LangSmith, from the LangChain team, is a platform for debugging, testing, evaluating, and monitoring LLM applications. You instrument your app, and LangSmith captures every trace — prompts, tool calls, token usage, latency — so you can inspect runs, build evaluation datasets, and track quality over time. It is framework-agnostic and not limited to coding agents; teams use it for chatbots, RAG pipelines, and any LLM workload.
Fleet is not an observability platform. It is the layer that actually runs the agents: a single Go binary that launches Claude Code agents in defined roles (developer, reviewer, release-manager), reacts to GitHub label events, and hands work between roles through its Fabric event bus. Where LangSmith answers 'what did my LLM calls do and how good were they?', Fleet answers 'who does what, in what order, and with what guardrails?'.