diff --git a/prompts/CODE_STYLEGUIDE_GO.md b/prompts/CODE_STYLEGUIDE_GO.md
index f859589..60a725c 100644
--- a/prompts/CODE_STYLEGUIDE_GO.md
+++ b/prompts/CODE_STYLEGUIDE_GO.md
@@ -229,6 +229,15 @@ last_modified: 2026-02-22
 
 1.  Define your struct types near their constructors.
 
+1.  Do not create packages whose sole purpose is to hold type definitions.
+    Packages named `types`, `domain`, or `models` that contain only structs and
+    interfaces (with no behavior) are a code smell. Define types alongside the
+    code that uses them. Type-only packages force consuming packages into alias
+    imports and circular-dependency gymnastics, and indicate that the package
+    boundaries were drawn around nouns instead of responsibilities. If multiple
+    packages need the same type, put it in the package that owns the behavior,
+    or in a small, focused interface package — not in a grab-bag types package.
+
 1.  Define your interface types near the functions that use them, or if you have
     multiple conformant types, put the interface(s) in their own file.