# Change: RAG hybrid search with RRF

## Why

Delivery 1 fixed chunk identity/dedup. Delivery 2 enables multi-path retrieval fused by RRF so category filters no longer whole-replace good results and lexical signals can complement dense ranks.

True Milvus sparse/BM25 schema migration remains staged: this change ships a **working hybrid mode on KnowledgeSearchPort** using multi-path dense (+ lexical rank path) and RRF, without thickening the legacy SDK API surface.

## What Changes

- `retrieval.search.mode=dense|hybrid` (default dense)
- Hybrid search on `KnowledgeSearchPort`:
  - dense unfiltered path
  - dense filtered path when category present
  - lexical rank path over the candidate union (sparse-lite until BM25 schema lands)
  - RRF / optional weighted RRF fusion by evidenceKey
- Preserve dense L2 scores for quality thresholds; RRF only orders
- Lookup tool uses search mode; filter low-quality retry remains as safety net
- Config for rrf-k and path weights
- Tests for RRF fusion ordering and hybrid adapter behavior

## Non-goals

- Full Milvus BM25/sparse collection rebuild (documented follow-up)
- Deleting legacy SDK mode entirely
- Cross-encoder model rerank
- Changing Agent tool schema name/input

## Capabilities

### New Capabilities

- `rag-hybrid-search`: hybrid mode, multi-path recall, RRF fusion via search port

### Modified Capabilities

- `rag-chunk-evidence-identity`: hybrid hits must keep chunk identity
- `rag-knowledge-retrieval`: filtered/unfiltered cooperation via fusion rather than only serial replace

## Impact

- Code: retrieval package, lookup wiring/config, tests
- Runtime default remains dense unless hybrid enabled
- Interface: L2 internal (search port behavior)

## Depends on

- Archived Delivery 1: `rag-chunk-evidence-identity-dedup`