diff --git a/tools/build_common_listings.js b/tools/build_common_listings.js
new file mode 100644
index 0000000..bb6cb5b
--- /dev/null
+++ b/tools/build_common_listings.js
@@ -0,0 +1,346 @@
+#!/usr/bin/env node
+"use strict";
+
+/*
+  Build a report of canonical SKUs and how many STORES carry each one.
+  - "Store" means storeLabel, NOT category. We union across categories per store.
+  - Uses sku_map canonicalization (same as alert tool).
+  - Writes: reports/common_listings.json
+  - Prints debug while scanning.
+
+  Usage:
+    node tools/build_common_listings.js [--top 50] [--min-stores 3] [--prefer-real-sku] [--require-all]
+
+  Notes:
+  - If --require-all is set, output includes only SKUs present in ALL stores (often empty).
+  - Otherwise, outputs top N by store coverage.
+*/
+
+const fs = require("fs");
+const path = require("path");
+
+/* ---------------- helpers ---------------- */
+
+function ensureDir(dir) {
+  fs.mkdirSync(dir, { recursive: true });
+}
+
+function readJson(filePath) {
+  try {
+    return JSON.parse(fs.readFileSync(filePath, "utf8"));
+  } catch {
+    return null;
+  }
+}
+
+function listDbFilesOnDisk() {
+  const dir = path.join(process.cwd(), "data", "db");
+  try {
+    return fs
+      .readdirSync(dir, { withFileTypes: true })
+      .filter((e) => e.isFile() && e.name.endsWith(".json"))
+      .map((e) => path.join(dir, e.name));
+  } catch {
+    return [];
+  }
+}
+
+function priceToNumber(v) {
+  const s = String(v ?? "").replace(/[^0-9.]/g, "");
+  const n = Number(s);
+  return Number.isFinite(n) ? n : null;
+}
+
+function hasRealSku6(s) {
+  return /\b\d{6}\b/.test(String(s || ""));
+}
+
+function isSyntheticSkuKey(k) {
+  return String(k || "").startsWith("u:");
+}
+
+/* ---------------- sku map + normalization ---------------- */
+
+function loadSkuMapOrNull() {
+  try {
+    // eslint-disable-next-line node/no-missing-require
+    const { loadSkuMap } = require(path.join(process.cwd(), "src", "utils", "sku_map"));
+    return loadSkuMap({ dbDir: path.join(process.cwd(), "data", "db") });
+  } catch {
+    return null;
+  }
+}
+
+function normalizeSkuKeyOrEmpty({ skuRaw, storeLabel, url }) {
+  try {
+    // eslint-disable-next-line node/no-missing-require
+    const { normalizeSkuKey } = require(path.join(process.cwd(), "src", "utils", "sku"));
+    const k = normalizeSkuKey(skuRaw, { storeLabel, url });
+    return k ? String(k) : "";
+  } catch {
+    // fallback: a 6-digit SKU if present, else synthetic from URL
+    const m = String(skuRaw ?? "").match(/\b(\d{6})\b/);
+    if (m) return m[1];
+    if (url) return `u:${String(storeLabel || "").toLowerCase()}:${String(url || "").toLowerCase()}`;
+    return "";
+  }
+}
+
+function canonicalize(skuKey, skuMap) {
+  if (!skuKey) return "";
+  if (skuMap && typeof skuMap.canonicalSku === "function") return String(skuMap.canonicalSku(skuKey) || skuKey);
+  return skuKey;
+}
+
+/* ---------------- args ---------------- */
+
+function parseArgs(argv) {
+  const out = {
+    top: 50,
+    minStores: 2,
+    preferRealSku: true,
+    requireAll: false,
+  };
+
+  for (let i = 0; i < argv.length; i++) {
+    const a = argv[i];
+    if (a === "--top" && argv[i + 1]) out.top = Math.max(1, Number(argv[++i]) || 50);
+    else if (a === "--min-stores" && argv[i + 1]) out.minStores = Math.max(1, Number(argv[++i]) || 2);
+    else if (a === "--prefer-real-sku") out.preferRealSku = true;
+    else if (a === "--no-prefer-real-sku") out.preferRealSku = false;
+    else if (a === "--require-all") out.requireAll = true;
+  }
+  return out;
+}
+
+/* ---------------- main ---------------- */
+
+function main() {
