Semantic Kernel
Microsoft open-source SDK for integrating AI models into .NET, Python, and Java applications. Officially superseded by Microsoft Agent Framework 1.0 (April 2026), which merges AutoGen and Semantic Kernel. Existing applications stay supported; new builds should target Agent Framework.
LangChain
Most widely used framework for building LLM applications. Chains, agents, RAG pipelines, and deep integrations with 300+ tools.