The four breakage classes — sort any LLM failure before you touch the prompt — step 8 of 9

Write classify_failure(trace) that returns one of "retrieval", "prompt", "hallucination", or "parse".

Apply the priority order from the chapter:

1. If trace["retrieved_chunks"] is non-empty AND trace["retrieved_chunks_match_query"] is False → "retrieval".
2. Else if trace["raw_output"] != trace["output_after_postprocess"] → "parse".
3. Else if trace["retrieved_chunks"] is empty (the list []) → "hallucination".
4. Else → "prompt".

Three traces run for you, one per branch. Expected output:

retrieval
parse
hallucination