+  const args = parseArgs(process.argv.slice(2));
+  const repoRoot = process.cwd();
+  const reportsDir = path.join(repoRoot, "reports");
+  ensureDir(reportsDir);
+
+  const dbFiles = listDbFilesOnDisk();
+  if (!dbFiles.length) {
+    console.error("No DB files found under data/db");
+    process.exitCode = 2;
+    return;
+  }
+
+  const skuMap = loadSkuMapOrNull();
+  console.log(`[debug] skuMap: ${skuMap ? "loaded" : "NOT loaded (will use raw sku keys)"}`);
+  console.log(`[debug] scanning ${dbFiles.length} db files...`);
+
+  // storeLabel -> Set(canonSku) (union across categories)
+  const storeToCanon = new Map();
+
+  // canonSku -> aggregate
+  const canonAgg = new Map(); // canonSku -> { stores:Set, listings:[], cheapest:{priceNum,item}|null }
+
+  const EXCLUDED_STORE_LABELS = new Set(["gull", "legacy", "strath", "vessel", "tudor"]);
+
+  let liveRows = 0;
+  let removedRows = 0;
+  let skippedNoSku = 0;
+
+  for (const abs of dbFiles.sort()) {
+    const obj = readJson(abs);
+    if (!obj) {
+      console.log(`[debug] skip unreadable: ${path.relative(repoRoot, abs)}`);
+      continue;
+    }
+
+    const storeLabel = String(obj.storeLabel || obj.store || "").trim();
+    const categoryLabel = String(obj.categoryLabel || obj.category || "").trim();
+    const rel = path.relative(repoRoot, abs).replace(/\\/g, "/");
+
+    if (!storeLabel) {
+      console.log(`[debug] skip no-storeLabel: ${rel}`);
+      continue;
+    }
+
+    if (!storeToCanon.has(storeLabel)) storeToCanon.set(storeLabel, new Set());
+
+    const items = Array.isArray(obj.items) ? obj.items : [];
+    console.log(`[debug] file ${rel} | store="${storeLabel}" cat="${categoryLabel}" items=${items.length}`);
+
+    for (const it of items) {
+      if (!it) continue;
+      if (Boolean(it.removed)) {
+        removedRows++;
+        continue;
+      }
+      liveRows++;
+
+      const skuRaw = String(it.sku || "");
+      const url = String(it.url || "");
+      const skuKey = normalizeSkuKeyOrEmpty({ skuRaw, storeLabel, url });
+      if (!skuKey) {
+        skippedNoSku++;
+        continue;
+      }
+      const canonSku = canonicalize(skuKey, skuMap);
+      if (!canonSku) {
+        skippedNoSku++;
+        continue;
+      }
+
+      storeToCanon.get(storeLabel).add(canonSku);
+
+      let agg = canonAgg.get(canonSku);
+      if (!agg) {
+        agg = { stores: new Set(), listings: [], cheapest: null };
+        canonAgg.set(canonSku, agg);
+      }
+      agg.stores.add(storeLabel);
+
+      const priceStr = String(it.price || "");
+      const priceNum = priceToNumber(priceStr);
+
+      const listing = {
+        canonSku,
+        skuKey,
+        skuRaw,
+        name: String(it.name || ""),
+        price: priceStr,
+        priceNum,
+        url,
+        img: String(it.img || it.image || it.thumb || ""),
+        storeLabel,
+        categoryLabel,
+        dbFile: rel,
+        hasRealSku6: hasRealSku6(skuRaw) && !isSyntheticSkuKey(skuKey),
+        excludedStore: EXCLUDED_STORE_LABELS.has(String(storeLabel || "").toLowerCase()),
+      };
+
+      agg.listings.push(listing);
+
+      if (priceNum !== null) {
+        if (!agg.cheapest || priceNum < agg.cheapest.priceNum) {
+          agg.cheapest = { priceNum, item: listing };
+        }
+      }
+    }
+  }
+
+  const stores = [...storeToCanon.keys()].sort((a, b) => a.localeCompare(b));
+  const storeCount = stores.length;
+
+  console.log(`[debug] stores=${storeCount} (${stores.join(", ")})`);
+  console.log(
+    `[debug] liveRows=${liveRows} removedRows=${removedRows} skippedNoSku=${skippedNoSku} canonSkus=${canonAgg.size}`
+  );
+
