adk-java

ADK (Agent Development Kit) for Java is Google's official toolkit for building, evaluating, and deploying AI agents. Define tools, orchestrate multi-step reasoning, handle conversation state, and evaluate agent performance all within your Java codebase.

Apache 2.0. This is early but backed by Google. It integrates with Google's AI models (Gemini) and supports the broader agent ecosystem.

Fully free and open source. No paid features in the toolkit. You pay for the AI models you call through it: Gemini API pricing, Vertex AI costs, or whatever LLM you connect.

The catch: Java in the AI agent space is unusual; most agent frameworks are Python or TypeScript. The ecosystem of examples, tutorials, and community plugins is small compared to LangChain or CrewAI. If your team is already in the Java ecosystem (Spring Boot, enterprise backends), this makes sense. If you're starting fresh, Python frameworks have 10x the community support. And it's early; the API is still evolving.