fix: dispatch client request handlers concurrently (#2489)

src/mcp/client/session.py

@@ -123,6 +123,9 @@ def __init__(
         experimental_task_handlers: ExperimentalTaskHandlers | None = None,
     ) -> None:
         super().__init__(read_stream, write_stream, read_timeout_seconds=read_timeout_seconds)
+        # Dispatch incoming server->client requests concurrently so a slow
+        # sampling/elicitation callback doesn't serialize other in-flight requests.
+        self._dispatch_requests_concurrently = True
         self._client_info = client_info or DEFAULT_CLIENT_INFO
         self._sampling_callback = sampling_callback or _default_sampling_callback
         self._sampling_capabilities = sampling_capabilities

src/mcp/shared/session.py

@@ -99,7 +99,8 @@ def __init__(
     def __enter__(self) -> RequestResponder[ReceiveRequestT, SendResultT]:
         """Enter the context manager, enabling request cancellation tracking."""
         self._entered = True
-        self._cancel_scope = anyio.CancelScope()
+        # Enter the scope created in __init__ so pre-entry cancel() targets
+        # the same scope the handler will later run under.
         self._cancel_scope.__enter__()
         return self
 
@@ -140,11 +141,12 @@ async def respond(self, response: SendResultT | ErrorData) -> None:
             )
 
     async def cancel(self) -> None:
-        """Cancel this request and mark it as completed."""
-        if not self._entered:  # pragma: no cover
-            raise RuntimeError("RequestResponder must be used as a context manager")
-        if not self._cancel_scope:  # pragma: no cover
-            raise RuntimeError("No active cancel scope")
+        """Cancel this request and mark it as completed.
+
+        Safe to call before the context manager has been entered.
+        """
+        if self._completed:
+            return
 
         self._cancel_scope.cancel()
         self._completed = True  # Mark as completed so it's removed from in_flight
@@ -158,6 +160,10 @@ async def cancel(self) -> None:
     def in_flight(self) -> bool:  # pragma: no cover
         return not self._completed and not self.cancelled
 
+    @property
+    def completed(self) -> bool:
+        return self._completed
+
     @property
     def cancelled(self) -> bool:
         return self._cancel_scope.cancel_called
@@ -185,6 +191,10 @@ class BaseSession(
     _progress_callbacks: dict[RequestId, ProgressFnT]
     _response_routers: list[ResponseRouter]
 
+    # When True, incoming requests are dispatched to the session's task group
+    # so handlers run concurrently with the receive loop.
+    _dispatch_requests_concurrently: bool = False
+
     def __init__(
         self,
         read_stream: ReadStream[SessionMessage | Exception],
@@ -348,6 +358,29 @@ def _receive_request_adapter(self) -> TypeAdapter[ReceiveRequestT]:
     def _receive_notification_adapter(self) -> TypeAdapter[ReceiveNotificationT]:
         raise NotImplementedError
 
+    async def _dispatch_request(
+        self,
+        responder: RequestResponder[ReceiveRequestT, SendResultT],
+    ) -> None:
+        """Run the per-request handler chain, translating handler exceptions
+        into a JSON-RPC error response so they can't wedge the peer.
+        """
+        request_id = responder.request_id
+        try:
+            await self._received_request(responder)
+            if not responder.completed:
+                await self._handle_incoming(responder)
+        except Exception:
+            logging.warning("Request handler raised an exception", exc_info=True)
+            if not responder.completed:
+                error_response = JSONRPCError(
+                    jsonrpc="2.0",
+                    id=request_id,
+                    error=ErrorData(code=INVALID_PARAMS, message="Invalid request parameters", data=""),
+                )
+                await self._write_stream.send(SessionMessage(message=error_response))
+            self._in_flight.pop(request_id, None)
+
     async def _receive_loop(self) -> None:
         async with self._read_stream, self._write_stream:
             try:
@@ -370,10 +403,6 @@ async def _handle_session_message(message: SessionMessage) -> None:
                                 context=sender_context,
                             )
                             self._in_flight[responder.request_id] = responder
-                            await self._received_request(responder)
-
-                            if not responder._completed:  # type: ignore[reportPrivateUsage]
-                                await self._handle_incoming(responder)
                         except Exception:
                             # For request validation errors, send a proper JSON-RPC error
                             # response instead of crashing the server
@@ -386,6 +415,12 @@ async def _handle_session_message(message: SessionMessage) -> None:
                             )
                             session_message = SessionMessage(message=error_response)
                             await self._write_stream.send(session_message)
+                            return
+
+                        if self._dispatch_requests_concurrently:
+                            self._task_group.start_soon(self._dispatch_request, responder)
+                        else:
+                            await self._dispatch_request(responder)
 
