{
  self,
  inputs,
  ...
}: {
  # Escape hatch for packages that move faster than the stable channel.
  #
  # `pkgs.unstable.<name>` resolves against nixos-unstable while the rest of the
  # system stays on nixos-26.05. Keep the number of consumers small and prefer
  # stable unless there is a concrete reason; every unstable package drags a bit
  # of the unstable closure (glibc, stdenv) in alongside the stable one.
  #
  # Bump with `nix flake update nixpkgs-unstable`.
  flake.overlays.unstable = _final: prev: {
    unstable = import inputs.nixpkgs-unstable {
      inherit (prev.stdenv.hostPlatform) system;
      # Carries allowUnfree (set in nix-settings.nix) into the unstable set.
      inherit (prev) config;
    };
  };

  # This flake has two independent `pkgs` instances, and the overlay has to be
  # registered on both or `pkgs.unstable` would exist in some modules only:
  #
  #   1. the NixOS module system's `pkgs` - plain modules such as
  #      modules/features/bun, modules/system/core
  #   2. flake-parts' perSystem `pkgs` - modules built with `moduleWithSystem`,
  #      such as modules/attrs/development, which ignore `nixpkgs.overlays`
  flake.nixosModules.overlays = {...}: {
    nixpkgs.overlays = [self.overlays.unstable];
  };

  perSystem = {system, ...}: {
    _module.args.pkgs = import inputs.nixpkgs {
      inherit system;
      overlays = [self.overlays.unstable];
      config.allowUnfree = true;
    };
  };
}