Add regression test for stateless request memory cleanup (#1140)

Author: felixweinberger

tests/server/test_streamable_http_manager.py

@@ -1,10 +1,11 @@
 """Tests for StreamableHTTPSessionManager."""
 
-from unittest.mock import AsyncMock
+from unittest.mock import AsyncMock, patch
 
 import anyio
 import pytest
 
+from mcp.server import streamable_http_manager
 from mcp.server.lowlevel import Server
 from mcp.server.streamable_http import MCP_SESSION_ID_HEADER
 from mcp.server.streamable_http_manager import StreamableHTTPSessionManager
@@ -197,3 +198,65 @@ async def mock_receive():
         "Session ID should be removed from _server_instances after an exception"
     )
     assert not manager._server_instances, "No sessions should be tracked after the only session crashes"
+
+
+@pytest.mark.anyio
+async def test_stateless_requests_memory_cleanup():
+    """Test that stateless requests actually clean up resources using real transports."""
+    app = Server("test-stateless-real-cleanup")
+    manager = StreamableHTTPSessionManager(app=app, stateless=True)
+
+    # Track created transport instances
+    created_transports = []
+
+    # Patch StreamableHTTPServerTransport constructor to track instances
+
+    original_constructor = streamable_http_manager.StreamableHTTPServerTransport
+
+    def track_transport(*args, **kwargs):
+        transport = original_constructor(*args, **kwargs)
+        created_transports.append(transport)
+        return transport
+
+    with patch.object(streamable_http_manager, "StreamableHTTPServerTransport", side_effect=track_transport):
+        async with manager.run():
+            # Mock app.run to complete immediately
+            app.run = AsyncMock(return_value=None)
+
+            # Send a simple request
+            sent_messages = []
+
+            async def mock_send(message):
+                sent_messages.append(message)
+
+            scope = {
+                "type": "http",
+                "method": "POST",
+                "path": "/mcp",
+                "headers": [
+                    (b"content-type", b"application/json"),
+                    (b"accept", b"application/json, text/event-stream"),
+                ],
+            }
+
+            # Empty body to trigger early return
+            async def mock_receive():
+                return {
+                    "type": "http.request",
+                    "body": b"",
+                    "more_body": False,
+                }
+
+            # Send a request
+            await manager.handle_request(scope, mock_receive, mock_send)
+
+            # Verify transport was created
+            assert len(created_transports) == 1, "Should have created one transport"
+
+            transport = created_transports[0]
+
+            # The key assertion - transport should be terminated
+            assert transport._terminated, "Transport should be terminated after stateless request"
+
+            # Verify internal state is cleaned up
+            assert len(transport._request_streams) == 0, "Transport should have no active request streams"