Flake registry set to a store path keeps copying

By default nixpkgs#hello will pull the nix unstable. But we can lock it on the command line by using nixpkgs/release-23.11#hello. This is fine but its too verbose. I can add a flake registry to lock nixpkgs like this:

nix registry add nixpkgs github:NixOS/nixpkgs/release-23.11

Now nixpkgs#hello should be following the release-23.11

The question is, what if I already have this in my store (for example by using home-manager). Currently I have this setting which ensures <nixpkgs> is the same pkgs version I have in home manager (within nix conf)

nix-path = nixpkgs=${pkgs.path}

I wish to do the same with flakes, but if I add this:

  nix.registry = {
    nixpkgs.to = {
      type = "path";
      path = "${pkgs.path}";

Then each time I run nix shell nixpkgs#hello it tries to copy pkgs.path to the store even though its already in store. I there any way to set the registry to a path and not have it copied each time (when its already in store). Alternatively are there further docs to describe all types of registry as I couldn’t find much in man pages. Currently the only option seems to be setting it to github and fix to a branch/rev

I had this for NixOS but after seeing your post I noticed that I didn’t set the nix-path for home-manager only installs. Something like this should work for hm:

  nix.registry = {
    # Make nix command use the same nixpkgs version: 'nix shell nixpkgs#curl'
    nixpkgs.flake = inputs.nixpkgs;
    nixpkgs-unstable.flake = inputs.nixpkgs-unstable; # 'nix shell nixpkgs-unstable#curl'

  nix.package = pkgs.nix;

  nix.settings = {
    experimental-features = "nix-command flakes";
    nix-path = ["nixpkgs=${inputs.nixpkgs.outPath}" "nixpkgs-unstable=${inputs.nixpkgs.outPath}"];
1 Like

Thanks this is exactly what I was looking for. btw I don’t use NixOS, but this worked like a charm on home manager non nixos too, note that I had to pass nixpkgs to home.nix as you pointed out and not use pkgs.path

extraSpecialArgs = {
  inherit nixpkgs;

This resulted in the following registry

  "exact": true,
  "from": {
    "id": "nixpkgs",
    "type": "indirect"
  "to": {
    "lastModified": 1701282334,
    "narHash": "sha256-MxCVrXY6v4QmfTwIysjjaX0XUhqBbxTWWB4HXtDYsdk=",
    "path": "/nix/store/j8han9cf3g8vba52yhiklaa6a500pcbv-source",
    "rev": "057f9aecfb71c4437d2b27d3323df7f93c010b7e",
    "type": "path"

Main difference being that it has a narHash. So if using path flakes already in store, must provide a narHash with exact = true