```yaml name: testcraft version: "0.1" base: ubuntu@24.04 platforms: all: parts: my-test: plugin: nil ``` ```bash craft-application/testcraft> python3 ./__main__.py pack testcraft internal error: 1 validation error for dict[literal['platforms'],dict[str,Platform]] platforms.all Input should be a valid dictionary or instance of Platform [type=model_type, input_value=None, input_type=NoneType] For further information visit https://errors.pydantic.dev/2.10/v/model_type ``` It's not surprising for a user to attempt to use `all` in `build-on`. Previously, Snapcraft would get to unmarshal the project and produce a user-friendly error like: ``` > snapcraft pack Bad snapcraft.yaml content: - 'all' cannot be used for 'build-on' (in field 'platforms') ``` Similar to #690