diff --git a/eslint.config.mjs b/eslint.config.mjs
index a43fee03..0586b65a 100644
--- a/eslint.config.mjs
+++ b/eslint.config.mjs
@@ -69,6 +69,25 @@ export default defineConfig([
             'no-unused-vars': 'warn',
             'no-case-declarations': 'off',
             'no-control-regex': 'warn',
+            // Store boundary rule:
+            // 1) Disallow `xxxStore.xxx = ...`
+            // 2) Disallow `xxxStore.xxx++ / --`
+            // Reason: prevent direct cross-store mutation and enforce owner-store actions.
+            'no-restricted-syntax': [
+                'error',
+                {
+                    selector:
+                        "AssignmentExpression[left.type='MemberExpression'][left.object.type='Identifier'][left.object.name=/Store$/]",
+                    message:
+                        'Do not mutate store state directly via *Store.* assignment. Use owner-store actions.'
+                },
+                {
+                    selector:
+                        "UpdateExpression[argument.type='MemberExpression'][argument.object.type='Identifier'][argument.object.name=/Store$/]",
+                    message:
+                        'Do not mutate store state directly via *Store.* update operators. Use owner-store actions.'
+                }
+            ],
 
             'vue/no-mutating-props': 'warn',
             'vue/multi-word-component-names': 'off',
diff --git a/src/stores/avatar.js b/src/stores/avatar.js
index 494bae16..789dbb87 100644
--- a/src/stores/avatar.js
+++ b/src/stores/avatar.js
@@ -117,28 +117,7 @@ export const useAvatarStore = defineStore('Avatar', () => {
         favoriteStore.applyFavorite('avatar', ref.id);
         if (favoriteStore.localAvatarFavoritesList.includes(ref.id)) {
             const avatarRef = ref;
-            for (
-                let i = 0;
-                i < favoriteStore.localAvatarFavoriteGroups.length;
-                ++i
-            ) {
-                const groupName = favoriteStore.localAvatarFavoriteGroups[i];
-                if (!favoriteStore.localAvatarFavorites[groupName]) {
-                    continue;
-                }
-                for (
-                    let j = 0;
-                    j < favoriteStore.localAvatarFavorites[groupName].length;
-                    ++j
-                ) {
-                    const favoriteRef =
-                        favoriteStore.localAvatarFavorites[groupName][j];
-                    if (favoriteRef.id === avatarRef.id) {
-                        favoriteStore.localAvatarFavorites[groupName][j] =
-                            avatarRef;
-                    }
-                }
-            }
+            favoriteStore.syncLocalAvatarFavoriteRef(avatarRef);
 
             // update db cache
             database.addAvatarToCache(avatarRef);
diff --git a/src/stores/favorite.js b/src/stores/favorite.js
index f35b0d4f..c7acd405 100644
--- a/src/stores/favorite.js
+++ b/src/stores/favorite.js
@@ -970,6 +970,27 @@ export const useFavoriteStore = defineStore('Favorite', () => {
         friendImportDialogInput.value = value;
     }
 
+    /**
+     * @param {object} avatarRef
+     */
+    function syncLocalAvatarFavoriteRef(avatarRef) {
+        if (!avatarRef?.id) {
+            return;
+        }
+        for (let i = 0; i < localAvatarFavoriteGroups.value.length; ++i) {
+            const groupName = localAvatarFavoriteGroups.value[i];
+            const group = localAvatarFavorites[groupName];
+            if (!group) {
+                continue;
+            }
+            for (let j = 0; j < group.length; ++j) {
+                if (group[j]?.id === avatarRef.id) {
+                    group[j] = avatarRef;
+                }
+            }
+        }
+    }
+
     /**
      *
      * @param {string} worldId
@@ -1892,6 +1913,7 @@ export const useFavoriteStore = defineStore('Favorite', () => {
         setAvatarImportDialogInput,
         setWorldImportDialogInput,
         setFriendImportDialogInput,
+        syncLocalAvatarFavoriteRef,
         addLocalWorldFavorite,
         hasLocalWorldFavorite,
         hasLocalAvatarFavorite,