[Python] uv run fails with "failed to spawn"

I have a module with a pretty regular layout, a main function in a sacrypt directory inside a __init__.py file. Then, pyproject.toml looks like this;

[project]
name = "sacrypt"
version = "0.1.0"
description = "Cryptanalysis examples for SAC"
readme = "README.md"
requires-python = ">=3.13"

[project.scripts]
sacrypt = "sacrypt:main"


uv run sacrypt, however, fails with:

error: Failed to spawn: `sacrypt`
Caused by: No such file or directory (os error 2)


I have tried changing it to a different name, as well as moving sacrypt to src/sacrypt to no avail. This is by the book following tutorials as well as the documentation. Any idea what is going on here or where I should poke to find the error? Running uv with any amount of -v is not giving any more information. Also, this is uv version 0.6.9.

Update: As instructed in the comment, I have added a build system this way

[build-system]
requires = ["setuptools>=42", "wheel"]
build-backend = "setuptools.build_meta"


And then issued uv build. Still an error, this time different:

Traceback (most recent call last):
File "/home/jmerelo/Tutoriales/cemed-green-software/code/Python/.venv/bin/sacrypt", line 4, in <module>
from sacrypt import main
ModuleNotFoundError: No module named 'sacrypt'


Running it from the command line, or running tests, works fine.

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