diff --git a/apps/api/tests/test_p0_diagnose_routing.py b/apps/api/tests/test_p0_diagnose_routing.py
new file mode 100644
index 00000000..2b10424b
--- /dev/null
+++ b/apps/api/tests/test_p0_diagnose_routing.py
@@ -0,0 +1,40 @@
+"""
+P0 DIAGNOSE Privacy-First Routing Tests
+========================================
+測試 AIRouter local chain 隔離 + DIAGNOSE timeout 路由
+
+建立時間: 2026-04-04 (台北時區)
+建立者: Claude Code (P0 DIAGNOSE Privacy-First)
+"""
+
+import os
+os.environ.setdefault("MOCK_MODE", "true")
+
+import pytest
+from unittest.mock import AsyncMock, MagicMock, patch
+
+
+class TestNemotronPerTaskTimeout:
+    """Nemotron 支援 per-task timeout"""
+
+    @pytest.mark.asyncio
+    async def test_diagnose_uses_diagnose_timeout(self):
+        """DIAGNOSE context 應使用 NEMOTRON_DIAGNOSE_TIMEOUT_SECONDS"""
+        from src.services.ai_providers.nemotron import NemotronProvider
+
+        provider = NemotronProvider()
+
+        # 建立 mock nvidia provider
+        mock_nvidia = MagicMock()
+        mock_result = MagicMock()
+        mock_result.tool_calls = []
+        mock_nvidia.tool_call = AsyncMock(return_value=mock_result)
+
+        with patch.object(provider, '_get_nvidia', return_value=mock_nvidia):
+            result = await provider.analyze(
+                prompt="測試診斷",
+                context={"task_type": "diagnose"},
+            )
+
+        assert result.success is True
+        mock_nvidia.tool_call.assert_called_once()