fix(stdio): drain responses after stdin EOF

Author: Epochex

src/mcp/server/lowlevel/server.py

@@ -390,29 +390,22 @@ async def run(
                 await stack.enter_async_context(task_support.run())
 
             async with anyio.create_task_group() as tg:
-                try:
-                    async for message in session.incoming_messages:
-                        logger.debug("Received message: %s", message)
-
-                        if isinstance(message, RequestResponder) and message.context is not None:
-                            context = message.context
-                        else:
-                            context = contextvars.copy_context()
-
-                        context.run(
-                            tg.start_soon,
-                            self._handle_message,
-                            message,
-                            session,
-                            lifespan_context,
-                            raise_exceptions,
-                        )
-                finally:
-                    # Transport closed: cancel in-flight handlers. Without this the
-                    # TG join waits for them, and when they eventually try to
-                    # respond they hit a closed write stream (the session's
-                    # _receive_loop closed it when the read stream ended).
-                    tg.cancel_scope.cancel()
+                async for message in session.incoming_messages:
+                    logger.debug("Received message: %s", message)
+
+                    if isinstance(message, RequestResponder) and message.context is not None:
+                        context = message.context
+                    else:
+                        context = contextvars.copy_context()
+
+                    context.run(
+                        tg.start_soon,
+                        self._handle_message,
+                        message,
+                        session,
+                        lifespan_context,
+                        raise_exceptions,
+                    )
 
     async def _handle_message(
         self,

src/mcp/server/session.py

@@ -85,7 +85,7 @@ def __init__(
         init_options: InitializationOptions,
         stateless: bool = False,
     ) -> None:
-        super().__init__(read_stream, write_stream)
+        super().__init__(read_stream, write_stream, close_write_stream_on_read_close=False)
         self._stateless = stateless
         self._initialization_state = (
             InitializationState.Initialized if stateless else InitializationState.NotInitialized

src/mcp/shared/session.py

@@ -191,16 +191,26 @@ def __init__(
         write_stream: WriteStream[SessionMessage],
         # If none, reading will never time out
         read_timeout_seconds: float | None = None,
+        # When True, closing/EOF on the read stream closes the write stream too.
+        #
+        # For full-duplex transports (e.g., stdio), an input EOF can be a
+        # half-close: the peer is done sending requests but still expects
+        # responses on the output stream. In that case, callers may opt out so
+        # in-flight handlers can drain their responses before shutdown.
+        close_write_stream_on_read_close: bool = True,
     ) -> None:
         self._read_stream = read_stream
         self._write_stream = write_stream
         self._response_streams = {}
         self._request_id = 0
         self._session_read_timeout_seconds = read_timeout_seconds
+        self._close_write_stream_on_read_close = close_write_stream_on_read_close
         self._in_flight = {}
         self._progress_callbacks = {}
         self._response_routers = []
         self._exit_stack = AsyncExitStack()
+        self._exit_stack.push_async_callback(self._read_stream.aclose)
+        self._exit_stack.push_async_callback(self._write_stream.aclose)
 
     def add_response_router(self, router: ResponseRouter) -> None:
         """Register a response router to handle responses for non-standard requests.
@@ -349,7 +359,10 @@ def _receive_notification_adapter(self) -> TypeAdapter[ReceiveNotificationT]:
         raise NotImplementedError
 
     async def _receive_loop(self) -> None:
-        async with self._read_stream, self._write_stream:
+        async with AsyncExitStack() as stack:
+            await stack.enter_async_context(self._read_stream)
+            if self._close_write_stream_on_read_close:
+                await stack.enter_async_context(self._write_stream)
             try:
 
                 async def _handle_session_message(message: SessionMessage) -> None:

tests/server/test_cancel_handling.py

@@ -19,6 +19,7 @@
     InitializeRequestParams,
     JSONRPCNotification,
     JSONRPCRequest,
+    JSONRPCResponse,
     ListToolsResult,
     PaginatedRequestParams,
     TextContent,
@@ -100,29 +101,18 @@ async def first_request():
 
 
 @pytest.mark.anyio
-async def test_server_cancels_in_flight_handlers_on_transport_close():
-    """When the transport closes mid-request, server.run() must cancel in-flight
-    handlers rather than join on them.
-
-    Without the cancel, the task group waits for the handler, which then tries
-    to respond through a write stream that _receive_loop already closed,
-    raising ClosedResourceError and crashing server.run() with exit code 1.
-
-    This drives server.run() with raw memory streams because InMemoryTransport
-    wraps it in its own finally-cancel (_memory.py) which masks the bug.
-    """
+async def test_server_drains_in_flight_handlers_on_transport_read_eof():
+    """When the transport's read side hits EOF (e.g., stdio stdin closes), the
+    server must drain already-started handlers so their responses reach the
+    peer via the still-open write side."""
     handler_started = anyio.Event()
-    handler_cancelled = anyio.Event()
+    handler_allowed_to_finish = anyio.Event()
     server_run_returned = anyio.Event()
 
     async def handle_call_tool(ctx: ServerRequestContext, params: CallToolRequestParams) -> CallToolResult:
         handler_started.set()
-        try:
-            await anyio.sleep_forever()
-        finally:
-            handler_cancelled.set()
-        # unreachable: sleep_forever only exits via cancellation
-        raise AssertionError  # pragma: no cover
+        await handler_allowed_to_finish.wait()
+        return CallToolResult(content=[TextContent(type="text", text="ok")])
 
     server = Server("test", on_call_tool=handle_call_tool)
 
