diff --git a/aircraft/aircraft_ws/src/mission/mission/mission_actions.py b/aircraft/aircraft_ws/src/mission/mission/mission_actions.py
new file mode 100644
index 00000000..c75934e4
--- /dev/null
+++ b/aircraft/aircraft_ws/src/mission/mission/mission_actions.py
@@ -0,0 +1,17 @@
+"""Mission leaf action name allowlist (import-safe offline)."""
+
+KNOWN_ACTIONS = frozenset({
+    "takeoff",
+    "land",
+    "orbit",
+    "wait",
+    "offboard",
+    "reposition",
+    "speed",
+    "check_blackboard",
+})
+
+
+def is_known_mission_action(action) -> bool:
+    """Return True if action is a supported mission tree leaf name."""
+    return isinstance(action, str) and action in KNOWN_ACTIONS
diff --git a/aircraft/aircraft_ws/src/mission/mission/test_mission_actions.py b/aircraft/aircraft_ws/src/mission/mission/test_mission_actions.py
new file mode 100644
index 00000000..9587f580
--- /dev/null
+++ b/aircraft/aircraft_ws/src/mission/mission/test_mission_actions.py
@@ -0,0 +1,26 @@
+#!/usr/bin/env python3
+import unittest
+
+try:
+    from mission.mission_actions import KNOWN_ACTIONS, is_known_mission_action
+except ImportError:  # plain unittest from this directory
+    from mission_actions import KNOWN_ACTIONS, is_known_mission_action
+
+
+class TestMissionActions(unittest.TestCase):
+    def test_known(self):
+        for name in ("takeoff", "land", "wait", "offboard"):
+            self.assertTrue(is_known_mission_action(name))
+
+    def test_unknown(self):
+        self.assertFalse(is_known_mission_action("fly_to_moon"))
+        self.assertFalse(is_known_mission_action(""))
+        self.assertFalse(is_known_mission_action(None))
+        self.assertFalse(is_known_mission_action(1))
+
+    def test_frozen_set_size(self):
+        self.assertEqual(len(KNOWN_ACTIONS), 8)
+
+
+if __name__ == "__main__":
+    unittest.main()
diff --git a/aircraft/aircraft_ws/src/mission/mission/tree_builder.py b/aircraft/aircraft_ws/src/mission/mission/tree_builder.py
index 1127d028..73feb556 100644
--- a/aircraft/aircraft_ws/src/mission/mission/tree_builder.py
+++ b/aircraft/aircraft_ws/src/mission/mission/tree_builder.py
@@ -1,6 +1,7 @@
 import uuid
 import py_trees
 from mission import behaviors
+from mission.mission_actions import is_known_mission_action
 
 def create_mission_tree(node_cfg, ros_node):
     # Recursively parse a YAML dictionary node into a py_trees object
@@ -27,7 +28,11 @@ def create_mission_tree(node_cfg, ros_node):
         elif action == 'check_blackboard':
             return behaviors.CheckBlackboardBehavior(name, ros_node, params)
         else:
-            ros_node.get_logger().error(f"Unknown action: {action}")
+            # Keep KNOWN_ACTIONS in mission_actions.py in sync with branches above
+            if not is_known_mission_action(action):
+                ros_node.get_logger().error(f"Unknown action: {action}")
+            else:
+                ros_node.get_logger().error(f"Unhandled known action branching: {action}")
             return py_trees.behaviours.Failure(name=f"Unknown_{action}")
 
     # Is it a composite node (a sequence or fallback/selector branch)?