diff --git a/tensorrt_llm/executor/proxy.py b/tensorrt_llm/executor/proxy.py
index 11451bad5b67..46c380ed74fe 100644
--- a/tensorrt_llm/executor/proxy.py
+++ b/tensorrt_llm/executor/proxy.py
@@ -573,7 +573,10 @@ def mpi_done_callback(future: concurrent.futures.Future):
             raise RuntimeError(
                 "Executor worker returned error") from ready_signal
 
-        if isinstance(self.mpi_session, MpiPoolSession) and len(status) == 3:
+        # Use the polymorphic predicate: the module-level ``MpiPoolSession``
+        # symbol may be rebound to a factory at runtime by test infrastructure
+        # (tests/test_common/session_reuse.py), breaking ``isinstance``.
+        if not self.mpi_session.is_comm_session() and len(status) == 3:
             worker_process_identities: List[WorkerProcessIdentity] = status[2]
             self._worker_process_monitor.register(worker_process_identities)