fix: Returns dict as result from McpTool

The `BaseTool` expects the run_async to return a json-serializable object. By model_dump the McpTool result explicitly can allow what ADK runtime sees is identical to what is persisted in the session event list.

Before the change, runtime sees CallToolResult instance and Session persists its serialized dict.

https://github.com/modelcontextprotocol/python-sdk/blob/main/src/mcp/types.py#L916-L922

PiperOrigin-RevId: 822465432

Author: Jacksunwei

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

@@ -80,7 +80,7 @@ def __init__(
           Callable[[ReadonlyContext], Dict[str, str]]
       ] = None,
   ):
-    """Initializes an MCPTool.
+    """Initializes an McpTool.
 
     This tool wraps an MCP Tool interface and uses a session manager to
     communicate with the MCP server.
@@ -186,7 +186,7 @@ async def run_async(
   @override
   async def _run_async_impl(
       self, *, args, tool_context: ToolContext, credential: AuthCredential
-  ):
+  ) -> Dict[str, Any]:
     """Runs the tool asynchronously.
 
     Args:
@@ -217,7 +217,7 @@ async def _run_async_impl(
     )
 
     response = await session.call_tool(self._mcp_tool.name, arguments=args)
-    return response
+    return response.model_dump(exclude_none=True, mode="json")
 
   async def _get_headers(
       self, tool_context: ToolContext, credential: AuthCredential
@@ -282,7 +282,7 @@ async def _get_headers(
             != APIKeyIn.header
         ):
           error_msg = (
-              "MCPTool only supports header-based API key authentication."
+              "McpTool only supports header-based API key authentication."
               " Configured location:"
               f" {self._credentials_manager._auth_config.auth_scheme.in_}"
           )

tests/unittests/tools/mcp_tool/test_mcp_tool.py

@@ -36,6 +36,8 @@
   from google.adk.tools.mcp_tool.mcp_tool import MCPTool
   from google.adk.tools.tool_context import ToolContext
   from google.genai.types import FunctionDeclaration
+  from mcp.types import CallToolResult
+  from mcp.types import TextContent
 except ImportError as e:
   if sys.version_info < (3, 10):
     # Create dummy classes to prevent NameError during test collection
@@ -47,6 +49,8 @@ class DummyClass:
     MCPTool = DummyClass
     ToolContext = DummyClass
     FunctionDeclaration = DummyClass
+    CallToolResult = DummyClass
+    TextContent = DummyClass
   else:
     raise e
 
@@ -150,9 +154,11 @@ async def test_run_async_impl_no_auth(self):
         mcp_session_manager=self.mock_session_manager,
     )
 
-    # Mock the session response
-    expected_response = {"result": "success"}
-    self.mock_session.call_tool = AsyncMock(return_value=expected_response)
+    # Mock the session response - must return CallToolResult
+    mcp_response = CallToolResult(
+        content=[TextContent(type="text", text="success")]
+    )
+    self.mock_session.call_tool = AsyncMock(return_value=mcp_response)
 
     tool_context = Mock(spec=ToolContext)
     args = {"param1": "test_value"}
@@ -161,7 +167,8 @@ async def test_run_async_impl_no_auth(self):
         args=args, tool_context=tool_context, credential=None
     )
 
-    assert result == expected_response
+    # Verify the result matches the model_dump output
+    assert result == mcp_response.model_dump(exclude_none=True, mode="json")
     self.mock_session_manager.create_session.assert_called_once_with(
         headers=None
     )
@@ -184,9 +191,11 @@ async def test_run_async_impl_with_oauth2(self):
         auth_type=AuthCredentialTypes.OAUTH2, oauth2=oauth2_auth
     )
 
-    # Mock the session response
-    expected_response = {"result": "success"}
-    self.mock_session.call_tool = AsyncMock(return_value=expected_response)
+    # Mock the session response - must return CallToolResult
+    mcp_response = CallToolResult(
+        content=[TextContent(type="text", text="success")]
+    )
+    self.mock_session.call_tool = AsyncMock(return_value=mcp_response)
 
     tool_context = Mock(spec=ToolContext)
     args = {"param1": "test_value"}
@@ -195,7 +204,7 @@ async def test_run_async_impl_with_oauth2(self):
         args=args, tool_context=tool_context, credential=credential
     )
 
