Langfuse

The most-used open-source LLM observability tool. Spec page: /tools/langfuse.

• Core value: self-hosting keeps sensitive traces off vendor servers, via OpenTelemetry so it is framework-agnostic.
• Spans tracing, prompt management, and evaluation, but its evaluation features trail dedicated eval tools (see evaluation and observability).
• Self-hosting runs on ClickHouse, which is real operational work at scale.
• Best paired with a dedicated eval tool if regression testing is the goal.

Contrast with LangSmith, which is tighter for LangChain stacks but proprietary and cloud-only.

Sources: 0004-langfuse-docs.md

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