refactor(streamable_http): remove writer notification on ClientDisconnect

src/mcp/server/streamable_http.py

@@ -649,11 +649,8 @@ async def sse_writer():
             # server error — log at WARNING and send a response so middleware chains
             # (e.g. Starlette BaseHTTPMiddleware) don't raise "No response returned".
             # The ASGI server will drop the response if the socket is already closed.
-            # Notify the writer so the inner session task can unblock cleanly.
+            # The connect() context manager will close streams, unblocking the session task.
             logger.warning("Client disconnected during POST request")
-            if writer is not None:
-                with suppress(Exception):
-                    await writer.send(ClientDisconnect())
             # 499 = Client Closed Request (nginx convention, not in stdlib HTTPStatus)
             response = self._create_json_response(None, 499)  # type: ignore[arg-type]
             with suppress(Exception):

tests/server/streamable_http/test_client_disconnect_post.py

@@ -12,7 +12,7 @@
 
 import logging
 from typing import Any
-from unittest.mock import AsyncMock, MagicMock, patch
+from unittest.mock import AsyncMock, MagicMock
 
 import pytest
 from starlette.requests import ClientDisconnect, Request
@@ -125,81 +125,4 @@ async def dummy_send(message):
         assert send_calls[0]["type"] == "http.response.start"
         assert send_calls[0]["status"] == 499
 
-    @pytest.mark.anyio
-    async def test_client_disconnect_notifies_writer(self):
-        """Writer should receive ClientDisconnect so the inner session task can unblock."""
-        transport = StreamableHTTPServerTransport(mcp_session_id=None)
-        scope = self._make_scope()
-
-        # Capture what the writer receives
-        writer_messages: list[Any] = []
-
-        async def capture_writer(msg):
-            writer_messages.append(msg)
-
-        # Patch the internal writer
-        with patch.object(transport, "_read_stream_writer", MagicMock(send=capture_writer)):
-            mock_request = MagicMock(spec=Request)
-            mock_request.body = AsyncMock(side_effect=ClientDisconnect())
-            mock_request.headers = {
-                "content-type": "application/json",
-                "accept": "application/json, text/event-stream",
-            }
-            mock_request.scope = scope
-
-            send_calls: list[Any] = []
-
-            async def dummy_receive():
-                return {"type": "http.request", "body": b""}
-
-            async def dummy_send(message):
-                send_calls.append(message)
-
-            await transport._handle_post_request(
-                scope, mock_request, dummy_receive, dummy_send
-            )
-
-        # Writer should have been notified with ClientDisconnect
-        assert len(writer_messages) == 1, (
-            f"Expected writer to receive 1 message, got {len(writer_messages)}"
-        )
-        assert isinstance(writer_messages[0], ClientDisconnect), (
-            f"Expected ClientDisconnect sent to writer, got {type(writer_messages[0])}"
-        )
 
-    @pytest.mark.anyio
-    async def test_client_disconnect_writer_suppresses_errors(self):
-        """If the writer itself is broken, we should not crash (suppress(Exception))."""
-        transport = StreamableHTTPServerTransport(mcp_session_id=None)
-        scope = self._make_scope()
-
-        broken_send = AsyncMock(side_effect=RuntimeError("writer is broken"))
-
-        with patch.object(transport, "_read_stream_writer", MagicMock(send=broken_send)):
-            mock_request = MagicMock(spec=Request)
-            mock_request.body = AsyncMock(side_effect=ClientDisconnect())
-            mock_request.headers = {
-                "content-type": "application/json",
-                "accept": "application/json, text/event-stream",
-            }
-            mock_request.scope = scope
-
-            async def dummy_receive():
-                return {"type": "http.request", "body": b""}
-
-            send_calls: list[Any] = []
-
-            async def dummy_send(message):
-                send_calls.append(message)
-
-            # Should not raise even though writer.send() fails
-            await transport._handle_post_request(
-                scope, mock_request, dummy_receive, dummy_send
-            )
-
-        # The broken writer.send was called once (suppressed)
-        broken_send.assert_called_once()
-        # Response is still sent even though writer was broken
-        assert len(send_calls) >= 1
-        assert send_calls[0]["type"] == "http.response.start"
-        assert send_calls[0]["status"] == 499