LangChain

LangChain is a developer framework for building applications on top of large language models, giving you standard building blocks for prompts, tool calling, retrieval, memory and agents so you can swap models without rewriting your app. It pairs with LangSmith for tracing, evaluation and monitoring and LangGraph for stateful agent workflows, and is widely used for chatbots, RAG systems and internal copilots. The libraries are open source and free, with paid cloud tiers.