Route by query shape

Two retrievers, one regex. A query that looks like a symbol goes to BM25; a sentence goes to the dense index. Hybrid fusion and reranking stay behind flags — both were measured and both lost.

Setting	Recall@8	MRR	TR prose R@8	p50
BM25 only	0.405	0.240	0.263	2 ms
hybrid (dense + BM25 → RRF)	0.786	0.604	0.684	40 ms
dense only	0.786	0.678	0.684	32 ms
auto: symbol → BM25, prose → dense ✓	0.786	0.690	0.684	34 ms

Why always-hybrid loses

RRF adds up ranks. When one channel has nothing useful — dense on QUEUE_NAMES, BM25 on "how does auth work" — its best guess is still promoted at full strength and pollutes the top of the list. Routing removes the channel that would have guessed.

What counts as a symbol

One token with an internal boundary: camelCase, snake_case, kebab-case, a.b.c, a/b, A::B, or ALLCAPS. Deliberately narrow — a false positive sends a real question to BM25, which is measurably bad; a false negative only forgoes a small win. Symbols score 1.0 / 1.0 on BM25 and 1.0 / 0.875 on dense.