                     elif isinstance(message.message, JSONRPCNotification):
                         try:

tests/shared/test_session.py

@@ -4,6 +4,7 @@
 from mcp import Client, types
 from mcp.client.session import ClientSession
 from mcp.server import Server, ServerRequestContext
+from mcp.shared._context import RequestContext
 from mcp.shared.exceptions import MCPError
 from mcp.shared.memory import create_client_server_memory_streams
 from mcp.shared.message import SessionMessage
@@ -416,3 +417,216 @@ async def make_request(client_session: ClientSession):
     # Pending request completed successfully
     assert len(result_holder) == 1
     assert isinstance(result_holder[0], EmptyResult)
+
+
+@pytest.mark.anyio
+async def test_concurrent_server_to_client_requests_run_in_parallel():
+    """Regression test for #2489.
+
+    A server tool fans out N concurrent ``ServerSession.create_message`` calls
+    via ``anyio.create_task_group``. The client sampling callback records the
+    peak number of concurrently-in-flight calls. Before the fix, requests were
+    serialized end-to-end by ``BaseSession._receive_loop`` and peak was 1.
+    """
+    n = 4
+
+    inflight = 0
+    peak = 0
+    started = anyio.Event()
+    release = anyio.Event()
+
+    async def sampling_callback(
+        context: RequestContext[ClientSession],
+        params: types.CreateMessageRequestParams,
+    ) -> types.CreateMessageResult:
+        nonlocal inflight, peak
+        inflight += 1
+        peak = max(peak, inflight)
+        if peak == n:
+            started.set()
+        try:
+            with anyio.fail_after(5):
+                await release.wait()
+        finally:
+            inflight -= 1
+        msg = params.messages[0].content
+        echo = msg.text if isinstance(msg, types.TextContent) else ""
+        return types.CreateMessageResult(
+            role="assistant",
+            content=types.TextContent(type="text", text=f"echo:{echo}"),
+            model="test-model",
+        )
+
+    async def handle_call_tool(ctx: ServerRequestContext, params: types.CallToolRequestParams) -> types.CallToolResult:
+        results: list[str] = [""] * n
+
+        async def one(i: int) -> None:
+            r = await ctx.session.create_message(
+                messages=[
+                    types.SamplingMessage(
+                        role="user",
+                        content=types.TextContent(type="text", text=str(i)),
+                    )
+                ],
+                max_tokens=8,
+            )
+            results[i] = r.content.text if isinstance(r.content, types.TextContent) else ""
+
+        async with anyio.create_task_group() as tg:  # pragma: no branch
+            for i in range(n):
+                tg.start_soon(one, i)
+        return types.CallToolResult(content=[types.TextContent(type="text", text=",".join(results))])
+
+    async def handle_list_tools(
+        ctx: ServerRequestContext, params: types.PaginatedRequestParams | None
+    ) -> types.ListToolsResult:
+        return types.ListToolsResult(tools=[types.Tool(name="fanout", input_schema={"type": "object"})])
+
+    server = Server(name="fanout", on_call_tool=handle_call_tool, on_list_tools=handle_list_tools)
+
+    async with Client(server, sampling_callback=sampling_callback) as client:
+        async with anyio.create_task_group() as tg:  # pragma: no branch
+
+            async def call() -> None:
+                await client.call_tool("fanout", {})
+
+            tg.start_soon(call)
+            with anyio.fail_after(5):
+                await started.wait()
+            release.set()
+
+    assert peak == n, f"server->client requests were serialized: peak in-flight={peak}, expected {n}"
+
+
+@pytest.mark.anyio
+async def test_sampling_callback_exception_returns_error_response():
+    """A raising sampling callback must produce a JSON-RPC error response so
+    the server-side ``await ctx.session.create_message(...)`` doesn't hang.
+    """
+
+    async def sampling_callback(
+        context: RequestContext[ClientSession],
+        params: types.CreateMessageRequestParams,
+    ) -> types.CreateMessageResult:
+        raise RuntimeError("boom")
+
+    caught: list[MCPError] = []
+
+    async def handle_call_tool(ctx: ServerRequestContext, params: types.CallToolRequestParams) -> types.CallToolResult:
+        try:
