tox-uv fails at uv sync when system uv is older than minimal required version

[ssbarnea]

Issue

I recently observed that if you add a minimal version of uv like below and run tox on a system that has an over uv installed it will fail to sync due to uv being older than minimal required.

Environment

# pyproject.toml
[tool.uv]
required-version = ">=0.8.13"

Output of running tox

Output of tox -rvv

Run tox -e py310-lint
ROOT: will run in automatically provisioned tox, host /opt/hostedtoolcache/Python/3.10.18/x64/bin/python3 is missing [requires (has)]: tox-extra>=2.1
ROOT: freeze> /opt/hostedtoolcache/Python/3.10.18/x64/bin/uv --color never pip freeze
ROOT: bindep==2.13.0,cachetools==6.2.0,chardet==5.2.0,colorama==0.4.6,distlib==0.4.0,distro==1.9.0,filelock==3.19.1,gitdb==4.0.12,gitpython==3.1.45,packaging==25.0,parsley==1.3,pbr==7.0.1,pip==25.2,platformdirs==4.3.8,pluggy==1.6.0,pyproject-api==1.9.1,setuptools==80.9.0,smmap==5.0.2,tomli==2.2.1,tox==4.28.4,tox-extra==2.2.0,tox-uv==1.28.0,typing-extensions==4.15.0,uv==0.8.13,virtualenv==20.34.0
ROOT: provision> .tox/.tox/bin/python -m tox -e py310-lint
py310-lint: uv-sync> uv sync --locked --python-preference system -p /home/runner/work/mk/mk/.tox/.tox/bin/python
error: Required uv version `>=0.8.13` does not match the running version `0.8.8`. Update `uv` by running `uv self update`.
py310-lint: exit 2 (0.01 seconds) /home/runner/work/mk/mk> uv sync --locked --python-preference system -p /home/runner/work/mk/mk/.tox/.tox/bin/python pid=3883

Seen at https://github.com/pycontribs/mk/actions/runs/17232944097/job/48891086328?pr=392

PS. I am working on it, filed here only for sharing.

[ssbarnea]

@gaborbernat One possible workaround for this would be to use the uv version from within .tox environment instead of the one from system PATH, especially because uv self update is not possible on all platforms (depends how uv was installed, uv, system-package, mise...).

The one inside .tox environment is almost for sure the latest anyway. Would it work or not?

Sadly I was not able to find a direct uv command that would identify if newer version of uv is required by the project, only when I run uv sync it would end-up complaining about outdated version.

I could add code to the plugin to read the pyproject.toml file and fail fast but I don't fancy duplicating an uv feature just to fail faster.

[gaborbernat]

https://github.com/tox-dev/tox-uv/blob/main/src/tox_uv/_venv.py#L180 should find the one inside the tox environment that should be new enough. I'd be ok to read the pyproject.toml and fail faster; but also ok with the status quo.