How do I configure multiple nixpkgs's instances in flake's

[Jump1n]

If I have a flake that provides a NixOS config, where I have the following inputs:

    nixpkgs.url = "github:nixos/nixpkgs?ref=nixos-unstable";
    nixpkgs-stable.url = "github:nixos/nixpkgs?ref=nixos-24.11";

and I for example set nixpkgs.config.allowUnfree = true; in my config, and use nixpkgs-stable using this overlay:

nixpkgs.overlays = [(final: prev:
  stable = nixpkgs-stable.legacyPackages.<system>;
)];

will packages from pkgs.stable also allow unfree packages as set in nixpkgs.config.allowUnfree?

[waffle8946]

@Jump1n

will packages from pkgs.stable also allow unfree packages as set in

No. See step #3 here for your choices, ultimately you have to manually import rather than using legacyPackages:

Mixing stable and unstable packages on flake-based NixOS system

Never set pkgs directly in the NixOS module system, as the other answers are indicating. Here’s a couple of concepts you need to know: If you want to access some inputs, or some other value across your config without creating an explicit option for it, use specialArgs or _module.args. (For things that you import, you must pass them in specialArgs specifically to avoid infrec.) Changing pkgs requires an overlay; however, for this application, you may not necessarily need one if you are willing…

But generally I wouldn’t recommend mixing stable and unstable.

[Jump1n]

Thanks, that’s annoying… why would you not recommend mixing them? I only am going to use stable for for example broken packages

[waffle8946]

The Myth of Stable Unstable

I will add another caveat to this - mixing stable and unstable is pretty much unsupported, so you are even more on your own* than using just one channel exclusively. And of course having multiple nixpkgs instances comes with its own performance and incompatibility problems, especially for GUI apps and “core” dependencies - complaints around glibc or qt version mismatches are common here. One other option for people that want to use stable would be to use a scheme similar to what I use for unst…