fix MCPServer tool result shapes

Author: Epochex

src/mcp/server/mcpserver/server.py

@@ -4,7 +4,6 @@
 
 import base64
 import inspect
-import json
 import re
 from collections.abc import AsyncIterator, Awaitable, Callable, Iterable, Sequence
 from contextlib import AbstractAsyncContextManager, asynccontextmanager
@@ -76,6 +75,8 @@
 
 _CallableT = TypeVar("_CallableT", bound=Callable[..., Any])
 
+ToolResult = CallToolResult | Sequence[ContentBlock] | tuple[Sequence[ContentBlock], dict[str, Any]]
+
 
 class Settings(BaseSettings, Generic[LifespanResultT]):
     """MCPServer settings.
@@ -322,14 +323,6 @@ async def _handle_call_tool(
                 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.
-            return CallToolResult(
-                content=[TextContent(type="text", text=json.dumps(result, indent=2))],
-                structured_content=result,
-            )
         return CallToolResult(content=list(result))
 
     async def _handle_list_resources(
@@ -399,7 +392,7 @@ 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]:
+    ) -> ToolResult:
         """Call a tool by name with arguments."""
         if context is None:
             context = Context(mcp_server=self)

tests/server/mcpserver/test_server.py

@@ -1,6 +1,6 @@
 import base64
 from pathlib import Path
-from typing import Any
+from typing import Any, get_type_hints
 from unittest.mock import AsyncMock, MagicMock, patch
 
 import pytest
@@ -15,12 +15,14 @@
 from mcp.server.mcpserver.exceptions import ToolError
 from mcp.server.mcpserver.prompts.base import Message, UserMessage
 from mcp.server.mcpserver.resources import FileResource, FunctionResource
+from mcp.server.mcpserver.server import ToolResult
 from mcp.server.mcpserver.utilities.types import Audio, Image
 from mcp.server.transport_security import TransportSecuritySettings
 from mcp.shared.exceptions import MCPError
 from mcp.types import (
     AudioContent,
     BlobResourceContents,
+    CallToolResult,
     Completion,
     CompletionArgument,
     CompletionContext,
@@ -304,6 +306,64 @@ async def test_tool_return_value_conversion(self):
             assert result.structured_content is not None
             assert result.structured_content == {"result": 3}
 
+    def test_call_tool_return_annotation_matches_reachable_shapes(self):
+        hints = get_type_hints(MCPServer.call_tool)
+        assert hints["return"] == ToolResult
+
+    async def test_call_tool_preserves_direct_call_tool_result(self):
+        mcp = MCPServer()
+
+        @mcp.tool()
+        def direct_result() -> CallToolResult:
+            return CallToolResult(content=[TextContent(type="text", text="direct")])
+
+        result = await mcp.call_tool("direct_result", {})
+        assert isinstance(result, CallToolResult)
+        assert result.content == [TextContent(type="text", text="direct")]
+
+        async with Client(mcp) as client:
+            handled = await client.call_tool("direct_result", {})
+            assert handled.content == result.content
+            assert handled.structured_content is None
+
+    async def test_call_tool_wraps_bare_content_sequence(self):
+        mcp = MCPServer()
+
+        def raw_blocks() -> list[ContentBlock]:
+            return [TextContent(type="text", text="raw")]
+
+        mcp.add_tool(raw_blocks, structured_output=False)
+
+        result = await mcp.call_tool("raw_blocks", {})
+        assert isinstance(result, list)
+        assert all(isinstance(item, ContentBlock) for item in result)
+
+        async with Client(mcp) as client:
+            handled = await client.call_tool("raw_blocks", {})
+            assert handled.content == result
+            assert handled.structured_content is None
+
+    async def test_call_tool_wraps_structured_tuple_result(self):
+        class UserOutput(BaseModel):
+            name: str
+            age: int
+
+        def get_user() -> UserOutput:
+            return UserOutput(name="John Doe", age=30)
+
+        mcp = MCPServer()
+        mcp.add_tool(get_user)
+
+        result = await mcp.call_tool("get_user", {})
+        assert isinstance(result, tuple)
+        unstructured_content, structured_content = result
+        assert structured_content == {"name": "John Doe", "age": 30}
+
+        async with Client(mcp) as client:
+            handled = await client.call_tool("get_user", {})
+            assert handled.content == list(unstructured_content)
+            assert handled.structured_content == structured_content
+
     async def test_tool_image_helper(self, tmp_path: Path):
         # Create a test image
         image_path = tmp_path / "test.png"