LangChain

LangChain is the most widely adopted open-source framework for building LLM applications, in Python and JavaScript. It standardizes the plumbing: a common interface over model providers, prompt templates, output parsers, document loaders, vector store integrations, retrievers, and agent runtimes, so prototypes do not hard-wire a single vendor.

LangGraph, its companion library, models agents as stateful graphs with checkpoints, retries, and human-in-the-loop steps, which is how production agent workflows survive failures. LangSmith, a separate service, adds tracing, datasets, and evaluation so prompts and runs can be inspected and scored.

The frameworks are MIT licensed and free; LangSmith is priced per seat or by usage above a free developer tier. LangChain suits teams moving a prototype toward a product, though its abstractions churn quickly, upgrades can break code, and debugging is easier with LangSmith than without.