With snapcraft latest/stable (Rev 8619), simply asking to list plugins with "snapcraft plugins" in a project with an "extensions:" line in snapcraft.yaml produces (log file): 2023-01-10 13:31:36.747 Starting Snapcraft 7.2.9 2023-01-10 13:31:36.766 Bad snapcraft.yaml content: - extra field 'extensions' not permitted in 'apps.proto-device-application' configuration 2023-01-10 13:31:36.780 Traceback (most recent call last): 2023-01-10 13:31:36.780 File "/snap/snapcraft/8619/lib/python3.8/site-packages/snapcraft/projects.py", line 567, in unmarshal 2023-01-10 13:31:36.780 project = Project(**data) 2023-01-10 13:31:36.780 File "pydantic/main.py", line 331, in pydantic.main.BaseModel.__init__ 2023-01-10 13:31:36.780 pydantic.error_wrappers.ValidationError: 1 validation error for Project 2023-01-10 13:31:36.780 apps -> proto-device-application -> extensions 2023-01-10 13:31:36.780 extra fields not permitted (type=value_error.extra) 2023-01-10 13:31:36.780 Full execution log: '/home/ubuntu/.cache/snapcraft/log/snapcraft-20230110-133136.746097.log' Thread in channel here, where someone else confirms: https://chat.canonical.com/canonical/channels/starcraft/pgjhyatkefb67ds7gxm9p8h7ra