fix: correct MCPServer call_tool result type (#2816)

Co-authored-by: Marcelo Trylesinski <marcelotryle@gmail.com>

tests/server/mcpserver/test_func_metadata.py

@@ -1038,7 +1038,8 @@ def test_structured_output_aliases():
 
     # Check that the actual output uses aliases too
     result = ModelWithAliases(**{"first": "hello", "second": "world"})
-    _, structured_content = meta.convert_result(result)
+    structured_content = meta.convert_result(result).structured_content
+    assert structured_content is not None
 
     # The structured content should use aliases to match the schema
     assert "first" in structured_content
@@ -1050,7 +1051,8 @@ def test_structured_output_aliases():
 
     # Also test the case where we have a model with defaults to ensure aliases work in all cases
     result_with_defaults = ModelWithAliases()  # Uses default None values
-    _, structured_content_defaults = meta.convert_result(result_with_defaults)
+    structured_content_defaults = meta.convert_result(result_with_defaults).structured_content
+    assert structured_content_defaults is not None
 
     # Even with defaults, should use aliases in output
     assert "first" in structured_content_defaults

tests/server/mcpserver/test_server.py

@@ -23,6 +23,7 @@ from mcp.types import (
     INVALID_PARAMS,
     AudioContent,
     BlobResourceContents,
+    CallToolResult,
     Completion,
     CompletionArgument,
     CompletionContext,
@@ -306,6 +307,29 @@ class TestServerTools:
             assert result.structured_content is not None
             assert result.structured_content == {"result": 3}
 
+    async def test_call_tool_always_returns_call_tool_result(self):
+        mcp = MCPServer()
+
+        @mcp.tool()
+        def direct() -> CallToolResult:
+            return CallToolResult(content=[TextContent(type="text", text="direct")])
+
+        @mcp.tool(structured_output=False)
+        def unstructured() -> str:
+            return "plain"
+
+        @mcp.tool()
+        def structured() -> int:
+            return 3
+
+        assert await mcp.call_tool("direct", {}) == CallToolResult(content=[TextContent(type="text", text="direct")])
+        assert await mcp.call_tool("unstructured", {}) == CallToolResult(
+            content=[TextContent(type="text", text="plain")]
+        )
+        assert await mcp.call_tool("structured", {}) == CallToolResult(
+            content=[TextContent(type="text", text="3")], structured_content={"result": 3}
+        )
+
     async def test_tool_image_helper(self, tmp_path: Path):
         # Create a test image
         image_path = tmp_path / "test.png"

tests/server/mcpserver/test_tool_manager.py

@@ -11,7 +11,7 @@ from mcp.server.mcpserver import Context, MCPServer
 from mcp.server.mcpserver.exceptions import ToolError
 from mcp.server.mcpserver.tools import Tool, ToolManager
 from mcp.server.mcpserver.utilities.func_metadata import ArgModelBase, FuncMetadata
-from mcp.types import TextContent, ToolAnnotations
+from mcp.types import CallToolResult, TextContent, ToolAnnotations
 
 
 class TestAddTools:
@@ -455,7 +455,8 @@ class TestStructuredOutput:
         manager.add_tool(get_user)
         result = await manager.call_tool("get_user", {"user_id": 1}, Context(), convert_result=True)
         # don't test unstructured output here, just the structured conversion
-        assert len(result) == 2 and result[1] == {"name": "John", "age": 30}
+        assert isinstance(result, CallToolResult)
+        assert result.structured_content == {"name": "John", "age": 30}
 
     @pytest.mark.anyio
     async def test_tool_with_primitive_output(self):
@@ -470,7 +471,8 @@ class TestStructuredOutput:
         result = await manager.call_tool("double_number", {"n": 5}, Context())
         assert result == 10
         result = await manager.call_tool("double_number", {"n": 5}, Context(), convert_result=True)
-        assert isinstance(result[0][0], TextContent) and result[1] == {"result": 10}
+        assert isinstance(result, CallToolResult)
+        assert isinstance(result.content[0], TextContent) and result.structured_content == {"result": 10}
 
     @pytest.mark.anyio
     async def test_tool_with_typeddict_output(self):
@@ -510,7 +512,8 @@ class TestStructuredOutput:
         manager.add_tool(get_person)
         result = await manager.call_tool("get_person", {}, Context(), convert_result=True)
         # don't test unstructured output here, just the structured conversion
-        assert len(result) == 2 and result[1] == expected_output
+        assert isinstance(result, CallToolResult)
+        assert result.structured_content == expected_output
 
     @pytest.mark.anyio
     async def test_tool_with_list_output(self):
@@ -528,7 +531,8 @@ class TestStructuredOutput:
         result = await manager.call_tool("get_numbers", {}, Context())
         assert result == expected_list
         result = await manager.call_tool("get_numbers", {}, Context(), convert_result=True)
-        assert isinstance(result[0][0], TextContent) and result[1] == expected_output
+        assert isinstance(result, CallToolResult)
+        assert isinstance(result.content[0], TextContent) and result.structured_content == expected_output
 
     @pytest.mark.anyio
     async def test_tool_without_structured_output(self):