Log warning for unhandled exceptions in default message handler

Transport errors that are not tied to a specific pending request (e.g.,
GET stream failures) were silently swallowed by the default message
handler. Add a warning log so these exceptions are at least visible in
logs as an observability safety net.

Github-Issue: #1401

Author: heyhayes

src/mcp/client/session.py

@@ -166,6 +166,8 @@ async def __call__(
 async def _default_message_handler(
     message: RequestResponder[types.ServerRequest, types.ClientResult] | types.ServerNotification | Exception,
 ) -> None:
+    if isinstance(message, Exception):
+        logger.warning("Unhandled exception in message handler: %s", message)
     await anyio.lowlevel.checkpoint()
 
 

tests/client/test_session.py

@@ -38,7 +38,7 @@
 from mcp import MCPError
 from mcp.client import ClientRequestContext
 from mcp.client.client import Client
-from mcp.client.session import DEFAULT_CLIENT_INFO, ClientSession
+from mcp.client.session import DEFAULT_CLIENT_INFO, ClientSession, _default_message_handler
 from mcp.client.subscriptions import ToolsListChanged, listen
 from mcp.server import Server, ServerRequestContext
 from mcp.shared.direct_dispatcher import create_direct_dispatcher_pair
@@ -1030,6 +1030,15 @@ async def handler(msg: object) -> None:
     assert "message_handler raised on transport exception" in caplog.text
 
 
+@pytest.mark.anyio
+async def test_default_message_handler_logs_unhandled_transport_exceptions(caplog: pytest.LogCaptureFixture):
+    """The default handler has no per-request waiter to resolve, so a transport-level `Exception`
+    item (e.g. a GET stream failure) would vanish silently; it logs a warning as a safety net."""
+    with caplog.at_level("WARNING", logger="client"):
+        await _default_message_handler(RuntimeError("boom"))
+    assert "Unhandled exception in message handler" in caplog.text
+
+
 @pytest.mark.anyio
 async def test_message_handler_awaiting_session_traffic_on_transport_exception_completes():
     """A `message_handler` that awaits session traffic on a transport `Exception` item completes: