Without MT_PORT_PATH the app runs 40250 system.py on the Python UI and 3D surfaces; MT_PORT_PATH=legacy keeps the GDScript AppFlow. The smoke/playable live tests stay on legacy, MT_TEST_MODE=python_ui remains a port alias, and an unset switch falls back to legacy (with a warning) when the Python host or the 40250 pack is missing. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>