AG2 is an open-source, async-first Python framework for building and orchestrating multi-agent AI systems. It provides a core Agent class with tool integration, middleware pipelines, human-in-the-loop hooks, and pluggable LLM backends (OpenAI, Anthropic, Gemini, Mistral, xAI, Bedrock, Ollama, and more). The framework supports complex agent topologies through a Network layer (Hub, channels, adapters) enabling conversation, consulting, workflow, and discussion patterns. It also implements multiple agent communication protocols (A2A, ACP, MCP, A2UI) for cross-agent and agent-to-UI interaction, real-time voice via LiveAgent, and a comprehensive evaluation suite for scoring agent traces. The architecture is modular and event-driven, with a rich set of built-in tools (code execution, web search, shell, file operations), sandboxed environments, knowledge stores, and streaming support.