Finally

tests/networkd-phobos.nix

@@ -2,17 +2,28 @@
 let
   system = "x86_64-linux";
 
-  # Grab nixpkgs lib for convenience
+  # Real nixpkgs for addressing lib
   pkgs = import <nixpkgs> { inherit system; };
-  lib = pkgs.lib;
+  realLib = pkgs.lib;
 
-  # Metanix world + addressing
+  # Minimal "testing lib" that makes mkIf actually behave like a real if
+  testLib = realLib // {
+    mkOption = x: x;
+    mkIf = cond: body: if cond then body else { };
+
+    types = {
+      str = "str";
+      nullOr = t: "nullOr " + builtins.toString t;
+    };
+  };
+
+  # Your world + addressing lib
   meta = import ../meta.nix;
-  addressing = import ../lib/addressing { inherit lib; };
+  addressing = import ../lib/addressing { lib = realLib; };
 
   network = addressing.mkNetworkFromSpec meta;
 
-  # Fake NixOS config that your module expects
+  # Fake NixOS config for this test
   config = {
     networking.hostName = "phobos";
     metanix = {
@@ -21,16 +32,18 @@ let
     };
   };
 
-  # Call the networkd module directly as a function
+  # Call the Metanix networkd module with the fake lib + config
   networkdModule = import ../modules/metanix/networkd.nix {
-    inherit lib config;
+    lib = testLib;
+    inherit config;
   };
+
 in
 {
-  # Raw Metanix view
+  # Raw Metanix view for sanity
   metanixHost = network.hosts.phobos;
   metanixSubnet = network.subnets."home-main";
 
-  # What the module actually emits
+  # What the module actually emits once mkIf is forced
   systemdNetwork = networkdModule.config.systemd.network;
 }