pypi/pipx tools installed with uv: which Python should the tool's venv use?

[halms]

A pypi:/pipx: tool installed with uv keeps a venv whose python links to the interpreter uv chose. When CI caches the mise data dir (for example with jdx/mise-action), the tool works on a warm cache run only if that interpreter is also restored. Today that depends on how the tool was installed:

1. Graph-locked install (mise lock with uv >= 0.12.10)

mise runs uv sync --frozen --no-python-downloads --python <mise python>. The venv links to installs/python/<version>/bin/python. Deterministic and self-contained.

2. Version-only install, python managed by mise

mise runs uv tool install <pkg>==<version> without --python and puts mise's Python first on PATH. uv's default preference is managed, so if a uv-managed Python already exists, uv uses it instead of mise's. One way this happens: with python.uv_venv_auto = "create|source", mise install created the project venv with a bare uv venv (fixed in #13981), uv downloaded CPython into ~/.local/share/uv/python, and the tool was built on that download. A restored cache then has the tool but not its interpreter, and the tool fails with couldn't exec process.

When uv does pick mise's Python, fix_venv_python_symlink rewrites the link to installs/python/X.Y, which looks like the intended outcome.

3. Version-only install, no Python managed by mise

uv uses a Python from PATH (/usr/bin/python3, or a setup-python toolcache path that includes the patch version) or downloads one into ~/.local/share/uv/python. Either way the venv depends on an interpreter outside the mise data dir.

Ideas

• For case 2: pass --python <mise python> to uv tool install when mise manages Python, like the graph path binds its interpreter. The result then no longer depends on what uv has downloaded before.
• For case 3: is an interpreter outside the mise data dir intended? Options I see: document that caching the mise data dir is not enough for these tools, or let mise's own uv calls put downloaded Pythons under the mise data dir so the tools are self-contained.

Happy to open a PR for case 2 if that direction fits.

AI-assisted — Tool: Claude Code; model: anthropic/claude-opus-5-5; version: 2.1.289.

[jdx]

Confirmed all three cases against the code, and the case 2 idea is right: the uv tool install call never passes --python, and fix_venv_python_symlink deliberately skips links that don't point into the mise installs dir, so a uv-downloaded interpreter went unnoticed.

Opened #14024 for case 2. Version-only installs now pass --python <mise python> when mise manages Python (a --python in uvx_args still wins). The e2e test links to the uv download before the change and to installs/python/ after.

One tradeoff: an explicit --python is a hard requirement, so a package whose requires-python excludes mise's Python fails instead of getting a uv download.

Case 3 I'd document rather than change: with no mise-managed Python, uv uses a PATH or downloaded interpreter. Redirecting UV_PYTHON_INSTALL_DIR into the data dir would change where uv stores Pythons for every tool, so it would need a setting.

AI-assisted — Tool: Claude Code; model: anthropic/claude-sonnet-5-5; version: 2.1.288.