A quick note about a small trick I implemented in my NixOS configurations.

The problem

I started using flakes a few months ago, and quickly began using them daily. Notably, I moved the NixOS configurations for all my machines in a single flake. But there has been a caveat in my workflow: using flakes for almost every project I compile made me end up with lots of different versions of Nixpkgs lying around. It became very annoying when compiling LaTeX documents: two LaTeX projects created a few hours apart could be using different versions of Nixpkgs, and thus different versions of TexLive. Downloading a few gigabytes every time I set up a LaTeX project was not very optimal. I began copying lock files between projects, but it only made me more frustrated.

The solution

Nix uses a registry to resolve flake URIs. For instance, it resolves the special flake URI “nixpkgs” to “github:nixpkgs/nixos-unstable”. We can configure the registry system-wide to map “nixpkgs” to a specific revision of the repository. Using the “rev” attribute of a flake input, we can even point it to the same revision as the one used by our configuration.

The module to configure the Nix registry looks like this:

  inputs = {
    nixpkgs.url = "github:nixos/nixpkgs";

  outputs = { self, nixpkgs }:
    nixosModule = { pkgs, ... }: {
      nix.registry.nixpkgs = {
        from = {
	  type = "indirect";
	  id = "nixpkgs";
	to = {
	  type = "github";
	  owner = "nixos";
	  repo = "nixpkgs";
	  inherit (nixpkgs) rev;

The Nix registry can also be configured at the user level, and this module can be used as a home-manager module.