What is LangChain?

LangChain is an open-source framework for building applications powered by large language models (LLMs). It provides composable abstractions - Models, Prompts, Chains, Agents, Memory, and Tools - that make it practical to connect LLMs with external data and systems without writing all the integration plumbing from scratch.

The framework is built around several key abstractions. Models give a unified interface to LLMs such as OpenAI, Anthropic, and Google regardless of their individual APIs. Prompts are templates that format inputs before they reach the model. Chains sequence calls to models, tools, or other Runnables. Agents let the LLM decide which tools to call and in what order. Memory stores conversation context so later turns can reference earlier ones. Tools are callable functions — web search, calculators, database queries — that models can invoke.

LangChain's declarative composition syntax, LCEL, uses the pipe operator | to connect components: prompt | model | parser. The ecosystem extends to LangSmith (tracing and evaluation), LangServe (REST deployment), and LangGraph (stateful multi-actor graph applications). Python and JavaScript/TypeScript are both supported.