Ok. That I have understood. Thank you :)

How do you add the "initial" base(s) to the project right after creation? Those have to be added manually to pyproject.toml?

I thought about this a bit yesterday. My understanding is mapping a base or multiple bases to a project is an entirely manual operation that will not be done in an automated way. As in, a human will do that.
Then, why not give the options upons creating (or modifying) a project to have some kind of a TUI in the polylith-cli command that lets you pick from a list of bases you already have.

Like:

uv run poly create project --name my-project

Creating new project "my-project"
Base "my-project" of the same name has been automatically added to pyproject.toml
Please select any other bases to be added:
Available bases /
    my-project    [x]
    my-other-base [ ]
    so-many-bases [ ]

Do components get directly added to projects or only through bases via "poly sync"? If it is only via indirection this TUI could already be sufficient.

It essentially seems like a way of managing the pyproject.toml files for projects through the poly cli tool.

Glad that you like the idea. What I like about the polylith is that it is so clean. I trade a bit of upfront training and an initially steeper learning curve for long-term control and effective code re-use. It just feels natural that the tooling should support an equally clean workflow :)

Great! I'll start working on a more interactive CLI workflow from your idea. Currently vacationing with the family, so will likely do this in a slower pace than usual 😄