diff --git a/src/hrm/bt_client.py b/src/hrm/bt_client.py
index 95de2a0..ee5db12 100644
--- a/src/hrm/bt_client.py
+++ b/src/hrm/bt_client.py
@@ -6,7 +6,7 @@
 import tempfile
 import time
 from datetime import datetime
-from typing import List
+from typing import List, Optional
 
 import matplotlib.pyplot as plt
 from bleak import BleakClient, BleakScanner
@@ -29,7 +29,7 @@
 logging.basicConfig(level=logging.INFO)
 
 
-def upload_file(file_path: str) -> str:
+def upload_file(file_path: str) -> Optional[str]:
     """Upload a file to QINIU Storage, it will let DeepChat to download the file."""
     load_dotenv()
     QINIU_ACCESS_KEY = os.getenv("QINIU_ACCESS_KEY")
@@ -67,6 +67,7 @@ def __init__(self):
         logger.info("BtClient initialized")
         # 50000 is the max length of the db
         self.db = TsDB(50000)
+        self.client: Optional[BleakClient] = None
 
     async def list_bluetooth_devices(self) -> dict[str, dict]:
         """Discover Bluetooth devices and filter by HRM profile. Returns a dic, key is the device id,
@@ -105,7 +106,6 @@ async def background_monitor(self, duration: int):
         if not self.client:
             return
         async with self.client:
-            await self.client.connect()
             self.db.clear()
             await self.client.start_notify(
                 HR_MEASUREMENT_CHAR_UUID, self.count_heart_rate
@@ -113,7 +113,6 @@ async def background_monitor(self, duration: int):
             # Keep listening for duration seconds
             await asyncio.sleep(duration)
             await self.client.stop_notify(HR_MEASUREMENT_CHAR_UUID)
-            await self.client.disconnect()
             logger.info(f"Stopped monitoring heart rate of {self.client.address}")
 
     def count_heart_rate(self, sender: int, data: bytearray):
@@ -258,6 +257,7 @@ def build_heart_rate_chart(self, since_from: float = 600.0) -> str:
         key = upload_file(debug_file)
         if key:
             logger.info(f"Debug PNG chart uploaded to {key}")
+            plt.close()
         else:
             logger.error("Failed to upload debug PNG chart")
             svg_buffer = io.StringIO()
diff --git a/src/hrm/ts_db.py b/src/hrm/ts_db.py
index 193b868..1d4b332 100644
--- a/src/hrm/ts_db.py
+++ b/src/hrm/ts_db.py
@@ -59,7 +59,7 @@ def clear(self):
         """
         self.data.clear()
 
-    def time_bucket(self, start: float, end: float, bucket_size: float) -> List[float]:
+    def time_bucket(self, start: float, end: float, bucket_size: float) -> List[tuple[float, float]]:
         """
         Bucket the data from the given start timestamp to the given end timestamp into the given time bucket size.
         """
diff --git a/tests/mcp/test_bt_client.py b/tests/mcp/test_bt_client.py
index d01c07e..a07d568 100644
--- a/tests/mcp/test_bt_client.py
+++ b/tests/mcp/test_bt_client.py
@@ -78,8 +78,8 @@ async def test_monitoring_heart_rate_already_connected(bt_client):
 async def test_background_monitor(bt_client):
     # Patch client and its methods
     mock_client = MagicMock()
-    mock_client.__aenter__.return_value = mock_client
-    mock_client.__aexit__.return_value = None
+    mock_client.__aenter__ = AsyncMock(return_value=mock_client)
+    mock_client.__aexit__ = AsyncMock(return_value=None)
     mock_client.connect = AsyncMock()
     mock_client.start_notify = AsyncMock()
     mock_client.stop_notify = AsyncMock()
@@ -87,11 +87,13 @@ async def test_background_monitor(bt_client):
     bt_client.client = mock_client
     with patch.object(bt_client.db, "clear") as mock_clear:
         await bt_client.background_monitor(duration=0.01)
-        mock_client.connect.assert_called_once()
+        mock_client.__aenter__.assert_called_once()
         mock_client.start_notify.assert_called_once()
         mock_client.stop_notify.assert_called_once()
-        mock_client.disconnect.assert_called_once()
+        mock_client.__aexit__.assert_called_once()
         mock_clear.assert_called_once()
+        mock_client.connect.assert_not_called()
+        mock_client.disconnect.assert_not_called()
 
 
 @pytest.mark.parametrize(