diff --git a/plan.md b/plan.md
index 4090e7b..a89025d 100644
--- a/plan.md
+++ b/plan.md
@@ -116,6 +116,15 @@ collisions with host app styles. The host can override via CSS custom properties
 ```
 
 
+## Code Structure
+
+All stateful components must be classes, not bare module-level variables. The
+current `app.mjs` has scanner state (`running`, `last_decoded_text`, `fps`,
+etc.) scattered as module globals — this needs to be encapsulated in
+`Barcode_Scanner` before the library refactor proceeds. Module scope is only
+for constants and exports.
+
+
 ## Lifecycle
 
 ```