fix: BM25 phrase boost + index freshness verification

src/search/bm25.js

@@ -48,7 +48,7 @@ export function buildBM25Index(documents) {
 
   for (const doc of documents) {
     const terms = tokenize(doc.text);
-    docs.push({ id: doc.id, terms, length: terms.length });
+    docs.push({ id: doc.id, terms, length: terms.length, originalText: doc.text });
     totalLength += terms.length;
 
     const seen = new Set();
@@ -68,11 +68,21 @@ export function buildBM25Index(documents) {
   };
 }
 
+// Phrase boost constants — supplement BM25, never replace it.
+// Exact: full query string found as substring in doc text.
+// Partial: any consecutive two-word query bigram found in doc text.
+const PHRASE_BOOST_EXACT = 5.0;
+const PHRASE_BOOST_PARTIAL = 2.0;
+
 /** Search BM25 index, return sorted {id, score} pairs */
 export function searchBM25(index, query, limit = 5) {
   const queryTerms = tokenize(query);
   if (queryTerms.length === 0) return [];
 
+  // Pre-compute phrase matching inputs once, outside the per-doc loop.
+  const queryLower = query.toLowerCase();
+  const queryWords = queryLower.replace(/[^\w\s-]/g, " ").split(/[\s\-_/]+/).filter((w) => w.length > 1 && !STOP_WORDS.has(w));
+
   const scores = [];
 
   for (const doc of index.docs) {
@@ -96,6 +106,23 @@ export function searchBM25(index, query, limit = 5) {
       score += idf * tfNorm;
     }
 
+    // Phrase boost: supplement BM25 — never replace it.
+    // Only apply when the document already has genuine BM25 relevance.
+    if (score > 0) {
+      const docLower = doc.originalText.toLowerCase();
+      if (docLower.includes(queryLower)) {
+        score += PHRASE_BOOST_EXACT;
+      } else if (queryWords.length >= 2) {
+        for (let i = 0; i < queryWords.length - 1; i++) {
+          const bigram = queryWords[i] + " " + queryWords[i + 1];
+          if (docLower.includes(bigram)) {
+            score += PHRASE_BOOST_PARTIAL;
+            break;
+          }
+        }
+      }
+    }
+
     if (score > 0) scores.push({ id: doc.id, score });
   }
 

workers/src/bm25.ts

@@ -44,6 +44,8 @@ export interface BM25Doc {
   id: string;
   terms: string[];
   length: number;
+  /** Original (pre-tokenization) text, used for phrase-level scoring. */
+  originalText: string;
 }
 
 export interface BM25Index {
@@ -63,7 +65,7 @@ export function buildBM25Index(
 
   for (const doc of documents) {
     const terms = tokenize(doc.text);
-    docs.push({ id: doc.id, terms, length: terms.length });
+    docs.push({ id: doc.id, terms, length: terms.length, originalText: doc.text });
     totalLength += terms.length;
 
     const seen = new Set<string>();
@@ -83,6 +85,12 @@ export function buildBM25Index(
   };
 }
 
+// Phrase boost constants — supplement BM25, never replace it.
+// Exact: full query string found as substring in doc text.
+// Partial: any consecutive two-word query bigram found in doc text.
+const PHRASE_BOOST_EXACT = 5.0;
+const PHRASE_BOOST_PARTIAL = 2.0;
+
 /** Search BM25 index, return sorted {id, score} pairs */
 export function searchBM25(
   index: BM25Index,
@@ -92,6 +100,10 @@ export function searchBM25(
   const queryTerms = tokenize(query);
   if (queryTerms.length === 0) return [];
 
+  // Pre-compute phrase matching inputs once, outside the per-doc loop.
+  const queryLower = query.toLowerCase();
+  const queryWords = queryLower.replace(/[^\w\s-]/g, " ").split(/[\s\-_/]+/).filter((w) => w.length > 1 && !STOP_WORDS.has(w));
+
   const scores: Array<{ id: string; score: number }> = [];
 
   for (const doc of index.docs) {
@@ -119,6 +131,23 @@ export function searchBM25(
       score += idf * tfNorm;
     }
 
+    // Phrase boost: supplement BM25 — never replace it.
+    // Only apply when the document already has genuine BM25 relevance.
+    if (score > 0) {
+      const docLower = doc.originalText.toLowerCase();
+      if (docLower.includes(queryLower)) {
+        score += PHRASE_BOOST_EXACT;
+      } else if (queryWords.length >= 2) {
+        for (let i = 0; i < queryWords.length - 1; i++) {
+          const bigram = queryWords[i] + " " + queryWords[i + 1];
+          if (docLower.includes(bigram)) {
+            score += PHRASE_BOOST_PARTIAL;
+            break;
+          }
+        }
+      }
+    }
+
     if (score > 0) scores.push({ id: doc.id, score });
   }
 

workers/src/zip-baseline-fetcher.ts

@@ -760,8 +760,22 @@ export class ZipBaselineFetcher {
     if (this.env.BASELINE_CACHE) {
       const cached = await this.env.BASELINE_CACHE.get(cacheKey, "json") as BaselineIndex | null;
       if (cached) {
-        // Content-addressed cache hit: SHA matches, content is truthful
-        return cached;
+        // Cloudflare KV is eventually consistent — two requests seconds apart
+        // can hit different edge nodes and return stale data even when the
+        // cache key looks correct. Cross-check the cached index's embedded
+        // commit SHAs against the SHAs we just resolved from the GitHub API.
+        // If they diverge, the cached entry is stale; discard and rebuild.
+        const baselineShaMatch = !baselineSha || cached.commit_sha === baselineSha;
+        const canonShaMatch = !canonSha || cached.canon_commit_sha === canonSha;
+        if (baselineShaMatch && canonShaMatch) {
+          // Content-addressed cache hit: SHA verified, content is truthful.
+          return cached;
+        }
+        console.warn(
+          `KV cache SHA mismatch — discarding stale index. ` +
+          `cached=${cached.commit_sha}/${cached.canon_commit_sha} ` +
+          `resolved=${baselineSha}/${canonSha ?? "none"}`
+        );
       }
     }