@@ -167,9 +157,13 @@ async def run_server():
             # handler gets CancelledError, server.run() returns.
             await to_server.aclose()
 
-            await server_run_returned.wait()
+            handler_allowed_to_finish.set()
+
+            response = await from_server.receive()
+            assert isinstance(response.message, JSONRPCResponse)
+            assert response.message.id == 2
 
-    assert handler_cancelled.is_set()
+            await server_run_returned.wait()
 
 
 @pytest.mark.anyio

tests/server/test_stdio.py

@@ -5,9 +5,27 @@
 import anyio
 import pytest
 
+from mcp.server import Server, ServerRequestContext
 from mcp.server.stdio import stdio_server
 from mcp.shared.message import SessionMessage
-from mcp.types import JSONRPCMessage, JSONRPCRequest, JSONRPCResponse, jsonrpc_message_adapter
+from mcp.types import (
+    LATEST_PROTOCOL_VERSION,
+    CallToolRequestParams,
+    CallToolResult,
+    ClientCapabilities,
+    Implementation,
+    InitializeRequestParams,
+    JSONRPCError,
+    JSONRPCMessage,
+    JSONRPCNotification,
+    JSONRPCRequest,
+    JSONRPCResponse,
+    ListToolsResult,
+    PaginatedRequestParams,
+    TextContent,
+    Tool,
+    jsonrpc_message_adapter,
+)
 
 
 @pytest.mark.anyio
@@ -92,3 +110,79 @@ async def test_stdio_server_invalid_utf8(monkeypatch: pytest.MonkeyPatch):
                 second = await read_stream.receive()
                 assert isinstance(second, SessionMessage)
                 assert second.message == valid
+
+
+@pytest.mark.anyio
+async def test_stdio_server_drains_in_flight_responses_on_stdin_eof():
+    """When stdin reaches EOF (e.g., bash-redirected input), already-received
+    requests must still be able to emit their responses on stdout."""
+    stdin = io.StringIO()
+    stdout = io.StringIO()
+
+    tool_started_count = 0
+    both_tools_started = anyio.Event()
+    allow_tools_to_finish = anyio.Event()
+
+    async def handle_list_tools(ctx: ServerRequestContext, params: PaginatedRequestParams | None) -> ListToolsResult:
+        return ListToolsResult(tools=[Tool(name="slow", description="test", input_schema={})])
+
+    async def handle_call_tool(ctx: ServerRequestContext, params: CallToolRequestParams) -> CallToolResult:
+        nonlocal tool_started_count
+        tool_started_count += 1
+        if tool_started_count == 2:
+            both_tools_started.set()
+        await allow_tools_to_finish.wait()
+        return CallToolResult(content=[TextContent(type="text", text="ok")])
+
+    server = Server("test", on_list_tools=handle_list_tools, on_call_tool=handle_call_tool)
+
+    init_req = JSONRPCRequest(
+        jsonrpc="2.0",
+        id=0,
+        method="initialize",
+        params=InitializeRequestParams(
+            protocol_version=LATEST_PROTOCOL_VERSION,
+            capabilities=ClientCapabilities(),
+            client_info=Implementation(name="test", version="1.0"),
+        ).model_dump(by_alias=True, mode="json", exclude_none=True),
+    )
+    initialized = JSONRPCNotification(jsonrpc="2.0", method="notifications/initialized")
+    call_1 = JSONRPCRequest(
+        jsonrpc="2.0",
+        id=1,
+        method="tools/call",
+        params=CallToolRequestParams(name="slow", arguments={}).model_dump(by_alias=True, mode="json"),
+    )
+    call_2 = JSONRPCRequest(
+        jsonrpc="2.0",
+        id=2,
+        method="tools/call",
+        params=CallToolRequestParams(name="slow", arguments={}).model_dump(by_alias=True, mode="json"),
+    )
+
+    for message in (init_req, initialized, call_1, call_2):
+        stdin.write(message.model_dump_json(by_alias=True, exclude_none=True) + "\n")
+    stdin.seek(0)
+
+    async with stdio_server(stdin=anyio.AsyncFile(stdin), stdout=anyio.AsyncFile(stdout)) as (
+        read_stream,
+        write_stream,
+    ):
+        with anyio.fail_after(5):
+            async with anyio.create_task_group() as tg:
+                tg.start_soon(server.run, read_stream, write_stream, server.create_initialization_options())
+                await both_tools_started.wait()
+                allow_tools_to_finish.set()
+
+    stdout.seek(0)
+    ids: set[int | str] = set()
+    for line in stdout.readlines():
+        line = line.strip()
+        if not line:
+            continue
+        message = jsonrpc_message_adapter.validate_json(line)
+        if isinstance(message, JSONRPCResponse | JSONRPCError):
+            assert message.id is not None
+            ids.add(message.id)
+    assert 1 in ids
+    assert 2 in ids