diff --git a/nix/module-eval/lib.nix b/nix/module-eval/lib.nix
index 2b6826f6..2a0b4708 100644
--- a/nix/module-eval/lib.nix
+++ b/nix/module-eval/lib.nix
@@ -37,11 +37,17 @@ let
   # Stub host, same shape ./docs/default.nix already uses: enough for a
   # `nixosSystem` to evaluate, nothing that pulls a real disk or
   # bootloader in.
+  #
+  # Every host fixture reuses the flake's `pkgs` rather than importing its own
+  # nixpkgs. `read-only.nix` makes any host module that defines top-level
+  # `nixpkgs.config`/`overlays`/`hostPlatform` fail the fixture's eval instead
+  # of being ignored.
   hive =
     extra:
     (nixosSystem {
-      system = pkgs.stdenv.hostPlatform.system;
       modules = [
+        (pkgs.path + "/nixos/modules/misc/nixpkgs/read-only.nix")
+        { nixpkgs.pkgs = pkgs; }
         self.nixosModules.default
         {
           fileSystems."/" = {