`uv run` discards free-threaded (3.13t) environments and recreates non-free-threaded ones

[Ravencentric]

I’ve been working on adding free-threaded support to my libraries. Like most libraries, mine support a wide range of Python versions (e.g., requires-python = ">=3.10").

My first step was running uv sync --python 3.13t and then testing my code with uv run .... To my surprise, uv run discarded the free-threaded environment and recreated a non-free-threaded one.

The only workarounds I’ve found are:

• Setting UV_PYTHON=3.13t. This applies globally, not just to the current project, which makes it unsuitable if I am juggling multiple projects.
• Adding --python 3.13t to every uv run invocation. This is repetitive and easy to forget.
• Activating the environment directly and skipping uv run. This forces me to think about venvs, which defeats the convenience of uv run.

Minimal Reproducible Example

❯ docker run --rm -it ghcr.io/astral-sh/uv:0.8.15-python3.13-bookworm /bin/sh
# uv init foobar --package --python ">=3.10"
Initialized project `foobar` at `/home/foobar`
# cd foobar
# uv sync --python 3.13t   # I want to use a free-threaded build
Using CPython 3.13.7
Creating virtual environment at: .venv
Resolved 1 package in 29ms
      Built foobar @ file:///home/foobar
Prepared 1 package in 21ms
Installed 1 package in 3ms
 + foobar==0.1.0 (from file:///home/foobar)
# uv run foobar   # uv run discards the free-threaded venv and rebuilds with a non-free-threaded interpreter
Using CPython 3.13.7 interpreter at: /usr/local/bin/python3.13
Removed virtual environment at: .venv
Creating virtual environment at: .venv
Installed 1 package in 11ms
Hello from foobar!
# uv run python -c 'import sys; print(sys._is_gil_enabled())'
True

Platform

Windows 11, Debian Bookworm

Version

0.8.15

Python version

Python 3.13t

[zanieb]

I think this is a duplicate of #12445

Thanks for the MRE!