LangSmith

LangSmith is a complete debugging, testing, and monitoring platform for LLM applications and AI agents, built by the LangChain authors and directly integrated into the framework. Its tracing system records every call in a chain or agent with inputs, outputs, and execution time, providing complete visibility into what actually happens. LangSmith offers LLM-as-judge or script-based evaluations to automatically score response quality, dataset creation to test different prompt and model variants side-by-side, and UI to replay traces and refine prompts without redeploying code. The Developer plan (free, 5k traces/month, 14d retention, 1 user) lets you start; the Plus plan ($39/user/month, 100k included traces, 400d retention, up to 10 users) suits small teams; Enterprise on request adds SSO, custom retention policies, and dedicated support. Pricing per trace with overage ($2.50/1k traces beyond quota) offers flexibility but makes costs unpredictable.

LangSmith does not provide a French-language interface and remains exclusively English-language. Its native integration to LangChain makes it essential for teams using that framework, but the tool also works independently via REST API and Python/TypeScript SDKs. The platform offers excellent debugging experience for complex LLM chains and agents, but requires good understanding of LLM concepts and evaluation to leverage advanced features. True cost can be hard to predict since it depends on trace volume generated by your production agents, which can vary significantly with load; teams must actively monitor consumption to avoid surprises.