LangChain

What it does

LangChain is the de facto framework for LLM apps: chains, retrievers, tool-calling agents and LangGraph for stateful orchestration. Most serious agent projects start here or with its rival for good reason.

LangSmith adds tracing and evaluation so behaviour stays observable in production. Free and open source; paid platform tiers.

Standout features

• Composable chains and tool-calling agents
• LangGraph for stateful multi-step flows
• LangSmith tracing and evals
• Enormous integration catalogue

Best for