Fix hybrid RRF weighting by query type

CHANGELOG.md

@@ -9,6 +9,7 @@
   Scoped `--force` clears only collection-owned vectors, preserves shared
   hashes referenced by sibling collections, and drops `vectors_vec` only
   when the scoped clear empties all vectors.
+- Hybrid search: weight RRF lists by query type so original FTS and original vector evidence get the intended 2x boost, instead of accidentally boosting the first lexical expansion. #591
 - GPU: respect explicit `QMD_LLAMA_GPU=metal|vulkan|cuda` backend overrides instead of always using auto GPU selection. #529
 - Fix: preserve original filename case in `handelize()`. The previous
   `.toLowerCase()` call made indexed paths unreachable on case-sensitive

src/store.ts

@@ -4158,6 +4158,21 @@ export type RankedListMeta = {
   query: string;
 };
 
+/**
+ * RRF list weights for hybridQuery.
+ *
+ * Original-query retrieval paths are the primary evidence and get 2x weight:
+ * - original FTS
+ * - original vector search
+ *
+ * Expansion-derived lists (lex/vec/hyde) stay at 1x regardless of list order,
+ * so a lex expansion inserted before original vector search cannot steal the
+ * original vector boost.
+ */
+export function getHybridRrfWeights(rankedListMeta: RankedListMeta[]): number[] {
+  return rankedListMeta.map(meta => meta.queryType === "original" ? 2.0 : 1.0);
+}
+
 /**
  * Hybrid search: BM25 + vector + query expansion + RRF + chunked reranking.
  *
@@ -4289,8 +4304,9 @@ export async function hybridQuery(
     }
   }
 
-  // Step 4: RRF fusion — first 2 lists (original FTS + first vec) get 2x weight
-  const weights = rankedLists.map((_, i) => i < 2 ? 2.0 : 1.0);
+  // Step 4: RRF fusion — original-query FTS and vector lists get 2x weight;
+  // expansion-derived lists stay at 1x independent of insertion order.
+  const weights = getHybridRrfWeights(rankedListMeta);
   const fused = reciprocalRankFusion(rankedLists, weights);
   const rrfTraceByFile = explain ? buildRrfTrace(rankedLists, weights, rankedListMeta) : null;
   const candidates = fused.slice(0, candidateLimit);

test/store.test.ts

@@ -52,10 +52,12 @@ import {
   insertDocument,
   generateEmbeddings,
   reindexCollection,
+  getHybridRrfWeights,
   type Store,
   type DocumentResult,
   type SearchResult,
   type RankedResult,
+  type RankedListMeta,
 } from "../src/store.js";
 import type { CollectionConfig } from "../src/collections.js";
 
@@ -2046,6 +2048,38 @@ describe("Reciprocal Rank Fusion", () => {
     expect(fused[0]!.file).toBe("doc1");
   });
 
+  test("hybrid RRF weights boost original vector evidence over expansion-only hits", () => {
+    const originalFtsOnly = makeResult("original-fts-only.md", 0.95);
+    const expansionOnly = makeResult("lex-expansion-only.md", 0.95);
+    const originalVector = makeResult("original-vector.md", 0.95);
+
+    // Mirrors hybridQuery's common list order when a lex expansion exists:
+    // original FTS, lex expansion FTS, original vector.
+    const rankedLists = [
+      [originalFtsOnly],
+      [expansionOnly],
+      [originalVector],
+    ];
+    const rankedListMeta: RankedListMeta[] = [
+      { source: "fts", queryType: "original", query: "user query" },
+      { source: "fts", queryType: "lex", query: "lex expansion" },
+      { source: "vec", queryType: "original", query: "user query" },
+    ];
+
+    const positionBasedWeights = rankedLists.map((_, i) => i < 2 ? 2.0 : 1.0);
+    const buggyOrder = reciprocalRankFusion(rankedLists, positionBasedWeights);
+
+    expect(buggyOrder.findIndex(r => r.file === "lex-expansion-only.md"))
+      .toBeLessThan(buggyOrder.findIndex(r => r.file === "original-vector.md"));
+
+    const semanticWeights = getHybridRrfWeights(rankedListMeta);
+    const fixedOrder = reciprocalRankFusion(rankedLists, semanticWeights);
+
+    expect(semanticWeights).toEqual([2.0, 1.0, 2.0]);
+    expect(fixedOrder.findIndex(r => r.file === "original-vector.md"))
+      .toBeLessThan(fixedOrder.findIndex(r => r.file === "lex-expansion-only.md"));
+  });
+
   test("RRF adds top-rank bonus", () => {
     // doc1 is #1 in list1, doc2 is #2 in list1
     const list1 = [makeResult("doc1", 0.9), makeResult("doc2", 0.8)];