Adds a rerank stage between the hybrid RRF fusion and the final top_k cut.
RRF is a heuristic over ranks — it can't tell which of two equally-ranked
candidates is actually more relevant. A cross-encoder scores each
(query, passage) pair directly and produces a true relevance order.
- voyage.rerank(query, documents, top_k) → list of {index, relevance_score}
sorted high→low. Defaults to the model in VOYAGE_RERANK_MODEL
(rerank-2.5, 32K tokens per pair, multilingual incl. Hebrew).
- search pulls top-30 from RRF, passes them to rerank, then returns the
reranker's top-k. Each returned item gets a rerank_score field.
- Fallback: if the rerank API errors (auth, 5xx, timeout), fall back to
the RRF ordering and log a warning — degraded precision beats outage.
- KB_RERANK_DISABLED=1 bypasses rerank for debugging.
Refs Task Master #2
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
chaim
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