Introduction In Chapter 2 (Evals) , we measured answer quality . Now we add Observability — making behavior visible . [Evals] Measure answer correctness with scores → quality measurement [Observability] Which tools were called, how many times, how long did each take → behavior visualization We'll use Langfuse v4 , an open-source observability tool. It has a free cloud tier and can also be self-hosted. Note: Langfuse v4 (from March 2026) has a significantly change...