diff --git a/backend/app/cache.py b/backend/app/cache.py
index 185400c..6fcf631 100644
--- a/backend/app/cache.py
+++ b/backend/app/cache.py
@@ -65,7 +65,8 @@ async def cache_set(key: str, value: Any, ttl: int) -> None:
     if c is None:
         return
     try:
-        await c.set(key, json.dumps(value, ensure_ascii=False), ex=max(1, ttl))
+        # default=str：value 里混入 datetime/date 也不炸（炸了会触发 _bail 毒死整个缓存层）
+        await c.set(key, json.dumps(value, ensure_ascii=False, default=str), ex=max(1, ttl))
     except Exception:  # noqa: BLE001
         _bail()
 
diff --git a/backend/app/data/market_overview.py b/backend/app/data/market_overview.py
index cbdb151..033056a 100644
--- a/backend/app/data/market_overview.py
+++ b/backend/app/data/market_overview.py
@@ -52,8 +52,9 @@ def _f(v) -> float | None:
     return None if math.isnan(f) else f
 
 
-def _d(v) -> date | None:
-    return datetime.strptime(str(v), "%Y%m%d").date() if v else None
+def _d(v) -> str | None:
+    """YYYYMMDD -> 'YYYY-MM-DD'（字符串便于 JSON 缓存；pydantic 响应模型自动 coerce）。"""
+    return datetime.strptime(str(v), "%Y%m%d").date().isoformat() if v else None
 
 
 def _get_pro():
@@ -152,7 +153,7 @@ async def fetch_overview() -> dict:
         errors.append(f"两市统计: {str(e)[:60]}")
 
     payload = {
-        "updated_at": datetime.now(),
+        "updated_at": datetime.now().isoformat(),  # 字符串：cache_set 走 json.dumps
         "indexes": indexes,
         "stats": stats,
         "errors": errors,
diff --git a/backend_run.log b/backend_run.log
index 6784f1c..7aa2b95 100644
--- a/backend_run.log
+++ b/backend_run.log
@@ -4,3 +4,8 @@ INFO:     Started reloader process [40700] using WatchFiles
 INFO:     Started server process [42044]
 INFO:     Waiting for application startup.
 INFO:     Application startup complete.
+INFO:     127.0.0.1:50860 - "GET /api/health HTTP/1.1" 200 OK
+INFO:     127.0.0.1:50992 - "GET /api/market/overview HTTP/1.1" 401 Unauthorized
+INFO:     127.0.0.1:50994 - "GET /api/market/overview HTTP/1.1" 401 Unauthorized
+WARNING:  WatchFiles detected changes in 'app\data\market_overview.py'. Reloading...
+ 
\ No newline at end of file