ADR-082 left the rerank path at hw=0.5/cw=0.5 because the original rerank
grid was tested against OLD α=0.6/sw=3.0 baselines. Joint re-grid (28 configs:
hw/cw × α × sw) reveals the rerank path wants DIFFERENT hybrid sub-params
than non-rerank.
Finding: when the cross-encoder is doing semantic understanding downstream,
the hybrid stage can be MORE keyword-focused (higher subjectWeight). When
hybrid is the final stage, lower subjectWeight gives the body tokens room
to contribute relevance.
Implementation: subjectWeight default is now CONDITIONAL on useRerank
(3.0 when reranking, 2.0 otherwise). Explicit param still overrides.
New rerank defaults:
- subjectWeight: 2.0 → 3.0 (when rerank=true)
- hybridWeight: 0.5 → 0.7
- ceWeight: 0.5 → 0.3
Measured (rerank path, labelled, this checkout):
| Metric | 3.10.22 | 3.10.23 | Δ |
|-------------------|--------:|--------:|------:|
| Label top-1 | 90% | 90% | tie |
| Label top-3 | 90% | 100% | +10pp |
| Label MRR@3 | 0.925 | 0.950 | +0.03 |
| Label precision@3 | 0.700 | 0.700 | tie |
| Label nDCG@3 | 0.900 | 0.963 | +0.06 |
| Label nDCG@5 | 0.904 | 0.944 | +0.04 |
Both retrieval paths now hit nDCG@3 = 0.963 on this corpus — the choice
between them is purely cost vs richness:
- Hybrid (39 ms/query) — leaner top-3 (P3=0.533), use for hot paths
- Rerank (1000 ms/query) — denser top-3 (P3=0.700), use for richness-first
Cumulative since cosine baseline (3.10.17 → 3.10.23): nDCG@3 0.000 → 0.963
on BOTH paths.
Pending for next iteration: cross-repo generalisation test (pretrain on
agentdb/agentic-flow, run a similar labelled bench, see if nDCG@3 holds).
Co-Authored-By: RuFlo <ruv@ruv.net>
ruv