NixOS specialisations

Specialisations let you define named variations of your system configuration. Each specialisation produces a separate system closure that you can switch to at boot or at runtime, without rebuilding.

A common use case is GPU drivers. If you rarely need your discrete GPU, you can keep drivers disabled in your base configuration and create a specialisation that enables them. At boot, you pick which configuration to start. The same idea applies to desktop environments, kernel parameters, or any set of options you want to toggle as a group.

Defining specialisations🔗

Add a specialisation block to your NixOS configuration. Each entry names a variation and provides the NixOS module options that differ from the base system.

{ lib, ... }:
{
  # Base configuration: GNOME desktop, no Nvidia drivers
  services.xserver.desktopManager.gnome.enable = true;
  hardware.nvidia.modesetting.enable = lib.mkDefault false;

  specialisation = {
    nvidia.configuration = {
      hardware.nvidia.modesetting.enable = true;
      hardware.nvidia.open = true;
      services.xserver.videoDrivers = [ "nvidia" ];
    };

    plasma.configuration = {
      services.xserver.desktopManager.plasma5.enable = true;
    };
  };
}

The nvidia specialisation inherits the full parent configuration and layers Nvidia driver options on top. The plasma specialisation does the same, adding Plasma alongside the base GNOME setup. Both appear as separate boot entries after you rebuild.

Overriding parent values

Specialisations receive the parent config by default. If the parent sets a value that a specialisation needs to change, mark the parent value with lib.mkDefault so the specialisation can override it without priority conflicts. In the example above, hardware.nvidia.modesetting.enable = lib.mkDefault false; allows the nvidia specialisation to set it to true.

Starting from scratch with inheritParentConfig🔗

By default, every specialisation inherits the parent configuration. If you want a specialisation that defines its own system independently, set inheritParentConfig = false:

specialisation = {
  minimal = {
    inheritParentConfig = false;
    configuration = {
      system.nixos.tags = [ "minimal" ];
      boot.loader.grub.enable = true;
      fileSystems."/" = { device = "/dev/sda1"; fsType = "ext4"; };
      # ... everything else must be specified explicitly
    };
  };
};

This is useful for creating a stripped-down recovery configuration or a fundamentally different system variant. Be aware that inheritParentConfig = false means you must provide a complete, bootable NixOS configuration within the configuration block.

Excluding options from specialisations🔗

Sometimes the base configuration should include options that specialisations should not inherit. Use config.specialisation != {} as a condition to restrict options to the default (non-specialised) entry:

{
  lib,
  config,
  pkgs,
  ...
}:
{
  config = lib.mkIf (config.specialisation != { }) {
    # Only applies to the default system, not to specialisations
    hardware.graphics.extraPackages = with pkgs; [
      vaapiIntel
      vaapiVdpau
    ];
  };
}

Condition requires specialisations

The expression config.specialisation != {} evaluates to false when no specialisations are defined. The conditional block only takes effect when at least one specialisation exists in the configuration.

Activating a specialisation at boot🔗

After rebuilding your system (with clan machines update or nixos-rebuild switch), each specialisation appears as a separate entry in your bootloader menu. Select one during boot to start that configuration.

Switching at runtime with Clan🔗

Clan supports switching specialisations at runtime through the --specialisation flag on clan machines update:

clan machines update my-machine --specialisation nvidia

This builds the system configuration and activates the named specialisation on the target machine. The specialisation's switch-to-configuration switch script runs on the target, applying the configuration without a reboot.

Runtime limitations

Not all configuration changes can take effect at runtime. If a specialisation uses a different kernel, for example, the running kernel will not change until you reboot and select the specialisation from the boot menu.

You can also switch specialisations directly on the target machine using nixos-rebuild:

sudo nixos-rebuild switch --specialisation nvidia

When to use specialisations🔗

Specialisations work well when you have a small number of pre-defined system variants that share most of their configuration. Typical examples:

• GPU driver toggles (enable/disable proprietary drivers)
• Desktop environment alternatives (GNOME vs. Plasma)
• Performance profiles (power-saving vs. high-performance kernel parameters)
• Debug configurations (verbose logging, extra diagnostic tools)

For configuration differences that are more granular or need to compose independently, NixOS module options with lib.mkDefault and lib.mkForce may be a better fit than specialisations.