LangChain

The most popular framework for building applications powered by large language models. LangChain provides the building blocks for chains, agents, RAG pipelines, and memory systems — with a massive ecosystem of integrations that has made it the default starting point for LLM application development.

• Chain composition — connect LLM calls, tools, and logic into complex workflows • Agent framework — build autonomous AI agents that use tools to accomplish goals • RAG support — built-in document loaders, embedders, and vector store integrations • Memory — conversation and entity memory for stateful applications • LangSmith — observability platform for debugging, testing, and monitoring LLM apps • Massive ecosystem — integrations with hundreds of tools, models, and data sources

Free and open-source. LangSmith platform has free and paid tiers.