Google Agent Development Kit (ADK) Python development guide. Build, evaluate, and deploy AI agents with Gemini models. Covers agents (LLM, Workflow, Custom, Multi-agent), tools (built-in, custom, MCP), runtime, deployment, observability, evaluation, sessions, memory, callbacks, artifacts, events, streaming, A2A protocol, and grounding. Keywords: google-adk, adk-python, agent, gemini, tool, workflow, multi-agent, deployment, evaluation, streaming, a2a, grounding