LangSmith is a unified DevOps platform for developing, debugging, testing, deploying, and monitoring LLM applications and AI agents built by LangChain. It provides framework-agnostic observability with comprehensive tracing, evaluation datasets, online/offline evaluations, prompt management, human-in-the-loop workflows, and production monitoring to help teams move models from prototype to production. LangSmith's core differentiator is its end-to-end visibility into agent execution via traces and runs, enabling developers to understand what happens inside the "black box" of complex multi-step LLM workflows through detailed logging, cost tracking, latency metrics (P50/P99), and automated evaluations at scale.