diff --git a/backend/app/services/trading_day.py b/backend/app/services/trading_day.py
index bfca3f9..8079241 100644
--- a/backend/app/services/trading_day.py
+++ b/backend/app/services/trading_day.py
@@ -113,9 +113,11 @@ def is_trading_day(now: datetime | None = None) -> bool | None:
         return False
 
     with _CACHE_LOCK:
+        # 「未知」(None) 也是一个结论, 同样按 TTL 缓存 —— 它正是 _TTL_UNKNOWN_S 要
+        # 挡住的场景 (未配 fuyao 且 tickflow 不可用时, 轮询每拍都会重打一次探测)。
+        # _CACHE.day 只在探测写回时设置, 因此「当天已探过」用它判定即可。
         if (
             _CACHE.day == now.date()
-            and _CACHE.verdict is not None
             and (time.monotonic() - _CACHE.probed_at) < _ttl_of(_CACHE.verdict)
         ):
             return _CACHE.verdict
diff --git a/backend/tests/test_trading_day.py b/backend/tests/test_trading_day.py
index 586c8a0..01508d9 100644
--- a/backend/tests/test_trading_day.py
+++ b/backend/tests/test_trading_day.py
@@ -254,3 +254,25 @@ def test_fuyao_provider_trading_days_conversion(monkeypatch):
     monkeypatch.setattr(fp, "get_api_key", lambda: "test-key")
     days = FuyaoProvider().trading_days()
     assert days == {date(2026, 9, 4), date(2026, 9, 7)}
+
+
+def test_unknown_verdict_is_cached_within_short_ttl(monkeypatch):
+    """未知结论也要按 _TTL_UNKNOWN_S 缓存: 轮询每拍重探会重复打 tickflow 请求。"""
+    monday = datetime(2026, 9, 7, 10, 0, tzinfo=CN)
+    calls = {"fuyao": 0, "tickflow": 0}
+
+    def _fuyao(now):
+        calls["fuyao"] += 1
+        return None
+
+    def _tickflow(now):
+        calls["tickflow"] += 1
+        return None
+
+    monkeypatch.setattr(trading_day, "_probe_fuyao", _fuyao)
+    monkeypatch.setattr(trading_day, "_probe_tickflow", _tickflow)
+
+    assert is_trading_day(monday) is None
+    assert is_trading_day(monday) is None
+    assert is_trading_day(monday) is None
+    assert calls == {"fuyao": 1, "tickflow": 1}