You are correct in that Nixpacks uses the languages own package manager instead of installing all dependencies with Nix using tools like node2nix, cargo2nix, or poetry2nix. This was a choice as the main problems Nixpacks aims to solve are with system dependencies and getting an environment where your app can install and build with as little configuration as possible. While continuing to use the same tooling that app authors are likely using locally (e.g. npm, pip, cargo). We have found that nixifying all possible deps often creates situations where the app author must know that Nix is being used under the hood and modify their app accordingly. Nothing against these tools, but using them didn't provide the experience we set out to have. If a user of Nixpacks wants to override the install command to `do_something && npm install`, they should be able to without worrying about how it affects the Nix store.

However, any Nix package can be installed, including from python-packages.nix, and we are planning on allowing more Nix config and customization in the future.