The reranker that hurt

The textbook says: retrieve 40, rerank with a cross-encoder, hand over 8. On this corpus the textbook was wrong, and the numbers said so.

Setting	Recall@8	MRR	TR prose	p50
auto + dense (no rerank)	0.786	0.690	0.684	34 ms
hybrid, k=40 candidates	0.952	—	0.895	39 ms
hybrid + bge-reranker-v2-m3	0.762	0.508	0.579	4389 ms
auto + bge-reranker-v2-m3	0.762	0.514	0.579	2050 ms

• The headroom is real. Recall@40 is 0.95 — the right chunk is almost always in the candidate pool. A good reranker has 17 points to win.
• This reranker spent them. MRR dropped from 0.690 to 0.514 and the Turkish slice fell furthest. It also cost two to four seconds per query.
• As a "no answer" gate it was worse. It caught 12/12 negatives — and flagged 12/42 real answers as junk. A 29% false alarm rate.