Lock Flattening

The Problem

Nix flakes fetch every input’s own lock file, which can result in many copies of the same package set (e.g. multiple nixpkgs versions) being downloaded and stored. Flattening forces all inputs to share common dependencies.

The lock-flattening hook is an alternative to automatic follows declarations with flake-edit. Flattening keeps generated follows out of flake.nix and records the deduplication only in flake.lock. See the comparison table before choosing; generally, enable one approach rather than both.

Built-in Modules

flake-file ships support for two established flattening tools:

allfollow

spikespaz/allfollow automatically adds follows entries for all transitive inputs.

{ inputs, ... }: {
  imports = [ inputs.flake-file.flakeModules.allfollow ];
}

Source: modules/prune-lock/allfollow.nix

nix-auto-follow

fzakaria/nix-auto-follow detects duplicate inputs and injects follows for them.

{ inputs, ... }: {
  imports = [ inputs.flake-file.flakeModules.nix-auto-follow ];
}

Source: modules/prune-lock/nix-auto-follow.nix

How it integrates

Both modules configure flake-file.prune-lock options, which instruct write-flake to run the flattener after writing flake.nix. You can also use the raw options for a custom tool:

{ pkgs, ... }: {
  flake-file.prune-lock = {
    enable = true;
    program = pkgs: pkgs.my-flattener;
  };
}

See the Options Reference for the full prune-lock schema.