diff --git a/litellm/responses/mcp/litellm_proxy_mcp_handler.py b/litellm/responses/mcp/litellm_proxy_mcp_handler.py
index a2b60ff8b4d..d970bd3c6d7 100644
--- a/litellm/responses/mcp/litellm_proxy_mcp_handler.py
+++ b/litellm/responses/mcp/litellm_proxy_mcp_handler.py
@@ -5,8 +5,7 @@ from typing import TYPE_CHECKING, Any, Final, Literal, Optional, TypeAlias, Type
 
 from openai.types.chat import ChatCompletionToolParam
 from openai.types.responses.function_tool_param import FunctionToolParam
-from typing_extensions import TypedDict as ExtraItemsTypedDict
-from typing_extensions import Unpack
+from typing_extensions import NotRequired, TypedDict as ExtraItemsTypedDict, Unpack
 
 from litellm._logging import verbose_logger
 from litellm.constants import MAXIMUM_TRACEBACK_LINES_TO_LOG
@@ -58,7 +57,7 @@ ToolParam: TypeAlias = Mapping[str, object]
 SplitTools: TypeAlias = tuple[list[ToolParam], list[Any]]
 
 
-class MCPToolResult(TypedDict, total=False):
+class MCPToolResult(TypedDict):
     tool_call_id: str | None
     result: str
     name: str | None
@@ -66,7 +65,8 @@ class MCPToolResult(TypedDict, total=False):
     # raised (guardrail block, HTTP failure, unexpected exception). Anthropic
     # native MCP consumers pass is_error=True on the tool_result block so the
     # model can react to the failure instead of reading it as a success (#49359).
-    is_error: bool
+    # Optional via NotRequired (PEP 655) so the three original keys stay required.
+    is_error: NotRequired[bool]
 
 
 class _AresponsesOptions(ExtraItemsTypedDict, total=False, extra_items=object):
diff --git a/tests/unit/responses/mcp/test_litellm_proxy_mcp_handler.py b/tests/unit/responses/mcp/test_litellm_proxy_mcp_handler.py
index 9a96550a231..69569eb4853 100644
--- a/tests/unit/responses/mcp/test_litellm_proxy_mcp_handler.py
+++ b/tests/unit/responses/mcp/test_litellm_proxy_mcp_handler.py
@@ -1997,3 +1997,24 @@ async def test_execute_tool_calls_marks_exception_results_as_error(monkeypatch):
             "is_error": True,
         }
     ]
+
+
+def test_mcp_tool_result_keeps_original_keys_required_statically():
+    """
+    PEP 655 negative control (#49359 review): ``is_error`` is the only optional
+    key; ``tool_call_id``, ``result``, and ``name`` must stay required, so a
+    bare ``{"is_error": True}`` literal is rejected at type-check time. This
+    pins the static contract the runtime tests cannot see.
+    """
+    from typing_extensions import assert_type
+
+    from litellm.responses.mcp.litellm_proxy_mcp_handler import MCPToolResult
+
+    # The three original keys keep Required status: total=False was reverted.
+    assert not MCPToolResult.__required_keys__ < {"tool_call_id", "result", "name"}
+    assert MCPToolResult.__required_keys__ == {"tool_call_id", "result", "name"}
+    assert MCPToolResult.__optional_keys__ == {"is_error"}
+
+    # Runtime shape of a full result is unchanged.
+    full: MCPToolResult = {"tool_call_id": "call-1", "result": "ok", "name": "echo"}
+    assert_type(full, MCPToolResult)