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[v1.x] fix: make request responder completion idempotent #2949

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[v1.x] fix: make request responder completion idempotent #2949

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7 changes: 5 additions & 2 deletions src/mcp/shared/session.py
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Original file line number Diff line number Diff line change
Expand Up @@ -122,11 +122,11 @@ async def respond(self, response: SendResultT | ErrorData) -> None:
Must be called within a context manager block.
Raises:
RuntimeError: If not used within a context manager
AssertionError: If request was already responded to
"""
if not self._entered: # pragma: no cover
raise RuntimeError("RequestResponder must be used as a context manager")
assert not self._completed, "Request already responded to"
if self._completed:
return

if not self.cancelled: # pragma: no branch
self._completed = True
Expand All @@ -143,6 +143,9 @@ async def cancel(self) -> None:
raise RuntimeError("No active cancel scope")

self._cancel_scope.cancel()
if self._completed:
return

self._completed = True # Mark as completed so it's removed from in_flight
# Send an error response to indicate cancellation
await self._session._send_response( # type: ignore[reportPrivateUsage]
Expand Down
44 changes: 44 additions & 0 deletions tests/shared/test_session.py
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Original file line number Diff line number Diff line change
@@ -1,5 +1,6 @@
from collections.abc import AsyncGenerator
from typing import Any
from unittest.mock import AsyncMock, MagicMock

import anyio
import pytest
Expand All @@ -10,11 +11,13 @@
from mcp.shared.exceptions import McpError
from mcp.shared.memory import create_client_server_memory_streams, create_connected_server_and_client_session
from mcp.shared.message import SessionMessage
from mcp.shared.session import RequestResponder
from mcp.types import (
CancelledNotification,
CancelledNotificationParams,
ClientNotification,
ClientRequest,
ClientResult,
EmptyResult,
ErrorData,
JSONRPCError,
Expand All @@ -30,6 +33,20 @@ def mcp_server() -> Server:
return Server(name="test server")


def make_request_responder() -> tuple[RequestResponder[ClientRequest, ClientResult], MagicMock]:
mock_session = MagicMock()
mock_session._send_response = AsyncMock()
request = ClientRequest(types.PingRequest())
responder: RequestResponder[ClientRequest, ClientResult] = RequestResponder(
request_id=1,
request_meta=None,
request=request,
session=mock_session,
on_complete=lambda responder: None,
)
return responder, mock_session


@pytest.fixture
async def client_connected_to_server(
mcp_server: Server,
Expand Down Expand Up @@ -128,6 +145,33 @@ async def make_request(client_session: ClientSession):
await ev_cancelled.wait()


@pytest.mark.anyio
async def test_request_responder_respond_after_cancel_does_not_raise():
responder, mock_session = make_request_responder()

with responder:
await responder.cancel()
await responder.respond(ClientResult(root=EmptyResult()))

mock_session._send_response.assert_awaited_once()
assert mock_session._send_response.await_args.kwargs["response"] == ErrorData(
code=0, message="Request cancelled", data=None
)


@pytest.mark.anyio
async def test_request_responder_cancel_after_respond_does_not_send_error():
responder, mock_session = make_request_responder()
response = ClientResult(root=EmptyResult())

with responder:
await responder.respond(response)
await responder.cancel()

mock_session._send_response.assert_awaited_once()
assert mock_session._send_response.await_args.kwargs["response"] == response


@pytest.mark.anyio
async def test_response_id_type_mismatch_string_to_int():
"""
Expand Down
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