RAG Failure Diagnosis

When a RAG answer is wrong, walk the layers backward from output to root cause. Most "the LLM hallucinated" reports are actually upstream failures that surface at generation.

Critical principle: don't fix downstream when upstream is broken. Tuning the reranker won't help if the parser dropped the table.

The diagnostic chain

For any wrong answer, walk in this order. The first layer where you find a problem is your root cause — stop walking there.