diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 4c0bcec..e4184f6 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -46,6 +46,13 @@ jobs:
           rm -f flake.lock
           nix flake lock
           nix flake check
+
+          # don't check custom conf relocation on darwin while we work with nix-darwin to make this a good experience
+          if [ "$(nix eval --raw --impure --expr 'builtins.currentSystem')" = "x86_64-linux" ]; then
+          # ensure relocating nix.conf to nix.custom.conf worked
+          nix build .#custom-conf --out-link /tmp/custom-conf
+          grep --quiet nix-community /tmp/custom-conf/etc/nix/nix.custom.conf
+          fi
       - run: |
           set -eux
 
diff --git a/modules/nixos.nix b/modules/nixos.nix
index 657945d..44d7294 100644
--- a/modules/nixos.nix
+++ b/modules/nixos.nix
@@ -15,6 +15,10 @@ in
   ];
 
   config = {
+    # Push the user's nix.conf into /etc/nix/nix.custom.conf,
+    # leaving determinate-nixd to manage /etc/nix/nix.conf
+    environment.etc."nix/nix.conf".target = "nix/nix.custom.conf";
+
     environment.systemPackages = [
       inputs.self.packages.${pkgs.stdenv.system}.default
     ];
diff --git a/tests/flake.nix b/tests/flake.nix
index 2c77cdb..2332642 100644
--- a/tests/flake.nix
+++ b/tests/flake.nix
@@ -6,37 +6,81 @@
       url = "github:LnL7/nix-darwin/nix-darwin-24.11";
       inputs.nixpkgs.follows = "nixpkgs";
     };
-    home-manager = {
-      url = "github:nix-community/home-manager/release-24.05";
-      inputs.nixpkgs.follows = "nixpkgs";
-    };
   };
 
-  outputs = { nixpkgs, determinate, home-manager, nix-darwin, ... }: {
-    checks.x86_64-linux.nixos = (nixpkgs.lib.nixosSystem {
-      system = "x86_64-linux";
-      modules = [
-        determinate.nixosModules.default
+  outputs =
+    {
+      nixpkgs,
+      determinate,
+      nix-darwin,
+      ...
+    }:
+    let
+      mkNixOS =
         {
-          fileSystems."/" = {
-            device = "/dev/bogus";
-            fsType = "ext4";
-          };
-          boot.loader.grub.devices = [ "/dev/bogus" ];
-          system.stateVersion = "24.11";
-        }
-      ];
-    }).config.system.build.toplevel;
-
-    checks.aarch64-darwin.nix-darwin = (nix-darwin.lib.darwinSystem {
-      system = "aarch64-darwin";
+          modules ? [ ],
+        }:
+        (nixpkgs.lib.nixosSystem {
+          system = "x86_64-linux";
+          modules = [
+            determinate.nixosModules.default
+            {
+              fileSystems."/" = {
+                device = "/dev/bogus";
+                fsType = "ext4";
+              };
+              boot.loader.grub.devices = [ "/dev/bogus" ];
+              system.stateVersion = "24.11";
+            }
+          ] ++ modules;
+        }).config.system.build.toplevel;
 
-      modules = [
-        determinate.darwinModules.default
+      mkNixDarwin =
         {
-          system.stateVersion = 5;
-        }
-      ];
-    }).system;
-  };
+          modules ? [ ],
+        }:
+        (nix-darwin.lib.darwinSystem {
+          system = "aarch64-darwin";
+
+          modules = [
+            determinate.darwinModules.default
+            {
+              system.stateVersion = 5;
+            }
+          ] ++ modules;
+        }).system;
+    in
+    {
+      packages.x86_64-linux = {
+        default = mkNixOS { };
+
+        custom-conf = mkNixOS {
+          modules = [
+            {
+              nix.settings = {
+                substituters = [ "https://nix-community.cachix.org" ];
+                trusted-substituters = [ "https://nix-community.cachix.org" ];
+                trusted-public-keys = [ "nix-community.cachix.org-1:mB9FSh9qf2dCimDSUo8Zy7bkq5CX+/rkCWyvRCYg3Fs=" ];
+              };
+            }
+          ];
+        };
+      };
+
+      packages.aarch64-darwin = {
+        default = mkNixDarwin { };
+
+        # custom-conf = mkNixDarwin {
+        #   modules = [
+        #     {
+        #       nix.settings = {
+        #         substituters = [ "https://nix-community.cachix.org" ];
+        #         trusted-substituters = [ "https://nix-community.cachix.org" ];
+        #         trusted-public-keys = [ "nix-community.cachix.org-1:mB9FSh9qf2dCimDSUo8Zy7bkq5CX+/rkCWyvRCYg3Fs=" ];
+        #       };
+        #     }
+        #   ];
+        # };
+      };
+    };
 }