Fix MCPServer.call_tool return type

src/mcp/server/mcpserver/server.py

@@ -4,11 +4,10 @@
 
 import base64
 import inspect
-import json
 import re
 from collections.abc import AsyncIterator, Awaitable, Callable, Iterable, Sequence
 from contextlib import AbstractAsyncContextManager, asynccontextmanager
-from typing import Any, Generic, Literal, TypeVar, overload
+from typing import Any, Generic, Literal, TypeAlias, TypeVar, cast, overload
 
 import anyio
 import pydantic_core
@@ -75,6 +74,9 @@
 logger = get_logger(__name__)
 
 _CallableT = TypeVar("_CallableT", bound=Callable[..., Any])
+_MCPServerToolResult: TypeAlias = (
+    Sequence[ContentBlock] | tuple[Sequence[ContentBlock], dict[str, Any]] | CallToolResult
+)
 
 
 class Settings(BaseSettings, Generic[LifespanResultT]):
@@ -316,19 +318,11 @@ async def _handle_call_tool(
             return CallToolResult(content=[TextContent(type="text", text=str(e))], is_error=True)
         if isinstance(result, CallToolResult):
             return result
-        if isinstance(result, tuple) and len(result) == 2:
-            unstructured_content, structured_content = result
-            return CallToolResult(
-                content=list(unstructured_content),  # type: ignore[arg-type]
-                structured_content=structured_content,  # type: ignore[arg-type]
-            )
-        if isinstance(result, dict):  # pragma: no cover
-            # TODO: this code path is unreachable — convert_result never returns a raw dict.
-            # The call_tool return type (Sequence[ContentBlock] | dict[str, Any]) is wrong
-            # and needs to be cleaned up.
+        if isinstance(result, tuple):
+            unstructured_content, structured_content = cast(tuple[Sequence[ContentBlock], dict[str, Any]], result)
             return CallToolResult(
-                content=[TextContent(type="text", text=json.dumps(result, indent=2))],
-                structured_content=result,
+                content=list(unstructured_content),
+                structured_content=structured_content,
             )
         return CallToolResult(content=list(result))
 
@@ -399,7 +393,7 @@ async def list_tools(self) -> list[MCPTool]:
 
     async def call_tool(
         self, name: str, arguments: dict[str, Any], context: Context[LifespanResultT, Any] | None = None
-    ) -> Sequence[ContentBlock] | dict[str, Any]:
+    ) -> _MCPServerToolResult:
         """Call a tool by name with arguments."""
         if context is None:
             context = Context(mcp_server=self)

tests/server/mcpserver/test_server.py

@@ -519,6 +519,23 @@ def calculate_sum(a: int, b: int) -> int:
             assert result.structured_content is not None
             assert result.structured_content == {"result": 12}
 
+    async def test_call_tool_direct_returns_structured_tuple(self):
+        """Test direct call_tool returns both content and structured output."""
+
+        def calculate_sum(a: int, b: int) -> int:
+            """Add two numbers"""
+            return a + b
+
+        mcp = MCPServer()
+        mcp.add_tool(calculate_sum)
+
+        result = await mcp.call_tool("calculate_sum", {"a": 5, "b": 7})
+
+        assert isinstance(result, tuple)
+        content, structured_content = result
+        assert list(content) == [TextContent(text="12")]
+        assert structured_content == {"result": 12}
+
     async def test_tool_structured_output_list(self):
         """Test tool with structured output returning list"""