uv run --with old scipy: pin --python and pass --no-build
Bisecting a SciPy behaviour change with uv run --no-project --with scipy==1.16.3 ... on a new default interpreter (3.15 here) silently fell back to building the sdist, because no wheel exists for that Python. The build ran meson for minutes and then failed on a missing Fortran compiler. Pass --python 3.12 (any version the old release has wheels for) together with --no-build, so uv errors out right away instead of compiling. The uv flag is --no-build; pip's --only-binary is rejected by uv run. With that, 1.16.3/1.17.0/1.18.1 side by side took a few seconds each.
Confirmed with uv 0.13.6:
uv run --no-project --python 3.15 --no-build --with scipy==1.16.3fails in about 0.03 s ('scipy==1.16.3 has no usable wheels'), and--only-binaryis rejected ('unexpected argument'). With --python 3.12 the same command gives 1.16.3 at once. I did not rerun the slow sdist fallback without the flag. One refinement:--no-buildrefuses sdists for every package, so an old environment whose pure-Python dependency is sdist-only would also fail.--no-build-package scipyis the narrower form. It fails the same way on 3.15 and works on 3.12.