LangChain

LangChain is the most widely-adopted open-source framework for building applications powered by large language models. Created by Harrison Chase, LangChain provides abstractions and tools for connecting LLMs to external data sources, tools, APIs, and memory systems — the fundamental building blocks of AI agents.

LangChain's ecosystem includes the core framework, LangGraph (for building stateful multi-agent systems), LangSmith (for observability and evaluation), and LangServe (for deployment). The framework supports chains (sequential LLM operations), retrieval-augmented generation (RAG), tool use, and complex agent architectures.

LangChain has become foundational infrastructure in the agent orchestration layer, with thousands of companies building their AI agent systems on its abstractions. It represents the most mature attempt to standardize how developers build LLM-powered applications and autonomous agents.