The issue is described in these 2 spots: https://github.com/pantsbuild/pex/blob/master/pex/version.py#L6-L9 https://github.com/pantsbuild/pex/blob/master/pex/package.py#L84-L87 The version.py setuptools constraint should limit pex to a compatible version of setuptools where

Requirement.specifier

is a valid attribute, but clearly that's no happening somehow. @most-wire-83210 maybe thats enough detail to help you debug further.