fix: Improve logic for checking if a MCP session is disconnected

Currently logic to check for a disconnected session only checks for certain headers but doesn't detect all cases, leading to situations where it tries to connect to a session that is down. This adds logic so that we ping the server to check if it is disconnected. Fixes https://github.com/google/adk-python/issues/3321.

Co-authored-by: Kathy Wu <wukathy@google.com>
PiperOrigin-RevId: 832460068

src/google/adk/tools/mcp_tool/mcp_session_manager.py

@@ -37,6 +37,7 @@ try:
   from mcp.client.sse import sse_client
   from mcp.client.stdio import stdio_client
   from mcp.client.streamable_http import streamablehttp_client
+  from mcp.types import EmptyResult
 except ImportError as e:
 
   if sys.version_info < (3, 10):
@@ -241,7 +242,7 @@ class MCPSessionManager:
 
     return base_headers
 
-  def _is_session_disconnected(self, session: ClientSession) -> bool:
+  async def _is_session_disconnected(self, session: ClientSession) -> bool:
     """Checks if a session is disconnected or closed.
 
     Args:
@@ -250,7 +251,24 @@ class MCPSessionManager:
     Returns:
         True if the session is disconnected, False otherwise.
     """
-    return session._read_stream._closed or session._write_stream._closed
+    if session._read_stream._closed or session._write_stream._closed:
+      return True
+
+    try:
+      response = await asyncio.wait_for(session.send_ping(), timeout=5.0)
+      if not isinstance(response, EmptyResult):
+        logger.info(
+            'Session ping returns illegal response %s, treating as'
+            ' disconnected',
+            response,
+        )
+        return True
+      return False
+    except Exception as e:
+      logger.info(
+          'Session ping failed with error %s, treating as disconnected', e
+      )
+      return True
 
   def _create_client(self, merged_headers: Optional[Dict[str, str]] = None):
     """Creates an MCP client based on the connection parameters.
@@ -325,7 +343,7 @@ class MCPSessionManager:
         session, exit_stack = self._sessions[session_key]
 
         # Check if the existing session is still connected
-        if not self._is_session_disconnected(session):
+        if not await self._is_session_disconnected(session):
           # Session is still good, return it
           return session
         else:

src/google/adk/tools/mcp_tool/mcp_toolset.py

@@ -175,7 +175,10 @@ class McpToolset(BaseToolset):
         else None
     )
     # Get session from session manager
-    session = await self._mcp_session_manager.create_session(headers=headers)
+    try:
+      session = await self._mcp_session_manager.create_session(headers=headers)
+    except Exception as e:
+      raise ConnectionError(f"Failed to create MCP session") from e
 
     # Fetch available tools from the MCP server
     timeout_in_seconds = (

tests/unittests/tools/mcp_tool/test_mcp_session_manager.py

@@ -54,6 +54,7 @@ except ImportError as e:
 # Import real MCP classes
 try:
   from mcp import StdioServerParameters
+  from mcp.types import EmptyResult
 except ImportError:
   # Create a mock if MCP is not available
   class StdioServerParameters:
@@ -62,6 +63,9 @@ except ImportError:
       self.command = command
       self.args = args or []
 
+  class EmptyResult:
+    pass
+
 
 class MockClientSession:
   """Mock ClientSession for testing."""
@@ -72,6 +76,7 @@ class MockClientSession:
     self._read_stream._closed = False
     self._write_stream._closed = False
     self.initialize = AsyncMock()
+    self.send_ping = AsyncMock()
 
 
 class MockAsyncExitStack:
@@ -206,19 +211,52 @@ class TestMCPSessionManager:
     }
     assert merged == expected
 
-  def test_is_session_disconnected(self):
-    """Test session disconnection detection."""
+  @pytest.mark.asyncio
+  async def test_is_session_disconnected_when_connected(self):
+    """Test session disconnection detection when session is connected."""
     manager = MCPSessionManager(self.mock_stdio_connection_params)
-
-    # Create mock session
     session = MockClientSession()
+    session.send_ping.return_value = EmptyResult()
+    assert not await manager._is_session_disconnected(session)
+    session.send_ping.assert_called_once()
 
-    # Not disconnected
-    assert not manager._is_session_disconnected(session)
-
-    # Disconnected - read stream closed
+  @pytest.mark.asyncio
+  async def test_is_session_disconnected_read_stream_closed(self):
+    """Test session disconnection detection when read stream is closed."""
+    manager = MCPSessionManager(self.mock_stdio_connection_params)
+    session = MockClientSession()
+    session.send_ping.return_value = EmptyResult()
     session._read_stream._closed = True
-    assert manager._is_session_disconnected(session)
+    assert await manager._is_session_disconnected(session)
+    session.send_ping.assert_not_called()
+
+  @pytest.mark.asyncio
+  async def test_is_session_disconnected_write_stream_closed(self):
+    """Test session disconnection detection when write stream is closed."""
+    manager = MCPSessionManager(self.mock_stdio_connection_params)
+    session = MockClientSession()
+    session.send_ping.return_value = EmptyResult()
+    session._write_stream._closed = True
+    assert await manager._is_session_disconnected(session)
+    session.send_ping.assert_not_called()
+
+  @pytest.mark.asyncio
+  async def test_is_session_disconnected_ping_fails(self):
+    """Test session disconnection detection when ping fails."""
+    manager = MCPSessionManager(self.mock_stdio_connection_params)
+    session = MockClientSession()
+    session.send_ping.side_effect = Exception("Ping failed")
+    assert await manager._is_session_disconnected(session)
+    session.send_ping.assert_called_once()
+
+  @pytest.mark.asyncio
+  async def test_is_session_disconnected_ping_returns_wrong_result(self):
+    """Test session disconnection detection when ping returns wrong result."""
+    manager = MCPSessionManager(self.mock_stdio_connection_params)
+    session = MockClientSession()
+    session.send_ping.return_value = "Wrong result"
+    assert await manager._is_session_disconnected(session)
+    session.send_ping.assert_called_once()
 
   @pytest.mark.asyncio
   async def test_create_session_stdio_new(self):
@@ -271,6 +309,7 @@ class TestMCPSessionManager:
     # Session is connected
     existing_session._read_stream._closed = False
     existing_session._write_stream._closed = False
+    existing_session.send_ping.return_value = EmptyResult()
 
     session = await manager.create_session()