+            await ctx.session.create_message(
+                messages=[
+                    types.SamplingMessage(
+                        role="user",
+                        content=types.TextContent(type="text", text="x"),
+                    )
+                ],
+                max_tokens=8,
+            )
+        except MCPError as e:
+            caught.append(e)
+        return types.CallToolResult(content=[types.TextContent(type="text", text="ok")])
+
+    async def handle_list_tools(
+        ctx: ServerRequestContext, params: types.PaginatedRequestParams | None
+    ) -> types.ListToolsResult:
+        return types.ListToolsResult(tools=[types.Tool(name="boom", input_schema={"type": "object"})])
+
+    server = Server(name="raise", on_call_tool=handle_call_tool, on_list_tools=handle_list_tools)
+
+    async with Client(server, sampling_callback=sampling_callback) as client:
+        with anyio.fail_after(5):
+            await client.call_tool("boom", {})
+
+    assert len(caught) == 1
+
+
+@pytest.mark.anyio
+async def test_double_cancel_does_not_send_second_response():
+    """Cancel called twice on the same responder must not emit a second response."""
+
+    class _Dummy:
+        _send_response_calls = 0
+
+        async def _send_response(self, *, request_id: types.RequestId, response: object) -> None:
+            self._send_response_calls += 1
+
+    dummy = _Dummy()
+    responder = RequestResponder[types.ServerRequest, types.ClientResult](
+        request_id=1,
+        request_meta=None,
+        request=types.PingRequest(method="ping"),
+        session=dummy,  # type: ignore[arg-type]
+        on_complete=lambda _r: None,
+    )
+    with responder:
+        await responder.cancel()
+        await responder.cancel()
+    assert dummy._send_response_calls == 1
+
+
+@pytest.mark.anyio
+async def test_cancel_before_context_entered_marks_scope_cancelled():
+    """Regression: with concurrent dispatch, a CancelledNotification can
+    arrive before the handler task has entered ``with responder:``.
+    ``cancel()`` must not raise, and the scope entered later must already
+    be cancelled.
+    """
+
+    class _Dummy:
+        async def _send_response(self, *, request_id: types.RequestId, response: object) -> None:
+            pass
+
+    responder = RequestResponder[types.ServerRequest, types.ClientResult](
+        request_id=7,
+        request_meta=None,
+        request=types.PingRequest(method="ping"),
+        session=_Dummy(),  # type: ignore[arg-type]
+        on_complete=lambda _r: None,
+    )
+
+    await responder.cancel()
+    assert responder.cancelled
+    assert responder._cancel_scope.cancel_called
+
+
+@pytest.mark.anyio
+async def test_handler_that_responds_then_raises_emits_no_duplicate_error():
+    """If a request handler completes the response and then raises, the
+    dispatch path must not emit a second JSON-RPC error for the same id.
+    """
+
+    class _RaiseAfterRespond(ClientSession):
+        async def _received_request(self, responder: RequestResponder[types.ServerRequest, types.ClientResult]) -> None:
+            with responder:
+                await responder.respond(types.EmptyResult())
+            raise RuntimeError("after respond")
+
+    class _CapturingWrite:
+        def __init__(self) -> None:
+            self.sent: list[SessionMessage] = []
+
+        async def send(self, msg: SessionMessage) -> None:
+            self.sent.append(msg)
+
+    async with create_client_server_memory_streams() as (client_streams, _server_streams):
+        client_read, client_write = client_streams
+        session = _RaiseAfterRespond(client_read, client_write)
+
+        capture = _CapturingWrite()
+        session._write_stream = capture  # type: ignore[assignment]
+
+        responder = RequestResponder[types.ServerRequest, types.ClientResult](
+            request_id=99,
+            request_meta=None,
+            request=types.PingRequest(method="ping"),
+            session=session,
+            on_complete=lambda r: session._in_flight.pop(r.request_id, None),
+        )
+        session._in_flight[99] = responder
+
+        await session._dispatch_request(responder)
+
+    assert len(capture.sent) == 1, capture.sent
+    assert isinstance(capture.sent[0].message, JSONRPCResponse)
+    assert capture.sent[0].message.id == 99