a FastAPI web UI with a Gatsby frontend for no-code agent prototyping. Key entry points include AssistantAgent, group chat managers, and various team orchestrators. The system relies heavily on LLM clients, Pydantic models, gRPC for distributed runtimes, and OpenTelemetry for tracing. Despite the maintenance mode status, the code shows solid engineering with clear separation of concerns, extensive test suites, and well-structured packages. However, the ecosystem is in transition, with active development shifted to the successor framework.