@btskinn I’m not using that, so I’m not sure what’s wrong, but I think I’ve read enough of what @cjerdonek says to understand his concerns about future-proofing. So the best way forward for now may be to just remove build-backend until the spec process gets worked out. I’ll figure out a way to use direct setuptools to build my distribution packages in CI.
I’ve found you can include toml in build-system.requires and then use that in setup.py to re-extract the build requirements and add them as an extra, such that pip install .[build] creates an environment where python setup.py is valid. It seems like a hack to recursively bootstrap the build systems like this, but I guess it’s better than relying on coincidentally useful undefined behavior.