diff --git a/tests/hermes_cli/conftest.py b/tests/hermes_cli/conftest.py
index 3eee1b2f32f2a..2848a8a9aa238 100644
--- a/tests/hermes_cli/conftest.py
+++ b/tests/hermes_cli/conftest.py
@@ -41,6 +41,16 @@ def _suppress_concurrent_hermes_gate(request, monkeypatch):
         from hermes_cli import main as _cli_main
     except Exception:
         return
+    # raising=False: under pytest's per-test spawn isolation, a concurrent
+    # xdist worker importing a module that transitively touches hermes_cli.main
+    # can briefly expose a partially-initialized module object here — one where
+    # _detect_concurrent_hermes_instances isn't defined yet. A bare setattr
+    # would raise AttributeError and error the (unrelated) test. The attribute
+    # always exists once main.py finishes importing, so a no-op when it's
+    # transiently absent is the correct, race-free default.
     monkeypatch.setattr(
-        _cli_main, "_detect_concurrent_hermes_instances", lambda *_a, **_k: []
+        _cli_main,
+        "_detect_concurrent_hermes_instances",
+        lambda *_a, **_k: [],
+        raising=False,
     )