LangFuse
langfuse.com
An open-source LLM engineering platform for tracing, evaluating, and monitoring AI applications in production, with prompt versioning, A/B testing, and user feedback collection.
Why it is useful
Fills the observability gap that appears when you move LLM apps to production — you can see exactly what prompts ran, what the model returned, how long it took, and how users rated it. Prompt versioning means you can roll back bad changes without digging through git history. The open-source self-hosted version has most features of the cloud tier, making it practical for teams with data residency concerns.