fix(mcp): close the HTTP transport when a remote connect fails

_connect_http entered streamablehttp_client and ClientSession on an
AsyncExitStack but never closed it when initialize or list_tools
failed. The half-open transport was left to the garbage collector,
which exited its AnyIO cancel scope from the wrong task, and the event
loop kept spinning: about 18% of a core per failed connect, growing
with each one, until restart.

Close the stack in the connecting task on any failure or cancellation,
the same way _connect_sse already does.

Fixes #5518

src/mcp_manager.py

@@ -10,6 +10,7 @@ import logging
 import os
 import re
 import asyncio 
+import contextlib
 from typing import Any, Dict, List, Optional, Set, Tuple
 from src.database import McpServer, SessionLocal
 
@@ -383,12 +384,21 @@ class McpManager:
 
             provider = build_provider(server_id, url, on_redirect=_on_redirect)
             stack = AsyncExitStack()
-            transport = await stack.enter_async_context(streamablehttp_client(url, auth=provider))
-            read_stream, write_stream, _get_session_id = transport
-            session = await stack.enter_async_context(ClientSession(read_stream, write_stream))
-            await session.initialize()
+            try:
+                transport = await stack.enter_async_context(streamablehttp_client(url, auth=provider))
+                read_stream, write_stream, _get_session_id = transport
+                session = await stack.enter_async_context(ClientSession(read_stream, write_stream))
+                await session.initialize()
 
-            tools_result = await session.list_tools()
+                tools_result = await session.list_tools()
+            except BaseException:
+                # Close the half-open transport here, in the task that entered
+                # it. Left for the garbage collector, its AnyIO cancel scope is
+                # exited from the wrong task and the event loop keeps spinning
+                # (#5518). Its own teardown error would only mask the real one.
+                with contextlib.suppress(Exception):
+                    await stack.aclose()
+                raise
             tools = []
             for tool in tools_result.tools:
                 tools.append({

tests/test_mcp_manager.py

@@ -39,3 +39,74 @@ def test_http_transport_routes_to_start_http_connect():
         result = asyncio.run(mgr.connect_server("id1", "n", "http", url="https://x/mcp"))
     assert result == "ROUTED"
     m.assert_called_once()
+
+
+def _patch_http_transport(monkeypatch, initialize):
+    """Fake the Streamable HTTP transport and record, for each close, whether
+    it ran in the task that opened it (AnyIO requires that)."""
+    import contextlib
+    import mcp
+    import mcp.client.streamable_http as streamable_http
+    import src.mcp_oauth as mcp_oauth
+
+    closes = []
+
+    @contextlib.asynccontextmanager
+    async def fake_client(url, auth=None):
+        opener = asyncio.current_task()
+        try:
+            yield (object(), object(), lambda: None)
+        finally:
+            closes.append(asyncio.current_task() is opener)
+
+    class FakeSession:
+        def __init__(self, *a):
+            pass
+
+        async def __aenter__(self):
+            return self
+
+        async def __aexit__(self, *exc):
+            return False
+
+    FakeSession.initialize = initialize
+    monkeypatch.setattr(streamable_http, "streamablehttp_client", fake_client)
+    monkeypatch.setattr(mcp, "ClientSession", FakeSession)
+    monkeypatch.setattr(mcp_oauth, "build_provider", lambda *a, **k: None)
+    return closes
+
+
+def test_failed_http_connect_closes_transport_in_its_own_task(monkeypatch):
+    """#5518: a failed connect left its transport for the GC, which exited the
+    AnyIO cancel scope from the wrong task and kept the event loop spinning."""
+    async def initialize(self):
+        raise RuntimeError("Session terminated")
+
+    closes = _patch_http_transport(monkeypatch, initialize)
+    mgr = McpManager()
+
+    ok = asyncio.run(mgr._start_http_connect("s1", "remote", "http://x/mcp/", wait=2))
+
+    assert ok is False
+    assert mgr._connections["s1"]["status"] == "error"
+    assert "Session terminated" in mgr._connections["s1"]["error"]
+    assert closes == [True]
+    assert "s1" not in mgr._stacks
+
+
+def test_cancelled_http_connect_closes_transport(monkeypatch):
+    """Deleting a server mid-connect cancels the connect task; the transport
+    must still be closed rather than leaked."""
+    async def initialize(self):
+        await asyncio.sleep(3600)
+
+    closes = _patch_http_transport(monkeypatch, initialize)
+    mgr = McpManager()
+
+    async def run():
+        await mgr._start_http_connect("s1", "remote", "http://x/mcp/", wait=0.05)
+        await mgr.disconnect_server("s1")
+        await asyncio.sleep(0)
+
+    asyncio.run(run())
+    assert closes == [True]