If I run build in an empty directory (or any directory that isn't a Python project), rather than reporting the error, the tool appears to run setuptools and creates an empty project called UNKNOWN. If there's no pyproject.toml, I guess it's assuming setuptools as a fallback. But if there's no setup.py or setup.cfg this is clearly incorrect, and so should fail.

As it stands, the build runs and results in unwanted (and useless) build, dist and UNKNOWN.egg-info directories being created in my project. Luckily, I wasn't already using build or dist as actual directories for my code...