-    assert result == expected_response
+    assert result == mcp_response.model_dump(exclude_none=True, mode="json")
     # Check that headers were passed correctly
     self.mock_session_manager.create_session.assert_called_once()
     call_args = self.mock_session_manager.create_session.call_args
@@ -322,7 +331,7 @@ async def test_get_headers_api_key_with_query_scheme_raises_error(self):
 
     with pytest.raises(
         ValueError,
-        match="MCPTool only supports header-based API key authentication",
+        match="McpTool only supports header-based API key authentication",
     ):
       await tool._get_headers(tool_context, auth_credential)
 
@@ -354,7 +363,7 @@ async def test_get_headers_api_key_with_cookie_scheme_raises_error(self):
 
     with pytest.raises(
         ValueError,
-        match="MCPTool only supports header-based API key authentication",
+        match="McpTool only supports header-based API key authentication",
     ):
       await tool._get_headers(tool_context, auth_credential)
 
@@ -460,9 +469,11 @@ async def test_run_async_impl_with_api_key_header_auth(self):
         auth_credential=auth_credential,
     )
 
-    # Mock the session response
-    expected_response = {"result": "authenticated_success"}
-    self.mock_session.call_tool = AsyncMock(return_value=expected_response)
+    # Mock the session response - must return CallToolResult
+    mcp_response = CallToolResult(
+        content=[TextContent(type="text", text="authenticated_success")]
+    )
+    self.mock_session.call_tool = AsyncMock(return_value=mcp_response)
 
     tool_context = Mock(spec=ToolContext)
     args = {"param1": "test_value"}
@@ -471,7 +482,7 @@ async def test_run_async_impl_with_api_key_header_auth(self):
         args=args, tool_context=tool_context, credential=auth_credential
     )
 
-    assert result == expected_response
+    assert result == mcp_response.model_dump(exclude_none=True, mode="json")
     # Check that headers were passed correctly with custom API key header
     self.mock_session_manager.create_session.assert_called_once()
     call_args = self.mock_session_manager.create_session.call_args
@@ -545,7 +556,7 @@ async def test_get_headers_api_key_error_logging(self):
       mock_logger.error.assert_called_once()
       logged_message = mock_logger.error.call_args[0][0]
       assert (
-          "MCPTool only supports header-based API key authentication"
+          "McpTool only supports header-based API key authentication"
           in logged_message
       )
 
@@ -652,8 +663,11 @@ async def test_run_async_impl_with_header_provider_no_auth(self):
         header_provider=header_provider,
     )
 
-    expected_response = {"result": "success"}
-    self.mock_session.call_tool = AsyncMock(return_value=expected_response)
+    # Mock the session response - must return CallToolResult
+    mcp_response = CallToolResult(
+        content=[TextContent(type="text", text="success")]
+    )
+    self.mock_session.call_tool = AsyncMock(return_value=mcp_response)
 
     tool_context = Mock(spec=ToolContext)
     tool_context._invocation_context = Mock()
@@ -663,7 +677,7 @@ async def test_run_async_impl_with_header_provider_no_auth(self):
         args=args, tool_context=tool_context, credential=None
     )
 
-    assert result == expected_response
+    assert result == mcp_response.model_dump(exclude_none=True, mode="json")
     header_provider.assert_called_once()
     self.mock_session_manager.create_session.assert_called_once_with(
         headers=expected_headers
@@ -688,8 +702,11 @@ async def test_run_async_impl_with_header_provider_and_oauth2(self):
         auth_type=AuthCredentialTypes.OAUTH2, oauth2=oauth2_auth
     )
 
-    expected_response = {"result": "success"}
-    self.mock_session.call_tool = AsyncMock(return_value=expected_response)
+    # Mock the session response - must return CallToolResult
+    mcp_response = CallToolResult(
+        content=[TextContent(type="text", text="success")]
+    )
+    self.mock_session.call_tool = AsyncMock(return_value=mcp_response)
 
     tool_context = Mock(spec=ToolContext)
     tool_context._invocation_context = Mock()
@@ -699,7 +716,7 @@ async def test_run_async_impl_with_header_provider_and_oauth2(self):
         args=args, tool_context=tool_context, credential=credential
     )
 
-    assert result == expected_response
+    assert result == mcp_response.model_dump(exclude_none=True, mode="json")
     header_provider.assert_called_once()
     self.mock_session_manager.create_session.assert_called_once()
     call_args = self.mock_session_manager.create_session.call_args