diff --git a/index.html b/index.html
index a811fb9..e1b2c09 100644
--- a/index.html
+++ b/index.html
@@ -257,6 +257,7 @@ function renderPerformance(){
     if(sands.fetched_at)notes.push(`Sands Cost 抓取于 ${esc(String(sands.fetched_at).replace('T',' '))}，不随每日自动更新`);
     if(sands.scope)notes.push(`Cost 范围：${esc(sands.scope)}`);
     if(sands.carried_over)notes.push('<b style="color:#b45309">Cost 沿用上次抓取值，需本地重新抓取才会更新</b>');
+    if(sands.stale)notes.push(`<b style="color:#b42318">Cost 数据滞后：抓取于 ${esc(sands.fetched_date||'')}，周期到 ${esc((report.period||{}).end||'')}，滞后约 ${num(sands.lag_days)} 天，GP 会偏高</b>`);
     if(Array.isArray(sands.missing_pkgs)&&sands.missing_pkgs.length)
       notes.push(`<b style="color:#b42318">Cost 不完整，缺少包：${esc(sands.missing_pkgs.join(', '))}</b>`);
   }
diff --git a/update_report_data.py b/update_report_data.py
index b19ce7d..f9f9123 100644
--- a/update_report_data.py
+++ b/update_report_data.py
@@ -458,6 +458,19 @@ def build_payload(now):
     except Exception as error:
         print(f"  Target 读取失败（{error}）；请把该表共享给服务账号或开放链接查看")
     sands = load_sands_cost()
+    # 判断 Cost 是否滞后于数据周期
+    if sands and sands.get("fetched_at"):
+        try:
+            fetched = datetime.fromisoformat(sands["fetched_at"]).date()
+        except ValueError:
+            fetched = None
+        if fetched:
+            lag = (end_date - fetched).days + 1
+            sands["fetched_date"] = fetched.isoformat()
+            sands["lag_days"] = max(0, lag)
+            sands["stale"] = lag > 0
+            if sands["stale"]:
+                print(f"  警告：Sands Cost 抓取于 {fetched}，可能未覆盖到 {end_date}，滞后约 {lag} 天")
 
     # 消耗表里可能出现 Cost_Client 未覆盖的 Campaign，一并纳入
     for key, _day in wide_costs: