fix(MultichainBalancesController): initialize balance entry when snap update arrives before account is in state

eslint-suppressions.json

@@ -311,7 +311,7 @@
       "count": 1
     },
     "no-restricted-syntax": {
-      "count": 2
+      "count": 1
     }
   },
   "packages/assets-controllers/src/NftController.test.ts": {

packages/assets-controllers/CHANGELOG.md

@@ -7,6 +7,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+
+- `MultichainBalancesController` no longer silently drops balance updates from `AccountsController:accountBalancesUpdated` when the account does not yet have an entry in `state.balances` ([#8484](https://github.com/MetaMask/core/pull/8484))
+  - Previously, if a Snap sent balance updates before `updateBalance` had initialized `state.balances[accountId]`, the update was ignored because of an `accountId in state.balances` guard. The handler now initializes the entry before merging, ensuring balances are always persisted.
+
 ## [104.1.0]
 
 ### Added

packages/assets-controllers/src/MultichainBalancesController/MultichainBalancesController.test.ts

@@ -377,6 +377,34 @@ describe('MultichainBalancesController', () => {
     );
   });
 
+  it('initializes and stores balances when "AccountsController:accountBalancesUpdated" fires before the account has an entry in state', async () => {
+    const { controller, messenger } = setupController({
+      state: { balances: {} },
+      mocks: {
+        listMultichainAccounts: [],
+      },
+    });
+
+    expect(controller.state.balances[mockBtcAccount.id]).toBeUndefined();
+
+    const balanceUpdate = {
+      balances: {
+        [mockBtcAccount.id]: mockBalanceResult,
+      },
+    };
+
+    messenger.publish(
+      'AccountsController:accountBalancesUpdated',
+      balanceUpdate,
+    );
+
+    await waitForAllPromises();
+
+    expect(controller.state.balances[mockBtcAccount.id]).toStrictEqual(
+      mockBalanceResult,
+    );
+  });
+
   it('updates balances when receiving "AccountsController:accountBalancesUpdated" event', async () => {
     const mockInitialBalances = {
       [mockBtcNativeAsset]: {

packages/assets-controllers/src/MultichainBalancesController/MultichainBalancesController.ts

@@ -384,9 +384,8 @@ export class MultichainBalancesController extends BaseController<
     this.update((state: Draft<MultichainBalancesControllerState>) => {
       Object.entries(balanceUpdate.balances).forEach(
         ([accountId, assetBalances]) => {
-          if (accountId in state.balances) {
-            Object.assign(state.balances[accountId], assetBalances);
-          }
+          state.balances[accountId] ??= {};
+          Object.assign(state.balances[accountId], assetBalances);
         },
       );
     });