WRITING / RAG / Evaluation
What I Learned Building My First RAG Evaluator
The mistakes, metrics, and architecture changes that made evaluation useful instead of decorative.
RAGEvaluation
- FORMAT
- lesson
- READ
- 7 min
- PUBLISHED
- Aug 27, 2026
- TOPICS
- RAG · Evaluation
THE TAKEAWAYAn evaluator becomes useful when its failures point to a specific system stage and a specific next action.
I started with one score
My first evaluator reduced every answer to a single quality number. It was easy to chart and almost impossible to debug.
- Retrieval failures looked like generation failures
- Averages hid hard query groups
- The score did not suggest a fix
The architecture changed
I split evaluation into retrieval, context, answer, citation, latency, and cost stages, then attached traces and dataset slices to every run.
- Stage-level metrics
- Versioned examples
- Side-by-side run comparison
What I use now
A smaller evaluation set with clear failure labels gives me more engineering value than a huge set of opaque scores.