diff --git a/backend/app/api/settings.py b/backend/app/api/settings.py
index 305b04d..299035c 100644
--- a/backend/app/api/settings.py
+++ b/backend/app/api/settings.py
@@ -1666,7 +1666,9 @@ async def test_endpoint(req: TestEndpointIn) -> dict:
     import statistics
 
     base = req.url.rstrip("/")
-    rounds = max(1, min(10, req.rounds or _endpoints_cache.get("data", {}).get("testRounds", 5)))
+    # 缓存初值的 "data" 是 None(键存在, get 的默认值不生效), 端点清单未预热时要兜底
+    manifest = _endpoints_cache.get("data") or {}
+    rounds = max(1, min(10, req.rounds or manifest.get("testRounds", 5)))
     health_url = base + "/health"
 
     latencies: list[float] = []
diff --git a/backend/tests/test_endpoint_latency_cold_cache.py b/backend/tests/test_endpoint_latency_cold_cache.py
new file mode 100644
index 0000000..ca1e531
--- /dev/null
+++ b/backend/tests/test_endpoint_latency_cold_cache.py
@@ -0,0 +1,48 @@
+"""端点测速接口: 端点清单缓存未预热时的默认轮数不能崩。
+
+_endpoints_cache 的初值是 {"ts": 0.0, "data": None} —— 进程启动后直到第一次
+GET /api/settings/endpoints 之前, "data" 键存在但值为 None。前端 endpoints 查询
+有 5 分钟 staleTime, 后端(看门狗自愈/重启)重启后用户再点测速, 就落在这个窗口里。
+"""
+from __future__ import annotations
+
+import pytest
+
+from app.api import settings as settings_api
+
+
+@pytest.fixture(autouse=True)
+def _cold_cache_and_fake_ping(monkeypatch):
+    """还原进程刚启动时的缓存初值, 并把网络探测换成固定延迟。"""
+    monkeypatch.setattr(settings_api, "_endpoints_cache", {"ts": 0.0, "data": None})
+
+    async def _ping(url: str, timeout: float = 10.0) -> float:
+        return 12.5
+
+    monkeypatch.setattr(settings_api, "_http_ping", _ping)
+
+
+async def test_latency_test_falls_back_to_default_rounds_on_cold_cache():
+    """缓存未预热且请求不带 rounds 时, 回退到默认 5 轮而不是 500。"""
+    result = await settings_api.test_endpoint(
+        settings_api.TestEndpointIn(url="https://api.example.com"),
+    )
+
+    assert result["ok"] is True
+    assert result["rounds"] == 5
+    assert result["success"] == 5
+    assert result["median_ms"] == 12.5
+
+
+async def test_latency_test_uses_manifest_rounds_when_cache_is_warm(monkeypatch):
+    """缓存已预热时仍取 endpoints.json 的 testRounds。"""
+    monkeypatch.setattr(
+        settings_api, "_endpoints_cache", {"ts": 0.0, "data": {"testRounds": 2}},
+    )
+
+    result = await settings_api.test_endpoint(
+        settings_api.TestEndpointIn(url="https://api.example.com"),
+    )
+
+    assert result["rounds"] == 2
+    assert result["success"] == 2