fix: correct MCPServer call_tool result type

Author: fengjikui

docs/migration.md

@@ -8,6 +8,17 @@ Version 2 of the MCP Python SDK introduces several breaking changes to improve t
 
 ## Breaking Changes
 
+### `MCPServer.call_tool()` return annotation corrected
+
+`MCPServer.call_tool()` no longer advertises a raw `dict[str, Any]`
+return. On v2 it returns exactly the shapes produced by the MCPServer
+tool conversion path: a direct `CallToolResult`, a sequence of
+`ContentBlock` values for unstructured tools, or a
+`(content, structured_content)` tuple for structured tools.
+
+If you subclass `MCPServer` or annotate wrappers around `call_tool()`,
+update those annotations to match the corrected return shape.
+
 ### `streamablehttp_client` removed
 
 The deprecated `streamablehttp_client` function has been removed. Use `streamable_http_client` instead.

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
@@ -76,6 +75,8 @@
 
 _CallableT = TypeVar("_CallableT", bound=Callable[..., Any])
 
+ToolResult: TypeAlias = CallToolResult | Sequence[ContentBlock] | tuple[Sequence[ContentBlock], dict[str, Any]]
+
 
 class Settings(BaseSettings, Generic[LifespanResultT]):
     """MCPServer settings.
@@ -317,18 +318,10 @@ async def _handle_call_tool(
         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.
+            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))
 
@@ -399,8 +392,15 @@ 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]:
-        """Call a tool by name with arguments."""
+    ) -> ToolResult:
+        """Call a tool by name with arguments.
+
+        Returns:
+            The tool result converted for the low-level handler:
+            - a `CallToolResult` returned directly by the tool,
+            - a sequence of content blocks for unstructured tools, or
+            - a `(content, structured_content)` tuple for tools with structured output.
+        """
         if context is None:
             context = Context(mcp_server=self)
         return await self._tool_manager.call_tool(name, arguments, context, convert_result=True)

tests/server/mcpserver/test_server.py

@@ -21,6 +21,7 @@
 from mcp.types import (
     AudioContent,
     BlobResourceContents,
+    CallToolResult,
     Completion,
     CompletionArgument,
     CompletionContext,
@@ -304,6 +305,30 @@ async def test_tool_return_value_conversion(self):
             assert result.structured_content is not None
             assert result.structured_content == {"result": 3}
 
+    async def test_call_tool_returns_declared_result_shapes(self):
+        mcp = MCPServer()
+
+        @mcp.tool()
+        def direct_result() -> CallToolResult:
+            return CallToolResult(content=[TextContent(text="direct")])
+
+        @mcp.tool(structured_output=False)
+        def unstructured() -> str:
+            return "plain"
+
+        @mcp.tool()
+        def structured() -> int:
+            return 3
+
+        direct = await mcp.call_tool("direct_result", {})
+        assert direct == CallToolResult(content=[TextContent(text="direct")])
+
+        bare_content = await mcp.call_tool("unstructured", {})
+        assert bare_content == [TextContent(text="plain")]
+
+        structured_result = await mcp.call_tool("structured", {})
+        assert structured_result == ([TextContent(text="3")], {"result": 3})
+
     async def test_tool_image_helper(self, tmp_path: Path):
         # Create a test image
         image_path = tmp_path / "test.png"