# LangChain — The most widely-used framework for building LLM apps and agents — with LangGraph for stateful control.

> Source: The Agents Index — https://theagentsindex.com/langchain (structured, researched, re-verified)
> Facts last verified: 2026-07-13

LangChain is an open-source framework for building applications and agents on top of LLMs. It provides the building blocks — model and tool abstractions, retrieval, memory — and, via LangGraph, a way to build stateful, controllable multi-step and multi-agent systems. LangSmith is its paid observability/eval layer.

| Fact | Value |
| --- | --- |
| Website | https://www.langchain.com |
| API | Yes |

## Strengths and weaknesses

- ✓ Unmatched integration ecosystem and community.
- ✓ LangGraph provides genuine stateful control, not just chains.
- ✗ The abstractions have churned across versions; a learning curve and some API instability.
- ✗ Powerful but heavier than minimal frameworks for simple agents.

## Key features

- **Rich integrations** — A large ecosystem of model, vector-store and tool integrations.
- **LangGraph** — Graph-based control for stateful, multi-step and multi-agent workflows.
- **LangSmith observability** — Tracing, evals and monitoring for agents in production (paid).

## Use cases

- **Build a custom agent** — A team composes retrieval, tools and control flow into a bespoke production agent.
- **Multi-agent systems** — Coordinate several agents with LangGraph’s stateful graph.