diff --git a/build/tradable.mjs b/build/tradable.mjs
index 70b9499bf..2ab2699dc 100644
--- a/build/tradable.mjs
+++ b/build/tradable.mjs
@@ -10,6 +10,22 @@ const builtUntradable = [
   'Launcher',
   'Sniper',
 ];
+/**
+ * Gate built Prime items (Warframes, weapons) from being marked tradable.
+ * Built Prime items are not directly tradable -- only their individual
+ * components/parts are (e.g. Loki Prime Blueprint). Components get their
+ * own tradable flag set independently during parsing.
+ *
+ * To determine if a Prime set is tradable, check item.components for
+ * parts with tradable: true.
+ *
+ * Item-specific tradability overrides live in config/overrides.json
+ * (keyed by uniqueName) and are applied by parser.mjs applyOverrides()
+ * after this check runs.
+ *
+ * @param {module:warframe-items.Item} item Item to check
+ * @returns {boolean}
+ */
 const tradableConditions = (item) => !(builtUntradable.includes(item.type) && item.name.match(/Prime/gi));
 
 const tradableArcanes = [