Google Agent Development Kit (ADK) is an open-source, code-first Python toolkit for building, evaluating, and deploying sophisticated AI agents with flexibility and control. While optimized for Gemini models, ADK is model-agnostic and supports multi-agent orchestration with hierarchical systems where specialized agents collaborate on complex tasks. It features built-in tool integration including custom functions, OpenAPI specs, and MCP (Model Context Protocol) tools, along with a development UI for testing and debugging. ADK also supports the A2A (Agent-to-Agent) protocol for remote agent communication and offers deployment flexibility across Cloud Run, Vertex AI Agent Engine, and custom infrastructure.