## Bug
config.rag.score_gate default 0.05 was incompatible with hybrid RRF
fusion_score: raw RRF tops out at num_retrievers / (k_rrf + 1) ≈
0.0328 at the default k_rrf=60, so every hybrid `kb ask` tripped
ScoreGate refusal even when the top hit was perfectly aligned across
both retrievers. Symptomatic on the post-P4-3 manual smoke at
/tmp/kb-smoke/ pointed at 192.168.0.47 Ollama:
$ kb ask "Rust ownership 모델의 핵심 규칙은 뭐야?" --mode hybrid
근거 부족. KB에 해당 내용 없음. # top fusion_score = 0.0164
Per-mode score_gate (lexical_score_gate / vector_score_gate /
hybrid_score_gate) was rejected because it forces every consumer
(CLI, eval, TUI) to know which mode picks which threshold. Score
normalization solves it at the source.
## Fix
crates/kb-search/src/hybrid.rs divides every fused score by
2 / (k_rrf + 1), the theoretical RRF maximum with two retrievers
each contributing rank 1. After normalization:
- both retrievers agree on rank 1 → fusion_score = 1.0
- only one retriever finds the chunk → caps near 0.5
- typical mixed ranks → falls between 0 and 0.5
RRF's rank-ordering invariants are preserved (every score divides
by the same positive constant), so sort + tiebreak behaviour is
unchanged. Wire schema label `fusion_score` keeps its slot in
RetrievalDetail; only the magnitude shifts, and only for hybrid
mode (lexical / vector were already in [0, 1]).
Verification: re-ran the four-scenario smoke at /tmp/kb-smoke/ with
default score_gate = 0.05 — all four (Korean→Korean, English→
English, cross-language Korean↔English, out-of-corpus) succeed
with the expected grounded / refusal classification, top
fusion_score now ≈ 0.5.
## Tests
One unit test (rrf_formula_matches_known_value) updated to expect
the normalized value `(1/61 + 1/62) / (2/61) ≈ 0.9919` instead of
the raw `1/61 + 1/62 ≈ 0.0325`. The integration snapshot fixture
crates/kb-search/tests/fixtures/search/hybrid/run-1.json already
used presence checks (fusion_score_positive: true) rather than
absolute values, so it doesn't need regeneration. Workspace 319
tests pass; clippy clean across both feature configs.
## Docs
This commit also adds tasks/HOTFIXES.md as a dated post-merge log
covering this fix and the two earlier --config-flag regressions
(P3-5 hotfix #20 across ingest/search/list/inspect/doctor; P4-3
follow-up #24 for kb ask). Original task specs in tasks/p<N>/
*.md stay frozen as the historical contract; HOTFIXES.md is the
live source of truth for post-merge deltas. Each affected task
spec gets a "Risks/notes" addendum pointing back to HOTFIXES.md
so a reader landing on the spec sees the active behaviour:
- tasks/INDEX.md gains a "Post-merge 핫픽스" section.
- tasks/phase-3-vector-hybrid.md updates the RRF formula text to
show the normalized form.
- tasks/p3/p3-4-hybrid-fusion.md "Behavior contract" RRF bullet
notes the normalization and reason.
- tasks/p3/p3-5-app-wiring.md "Risks/notes" notes the --config
fix.
- tasks/p4/p4-3-rag-pipeline.md "Risks/notes" notes the kb-ask
--config fix and the score_gate-RRF incompatibility (closed by
the normalization in p3-4).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
