From d40d00ea26102d73685f423346efadb7bd9ae254 Mon Sep 17 00:00:00 2001
From: Sarvesh <sarveshagl1327@gmail.com>
Date: Tue, 16 Jun 2026 11:04:35 +0530
Subject: [PATCH] fix: tighten mcpserver tool result typing

---
 src/mcp/server/mcpserver/server.py           | 11 +--------
 tests/server/mcpserver/test_func_metadata.py | 24 ++++++++++++++++++--
 2 files changed, 23 insertions(+), 12 deletions(-)

diff --git a/src/mcp/server/mcpserver/server.py b/src/mcp/server/mcpserver/server.py
index fdb69571d8..f4c971ff7c 100644
--- a/src/mcp/server/mcpserver/server.py
+++ b/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
@@ -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(
@@ -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)
diff --git a/tests/server/mcpserver/test_func_metadata.py b/tests/server/mcpserver/test_func_metadata.py
index c57d1ee9f0..f9eaaa2596 100644
--- a/tests/server/mcpserver/test_func_metadata.py
+++ b/tests/server/mcpserver/test_func_metadata.py
@@ -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):
@@ -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."""