If you've never measured your chain's quality with something more rigorous than "I tried a few prompts by hand and it looked fine," you don't have an evaluation system — you have an impression.
Offline evaluation runs against curated datasets during development to catch regressions before deployment, and functions as unit tests for your LLM application. Offline evaluations are used for pre-deployment testing, including benchmarking, regression testing, unit testing, and backtesting — they target dataset examples: curated test cases with reference outputs that define what "good" means for your specific case.
Online evaluation runs in production and continuously evaluates real user interactions as they happen, to monitor quality over live data. Online evaluations can identify issues to add to offline datasets — closing the loop: what fails in production becomes the next offline test case, preventing the same problem from repeating undetected.
LangSmith lets you evaluate output quality with offline datasets and LLM-as-judge online evaluators — using an LLM to judge another LLM's response quality against explicitly defined criteria is what makes evaluating at scale viable without a human reviewing every individual case.
By 2026, LangSmith had expanded beyond pure observability into a full agent operations stack — including LangSmith Fleet (formerly Agent Builder) for deployment, a unified cost view across complete agent workflows, and availability on AWS Marketplace for enterprise procurement. It's no longer just a debugging tool, it's the full operational management layer for agents in production.
You don't need an exhaustive evaluation program from day one — start with a small offline dataset (10-20 cases representative of the real failures you've already seen), run offline evaluation before every significant change to your chain, and add online evaluation only once you already have real production traffic worth monitoring continuously.
Carlos Montiel is an enterprise AI solutions architect. He implements LLMs, Agents, RAG, and orchestrators for companies across Guatemala and Latin America. Reach out for a consultation.
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