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fix: tighten MCPServer tool result typing #2881

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fix: tighten MCPServer tool result typing #2881

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11 changes: 1 addition & 10 deletions src/mcp/server/mcpserver/server.py
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Original file line number Diff line number Diff line change
Expand Up @@ -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
Expand Down Expand Up @@ -322,14 +321,6 @@ 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(
Expand Down Expand Up @@ -399,7 +390,7 @@ 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]:
) -> CallToolResult | Sequence[ContentBlock] | tuple[Sequence[ContentBlock], dict[str, Any]]:
"""Call a tool by name with arguments."""
if context is None:
context = Context(mcp_server=self)
Expand Down
24 changes: 22 additions & 2 deletions tests/server/mcpserver/test_func_metadata.py
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Original file line number Diff line number Diff line change
Expand Up @@ -13,8 +13,8 @@
from pydantic import BaseModel, Field

from mcp.server.mcpserver.exceptions import InvalidSignature
from mcp.server.mcpserver.utilities.func_metadata import func_metadata
from mcp.types import CallToolResult
from mcp.server.mcpserver.utilities.func_metadata import FuncMetadata, func_metadata
from mcp.types import CallToolResult, TextContent


class SomeInputModelA(BaseModel):
Expand Down Expand Up @@ -203,6 +203,26 @@ def func_with_many_params(keep_this: int, skip_this: str, also_keep: float, also
assert model.also_keep == 2.5 # type: ignore


def test_convert_result_serializes_unstructured_dict_to_text_content():
"""Unstructured dict returns are content blocks, not raw dict values."""

def func_dict(): # pragma: no cover
return {"ok": True, "count": 2}

meta: FuncMetadata = func_metadata(func_dict)

assert meta.output_schema is None
assert meta.convert_result({"ok": True, "count": 2}) == [
TextContent(
type="text",
text="""{
"ok": true,
"count": 2
}""",
)
]


def test_structured_output_dict_str_types():
"""Test that dict[str, T] types are handled without wrapping."""

Expand Down
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