The `nixpkgs.overlays` defined in `darwin-configuration.nix` is not being applied to `perSystem`.

[shirakami-chigusa]

I'm attempting to configure nixpkgs.overlays on a per-host basis, rather than globally.

I've confirmed that if nixpkgs.overlays is defined in darwin-configuration.nix instead of directly under inputs.blueprint in flake.nix, the overlays are not being applied within packages/<pname>/default.nix via perSystem.<pname>.

https://github.com/shirakami-chigusa/test-blueprint-overlays/tree/main

System Info

• blueprint: 7ae2142c8b5a47bed6d403fdd5f5a1215961e10c
• system: "aarch64-darwin"
• host os: Darwin 24.3.0, macOS 15.3.2
• multi-user?: yes
• sandbox: no
• version: nix-env (Nix) 2.24.12
• nixpkgs: /nix/store/bgkmh40ahkw48wxwmwbilnvjdjgcn3n6-source

packages/my-hello/default.nix

This package is designed to test the validity of nixpkgs.overlays within package definitions. It simply extracts pkgs.hello as is. If pkgs.hello is overlaid with pkgs.htop, the output will be htop.

# packages/my-hello/default.nix
{ pkgs, ... }:
pkgs.stdenv.mkDerivation {
  name = "my-hello";
  src = ./.;

  buildInputs = [ pkgs.hello ];

  installPhase = ''
    mkdir -p $out/bin
    cp ${pkgs.hello}/bin/* $out/bin/
  '';
}

1. overlays in flake.nix + nix build .#my-hello = OK.

set nixpkgs.overlays in flake.nix (global).

https://github.com/numtide/blueprint/blob/main/docs/configuration.md#nixpkgsoverlays

# flake.nix
    inputs.blueprint {
      inherit inputs;
      nixpkgs.overlays = [
        (final: prev: {
          hello = prev.htop;
        })
      ];
    };

run nix build .#my-hello.

result is ./result/bin/htop. OK.

2. overlays in flake.nix + perSystem.self.my-hello = OK.

set nixpkgs.overlays in flake.nix (global).

# flake.nix
    inputs.blueprint {
      inherit inputs;
      nixpkgs.overlays = [
        (final: prev: {
          hello = prev.htop;
        })
      ];
    };

install the overlaid package perSystem.self.my-hello.

# hosts/myhost/configuration.nix
  environment.systemPackages = with pkgs; [
    perSystem.self.my-hello
  ];

run darwin-rebuild build --flake .#myhost

result is ./result/sw/bin/htop. OK.

3. overlays in darwin-configuration.nix + perSystem.self.my-hello = NG!!!

set nixpkgs.overlays in hosts/myhost/darwin-configuration.nix.

install the overlaid package perSystem.self.my-hello.

# hosts/myhost/darwin-configuration.nix
  nixpkgs.overlays = [
    (final: prev: {
      hello = prev.htop;
    })
  ];
  environment.systemPackages = with pkgs; [
    perSystem.self.my-hello
  ];

run darwin-rebuild build --flake .#myhost

result is ./result/sw/bin/hello. pkgs.hello in packages/my-hello is not overlaid. NG!!!

4. overlays in darwin-configuration.nix + overlaid pkgs.hello = OK.

set nixpkgs.overlays in hosts/myhost/darwin-configuration.nix.

install the overlaid package pkgs.hello.

# darwin-configuration.nix
  nixpkgs.overlays = [
    (final: prev: {
      hello = prev.htop;
    })
  ];
  environment.systemPackages = with pkgs; [
    pkgs.hello
  ];

run darwin-rebuild build --flake .#myhost. result is ./result/sw/bin/htop. OK. pkgs.hello is overlaid correctly.

Conclusion

The nixpkgs.overlays defined in darwin-configuration.nix is not being applied to internal packages.

Is this my fault or a bug?

[shirakami-chigusa]

5. overlays in darwin-configuration.nix + (pkgs.callPackage ../../packages/my-hello { }) = OK!

It worked fine when I used pkgs.callPackage without using perSystem.

# darwin-configuration.nix  
  nixpkgs.overlays = [
    (final: prev: {
      hello = prev.htop;
    })
  ];

  environment.systemPackages = with pkgs; [
    (callPackage ../../packages/my-hello { }) # OK!
  ];

run darwin-rebuild build --flake .#myhost. result is ./result/sw/bin/htop. OK. pkgs.hello in packages/my-hello/default.nix is overlaid correctly.

Since I don't know the correct way to overlay nixpkgs within perSystem from host/myhost/darwin-configuration.nix, I will use pkgs.callPackage to avoid this issue.