diff --git a/src/dotfiles/guests.nix b/src/dotfiles/guests.nix
index 896ff97..e0e5deb 100644
--- a/src/dotfiles/guests.nix
+++ b/src/dotfiles/guests.nix
@@ -1,6 +1,5 @@
 {
   inputs,
-  lib,
   root,
   specialArgs,
   repositoryModules,
@@ -276,7 +275,7 @@ let
       };
     };
 
-  names = map (path: lib.removeSuffix ".nix" (builtins.baseNameOf path)) files;
+  names = map util.removeNixSuffix files;
 in
 {
   imports = files ++ map mkGuest names;
diff --git a/src/dotfiles/registry.nix b/src/dotfiles/registry.nix
index 6950765..c8ce56e 100644
--- a/src/dotfiles/registry.nix
+++ b/src/dotfiles/registry.nix
@@ -6,13 +6,11 @@
 }:
 let
   definitions = lib.listToAttrs (
-    map (path: lib.nameValuePair (registryName (builtins.baseNameOf path)) (mkRegistry path)) (
+    map (path: lib.nameValuePair (util.removeNixSuffix path) (mkRegistry path)) (
       util.collectNixFiles root
     )
   );
 
-  registryName = fileName: lib.removeSuffix ".nix" fileName;
-
   mkRegistry =
     path:
     let
diff --git a/src/dotfiles/util.nix b/src/dotfiles/util.nix
index df0b3a5..56c5e99 100644
--- a/src/dotfiles/util.nix
+++ b/src/dotfiles/util.nix
@@ -20,6 +20,8 @@ in
 {
   inherit collectNixFiles;
 
+  removeNixSuffix = path: lib.removeSuffix ".nix" (builtins.baseNameOf path);
+
   bridgeName = id: "br-vlan${toString id}";
 
   deriveMac =
diff --git a/src/skills/style/nix/rules/general.md b/src/skills/style/nix/rules/general.md
index 3b51109..98d33a6 100644
--- a/src/skills/style/nix/rules/general.md
+++ b/src/skills/style/nix/rules/general.md
@@ -10,3 +10,14 @@ Pass `util` to Nix functions that need the helper.
 
 `util.collectNixFiles` centralizes recursive discovery of Nix files.
 Repository Nix code uses one implementation.
+
+## Remove Nix file suffixes with the utility
+
+### Rule
+
+Use `util.removeNixSuffix` to remove a `.nix` suffix from a path.
+Do not call `builtins.baseNameOf` or `lib.removeSuffix ".nix"` outside the utility definition.
+
+### Rationale
+
+The utility centralizes Nix file-name normalization.