diff --git a/src/cloudai/_core/test_scenario.py b/src/cloudai/_core/test_scenario.py
index b8bdded3a..82315fb7d 100644
--- a/src/cloudai/_core/test_scenario.py
+++ b/src/cloudai/_core/test_scenario.py
@@ -217,6 +217,10 @@ def apply_params_set(self, action: dict[str, Any], env_params: dict[str, Any] |
         for key, value in full_action.items():
             if key.startswith("extra_env_vars."):
                 tdef.extra_env_vars[key[len("extra_env_vars.") :]] = value
+            elif key == "NUM_NODES":
+                # Handled below via new_tr.num_nodes; cmd_args has no such field, and CmdArgs'
+                # extra="allow" would otherwise let this create a phantom attribute on it.
+                continue
             else:
                 attrs = key.split(".")
                 obj = tdef.cmd_args
diff --git a/tests/test_cloudaigym.py b/tests/test_cloudaigym.py
index c1d8c3153..174e7049a 100644
--- a/tests/test_cloudaigym.py
+++ b/tests/test_cloudaigym.py
@@ -335,6 +335,7 @@ def test_params_set(setup_env: tuple[TestRun, Runner], num_nodes: int):
                 assert new_tr.test.extra_env_vars[key[len("extra_env_vars.") :]] == value
             elif key == "NUM_NODES":
                 assert new_tr.num_nodes == value
+                assert "NUM_NODES" not in cmd_args
             else:
                 assert cmd_args[key] == value