It's totally possible to introduce mutable, non-reproducible state on a system that uses Nix, even if that system is NixOS. However, you can't do so in your Nix

It's totally possible to introduce mutable, non-reproducible state on a system that uses Nix, even if that system is NixOS. However, you can't do so in your Nix store, which is where all the Nix-installed things live (at least, not without a lot of hackery - Nix tries very hard to make the entire store immutable). Basically this works out to: anything you install or configure with Nix, is immutable and reproducible. Anything you don't is at your own risk. Unfortunately there are still some "pockets of state" that haven't been solved yet, most notably applications storing state in ~. For most every language dependency management system, there's a Nix equivalent of some sort that allows you to manage those dependencies without invoking the non-reproducible mutable tooling.