Hi, I can't use pyproject.toml for some projects with "optional build dependencies" because of build isolation. However, it would be very useful because we have also real build dependencies. I created an issue in pip repository (https://github.com/pypa/pip/issues/6144) but it seems to be a more general problem. I didn't find anything about such potential problem in PEP 517/518. Any thoughts how it can be solved? In the issue, I propose this: ``` [build-system] # Minimum requirements for the build system to execute. requires = [ "setuptools", "wheel", "numpy", "cython", "jinja2", "fluidpythran>=0.1.7" ] # packages that are used during the build only if they are installed: optional = ["mpi4py", "pythran"] ``` Pierre Augier