This module does NOT make consuming an overlay easy. This module is intended for creating overlays. While it is possible to consume the overlay created by this module using the final module argument, this is somewhat unconventional. Instead:

  • Avoid overlays. Many flakes can do without them.
  • Initialize pkgs yourself:
    perSystem = { system, ... }: {
      _module.args.pkgs = import inputs.nixpkgs {
        inherit system;
        overlays = [
          (final: prev: {
            # ... things you really need to patch ...
        config = { };

Who this is for

This module is for flake authors who need to provide a simple overlay in addition to the common flake attributes. It is not for users who want to consume an overlay.

What it actually does

This module overrides the pkgs module argument and provides the final module argument so that the perSystem module can be evaluated as an overlay. Attributes added by the overlay must be defined in overlayAttrs. The resulting overlay is defined in the overlays.default output.

The resulting behavior tends to be not 100% idiomatic. A hand-written overlay would usually use final more often, but nonetheless it gets the job done for simple use cases; certainly the simple use cases where overlays aren't strictly necessary.

The status of this module

It has an unfortunate name and may be renamed. Alternatively, its functionality may be moved out of flake-parts, into some Nixpkgs module. Certainly until then, feel free to use the module if you understand what it does.


To use these options, add inside the mkFlake:

imports = [

Run nix flake lock and you're set.



A function from system to flake-like attributes omitting the <system> attribute.

Modules defined here have access to the suboptions and some convenient module arguments.

Type: module

Declared by:


Attributes to add to overlays.default.

The overlays.default overlay will re-evaluate perSystem with the “prev” (or “super”) overlay argument value as the pkgs module argument. The easyOverlay module also adds the final module argument, for the result of applying the overlay.

When not in an overlay, final defaults to pkgs plus the generated overlay. This requires Nixpkgs to be re-evaluated, which is more expensive than setting pkgs to a Nixpkgs that already includes the necessary overlays that are required for the flake itself.

See Overlays.

Type: lazy attribute set of raw value

Default: { }

Declared by: