LangChain is an open-source framework and ecosystem for building applications and agents powered by language models. It provides abstractions for working with models, tools, prompts, retrieval systems, structured outputs, and agent workflows, helping developers connect LLM reasoning with external data and actions.

The broader LangChain ecosystem includes components for building reliable agentic applications, tracing and observability, evaluation, and deployment. It is commonly used for assistants, RAG systems, tool-using agents, and multi-step AI workflows where developers need reusable integrations and orchestration rather than calling a model API directly for every step.