+  function pickRepresentative(agg) {
+    // prefer: real 6-digit + non-excluded store + cheapest among those
+    const candidates = agg.listings.slice();
+
+    const byPrice = (a, b) => {
+      const ap = a.priceNum;
+      const bp = b.priceNum;
+      if (ap === null && bp === null) return 0;
+      if (ap === null) return 1;
+      if (bp === null) return -1;
+      return ap - bp;
+    };
+
+    const preferred = candidates
+      .filter((x) => x.hasRealSku6 && !x.excludedStore)
+      .sort(byPrice);
+
+    if (args.preferRealSku && preferred.length) return preferred[0];
+
+    // else: cheapest overall if available
+    if (agg.cheapest && agg.cheapest.item) return agg.cheapest.item;
+
+    // else: deterministic fallback
+    candidates.sort((a, b) => {
+      const ak = `${a.storeLabel}|${a.name}|${a.url}`;
+      const bk = `${b.storeLabel}|${b.name}|${b.url}`;
+      return ak.localeCompare(bk);
+    });
+    return candidates[0] || null;
+  }
+
+  const rows = [];
+  for (const [canonSku, agg] of canonAgg.entries()) {
+    const rep = pickRepresentative(agg);
+    rows.push({
+      canonSku,
+      storeCount: agg.stores.size,
+      stores: [...agg.stores].sort((a, b) => a.localeCompare(b)),
+      representative: rep
+        ? {
+            name: rep.name,
+            price: rep.price,
+            priceNum: rep.priceNum,
+            storeLabel: rep.storeLabel,
+            skuRaw: rep.skuRaw,
+            skuKey: rep.skuKey,
+            url: rep.url,
+            dbFile: rep.dbFile,
+            categoryLabel: rep.categoryLabel,
+          }
+        : null,
+      cheapest: agg.cheapest
+        ? {
+            price: agg.cheapest.item.price,
+            priceNum: agg.cheapest.priceNum,
+            storeLabel: agg.cheapest.item.storeLabel,
+            url: agg.cheapest.item.url,
+          }
+        : null,
+    });
+  }
+
+  // Sort by coverage desc, then cheapest asc, then canonSku
+  rows.sort((a, b) => {
+    if (b.storeCount !== a.storeCount) return b.storeCount - a.storeCount;
+    const ap = a.cheapest ? a.cheapest.priceNum : null;
+    const bp = b.cheapest ? b.cheapest.priceNum : null;
+    if (ap !== null && bp !== null && ap !== bp) return ap - bp;
+    if (ap !== null && bp === null) return -1;
+    if (ap === null && bp !== null) return 1;
+    return String(a.canonSku).localeCompare(String(b.canonSku));
+  });
+
+  const allStoresRows = rows.filter((r) => r.storeCount === storeCount);
+  const filtered = args.requireAll ? allStoresRows : rows.filter((r) => r.storeCount >= args.minStores);
+  const top = filtered.slice(0, args.top);
+
+  console.log(
+    `[debug] all-stores=${allStoresRows.length} minStores>=${args.minStores} filtered=${filtered.length} top=${top.length}`
+  );
+  if (top.length) {
+    console.log("[debug] sample:");
+    for (const r of top.slice(0, Math.min(10, top.length))) {
+      const rep = r.representative;
+      console.log(
+        `  - stores=${r.storeCount}/${storeCount} canon=${r.canonSku} | rep="${rep?.name || "?"}" ${rep?.price || ""} @ ${rep?.storeLabel || "?"}`
+      );
+    }
+  }
+
+  const payload = {
+    generatedAt: new Date().toISOString(),
+    args,
+    storeCount,
+    stores,
+    totals: {
+      liveRows,
+      removedRows,
+      skippedNoSku,
+      canonSkus: canonAgg.size,
+      allStores: allStoresRows.length,
+      outputCount: top.length,
+    },
+    rows: top,
+  };
+
+  const outPath = path.join(reportsDir, "common_listings.json");
+  fs.writeFileSync(outPath, JSON.stringify(payload, null, 2) + "\n", "utf8");
+  console.log(`Wrote ${path.relative(repoRoot, outPath)} (${top.length} rows)`);
+}
+
+main();