fix(mcp): re-raise CancelledError explicitly in MCPServerTask.run (#21318)

On Python 3.11+, `asyncio.CancelledError` inherits from `BaseException`
(not `Exception`), so the broad `except Exception as exc:` in
`MCPServerTask.run`'s transport loop did NOT catch it. Task cancellation
from gateway restart / explicit `task.cancel()` silently escaped past
the reconnect logic — the MCP server task died without going through
the shutdown/reconnect code paths that check `_shutdown_event`.

Add an explicit `except asyncio.CancelledError: raise` before the broad
catch so cancellation propagation is self-documenting rather than an
accident of exception hierarchy, and future sibling-site work (e.g.
distinguishing shutdown-cancel from transport-cancel) has an obvious
hook. Behavior on pre-3.8 Pythons where CancelledError WAS an Exception
subclass is also corrected: the old path would have caught it and
treated it as a connection failure worth retrying.

Closes #9930.

tests/tools/test_mcp_cancelled_error_propagation.py

@@ -0,0 +1,92 @@
+"""Regression tests for ``MCPServerTask.run`` + ``asyncio.CancelledError``.
+
+Background
+==========
+On Python 3.11+, ``asyncio.CancelledError`` inherits from ``BaseException``
+rather than ``Exception``, so a bare ``except Exception`` does NOT catch it.
+``MCPServerTask.run`` had a broad ``except Exception`` around the transport
+loop which meant a task cancellation (gateway restart, explicit
+``task.cancel()``) caused the reconnect loop to exit silently — the MCP
+server stayed dead until Hermes was restarted. See #9930.
+
+The fix adds an explicit ``except asyncio.CancelledError: raise`` BEFORE
+the broad catch so cancellation propagates cleanly to asyncio's task
+machinery and ``MCPServerTask.shutdown()``'s ``await self._task`` completes
+without hanging the reconnect loop.
+"""
+
+from __future__ import annotations
+
+import asyncio
+from unittest.mock import patch
+
+import pytest
+
+
+async def _hanging_run(self, cfg):
+    """Stand-in transport that hangs forever so we can cancel it."""
+    await asyncio.sleep(3600)
+
+
+class TestCancelledErrorPropagation:
+    def test_cancelled_error_is_not_swallowed_by_except_exception(self):
+        """CancelledError raised inside the transport call must re-raise
+        so the reconnect loop terminates cleanly on cancel — not stay wedged."""
+        from tools.mcp_tool import MCPServerTask
+
+        server = MCPServerTask("cancel-test")
+
+        async def drive():
+            with patch.object(MCPServerTask, "_run_stdio", _hanging_run), \
+                 patch.object(MCPServerTask, "_is_http", lambda self: False):
+                task = asyncio.create_task(server.run({"command": "fake"}))
+                # Let the run loop enter the try/except and start awaiting.
+                await asyncio.sleep(0.05)
+                task.cancel()
+                # The fix guarantees the task completes (either via
+                # CancelledError propagation or clean exit) rather than
+                # hanging forever.
+                try:
+                    await asyncio.wait_for(task, timeout=2.0)
+                except asyncio.CancelledError:
+                    return "cancelled_cleanly"
+                except asyncio.TimeoutError:
+                    # If we hit this, the reconnect loop swallowed the cancel
+                    # and stayed wedged — the exact #9930 bug.
+                    task.cancel()
+                    try:
+                        await task
+                    except Exception:
+                        pass
+                    return "wedged"
+                return "clean_return"
+
+        outcome = asyncio.run(drive())
+        assert outcome in ("cancelled_cleanly", "clean_return"), (
+            f"MCPServerTask.run wedged on cancel (outcome={outcome}) — "
+            f"#9930 regression"
+        )
+
+    def test_shutdown_completes_promptly_when_task_is_cancelled(self):
+        """``shutdown()`` falls through to ``task.cancel()`` + ``await self._task``
+        after a grace period. That cancel must unwedge the reconnect loop —
+        otherwise ``await self._task`` hangs indefinitely."""
+        from tools.mcp_tool import MCPServerTask
+
+        server = MCPServerTask("shutdown-cancel-test")
+
+        async def drive():
+            with patch.object(MCPServerTask, "_run_stdio", _hanging_run), \
+                 patch.object(MCPServerTask, "_is_http", lambda self: False):
+                server._task = asyncio.ensure_future(server.run({"command": "fake"}))
+                await asyncio.sleep(0.05)
+                server._shutdown_event.set()
+                server._task.cancel()
+                try:
+                    await asyncio.wait_for(server._task, timeout=2.0)
+                except (asyncio.CancelledError, asyncio.TimeoutError):
+                    pass
+                return server._task.done()
+
+        done = asyncio.run(drive())
+        assert done, "MCPServerTask did not finish after cancel — #9930 regression"

tools/mcp_tool.py

@@ -1399,6 +1399,18 @@ class MCPServerTask:
                 # still detect a transient in-flight state — it'll be
                 # re-set after the fresh session initializes.
                 continue
+            except asyncio.CancelledError:
+                # Task was cancelled (shutdown, gateway restart, explicit
+                # task.cancel()). Don't treat this as a connection failure —
+                # CancelledError inherits from BaseException (not Exception)
+                # in Python 3.11+, so the broad ``except Exception`` below
+                # would NOT catch it; we'd silently exit the reconnect loop
+                # and the MCP server would stay dead until Hermes is fully
+                # restarted. Re-raise so the task's cancellation propagates
+                # correctly to asyncio's task machinery and ``shutdown()``'s
+                # ``await self._task`` completes. See #9930.
+                self.session = None
+                raise
             except Exception as exc:
                 self.session = None