Fix for issue #1401

src/mcp/client/session.py

@@ -85,6 +85,8 @@ async def __call__(
 async def _default_message_handler(
     message: RequestResponder[types.ServerRequest, types.ClientResult] | types.ServerNotification | Exception,
 ) -> None:
+    if isinstance(message, Exception):
+        raise message
     await anyio.lowlevel.checkpoint()
 
 

tests/client/test_session.py

@@ -1476,3 +1476,14 @@ async def test_send_notification_after_close_is_dropped_silently():
     finally:
         for s in (s2c_send, s2c_recv, c2s_send, c2s_recv):
             s.close()
+
+
+@pytest.mark.anyio
+async def test_default_message_handler():
+    from mcp.client.session import _default_message_handler
+
+    with pytest.raises(ValueError, match="test error"):
+        await _default_message_handler(ValueError("test error"))
+
+    # Should not raise for non-exception
+    await _default_message_handler(types.ToolListChangedNotification())

tests/transports/stdio/test_lifecycle.py

@@ -228,7 +228,7 @@ def test_fallback_process_reports_death_through_returncode_without_a_wait_call()
     try:
         process = FallbackProcess(popen)
 
-        os.waitid(os.P_PID, popen.pid, os.WEXITED | os.WNOWAIT)
+        os.waitid(os.P_PID, popen.pid, os.WEXITED | os.WNOWAIT)  # pyright: ignore[reportAttributeAccessIssue, reportUnknownMemberType]
         assert process.returncode == 0
     finally:
         popen.stdin.close()