where is the constraint to use python 3.8.6 can come from ? it is not in pants.toml (in any of the interpreter constraints settings) where else could it be defined?

Try using

py-constraints

if you haven't yet to audit targets: https://www.pantsbuild.org/v2.5/docs/python-interpreter-compatibility#tip-activate-pantsbackendpythonmixed_interpreter_constraints Also double check if you are setting an env var with 3.8.6

nvm. it was an issue with the pants install itself...

rm -rf ~/.cache/pants

fixed it.