feat: Add StorageService for offloading large controller data

.github/CODEOWNERS

@@ -100,6 +100,7 @@
 /packages/permission-controller                 @MetaMask/wallet-integrations @MetaMask/core-platform
 /packages/permission-log-controller             @MetaMask/wallet-integrations @MetaMask/core-platform
 /packages/remote-feature-flag-controller        @MetaMask/extension-platform @MetaMask/mobile-platform @MetaMask/core-platform
+/packages/storage-service                       @MetaMask/extension-platform @MetaMask/mobile-platform @MetaMask/core-platform
 
 ## Package Release related
 /packages/account-tree-controller/package.json  @MetaMask/accounts-engineers @MetaMask/core-platform
@@ -168,6 +169,8 @@
 /packages/bridge-controller/CHANGELOG.md                 @MetaMask/swaps-engineers @MetaMask/core-platform
 /packages/remote-feature-flag-controller/package.json @MetaMask/extension-platform @MetaMask/mobile-platform @MetaMask/core-platform
 /packages/remote-feature-flag-controller/CHANGELOG.md @MetaMask/extension-platform @MetaMask/mobile-platform @MetaMask/core-platform
+/packages/storage-service/package.json @MetaMask/extension-platform @MetaMask/mobile-platform @MetaMask/core-platform
+/packages/storage-service/CHANGELOG.md @MetaMask/extension-platform @MetaMask/mobile-platform @MetaMask/core-platform
 /packages/bridge-status-controller/package.json         @MetaMask/swaps-engineers @MetaMask/core-platform
 /packages/bridge-status-controller/CHANGELOG.md         @MetaMask/swaps-engineers @MetaMask/core-platform
 /packages/app-metadata-controller/package.json        @MetaMask/mobile-platform @MetaMask/core-platform

packages/storage-service/CHANGELOG.md

@@ -0,0 +1,14 @@
+# Changelog
+
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
+and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## [Unreleased]
+
+### Added
+
+- Initial release of `@metamask/storage-service` ([#7192](https://github.com/MetaMask/core/pull/7192))
+
+[Unreleased]: https://github.com/MetaMask/core/

packages/storage-service/LICENSE

@@ -0,0 +1,6 @@
+This project is licensed under either of
+
+ * MIT license ([LICENSE.MIT](LICENSE.MIT))
+ * Apache License, Version 2.0 ([LICENSE.APACHE2](LICENSE.APACHE2))
+
+at your option. 

packages/storage-service/LICENSE.APACHE2

@@ -0,0 +1,201 @@
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packages/storage-service/LICENSE.MIT

@@ -0,0 +1,21 @@
+MIT License
+
+Copyright (c) 2025 MetaMask
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packages/storage-service/README.md

@@ -0,0 +1,131 @@
+# `@metamask/storage-service`
+
+A platform-agnostic service for storing large, infrequently accessed controller data outside of memory.
+
+## When to Use
+
+✅ **Use StorageService for:**
+
+- Large data (> 100 KB)
+- Infrequently accessed data
+- Data that doesn't need to be in Redux state
+- Examples: Snap source code, cached API responses
+
+❌ **Don't use for:**
+
+- Frequently accessed data (use controller state)
+- Small data (< 10 KB - overhead not worth it)
+- Data needed for UI rendering
+
+## Installation
+
+`yarn add @metamask/storage-service`
+
+or
+
+`npm install @metamask/storage-service`
+
+## Usage
+
+### Controller Setup
+
+```typescript
+import type {
+  StorageServiceSetItemAction,
+  StorageServiceGetItemAction,
+} from '@metamask/storage-service';
+
+// Grant access to storage actions
+type AllowedActions =
+  | StorageServiceSetItemAction
+  | StorageServiceGetItemAction;
+
+class MyController extends BaseController<...> {
+  async storeData(id: string, data: string) {
+    await this.messenger.call(
+      'StorageService:setItem',
+      'MyController',
+      `${id}:data`,
+      data,
+    );
+  }
+
+  async getData(id: string): Promise<string | undefined> {
+    const { result, error } = await this.messenger.call(
+      'StorageService:getItem',
+      'MyController',
+      `${id}:data`,
+    );
+    if (error) {
+      throw error;
+    }
+    // result is undefined if key doesn't exist
+    return result as string | undefined;
+  }
+}
+```
+
+### Service Initialization
+
+The service accepts an optional `StorageAdapter` for platform-specific storage:
+
+```typescript
+import { StorageService, type StorageAdapter } from '@metamask/storage-service';
+
+// Production: Provide a platform-specific adapter
+const service = new StorageService({
+  messenger: storageServiceMessenger,
+  storage: myPlatformAdapter, // FilesystemStorage, IndexedDB, etc.
+});
+
+// Testing: Uses in-memory storage by default
+const testService = new StorageService({
+  messenger: testMessenger,
+  // No adapter needed - data isolated per test
+});
+```
+
+### Events
+
+Subscribe to storage changes:
+
+```typescript
+this.messenger.subscribe(
+  'StorageService:itemSet:MyController',
+  (key, value) => {
+    console.log(`Data stored: ${key}`);
+  },
+);
+```
+
+## StorageAdapter Interface
+
+Implement this interface to provide platform-specific storage:
+
+```typescript
+import type { Json } from '@metamask/utils';
+
+// Response type for getItem - distinguishes found, not found, and error
+type StorageGetResult =
+  | { result: Json; error?: never } // Data found
+  | { result?: never; error: Error } // Error occurred
+  | Record<string, never>; // Key doesn't exist (empty object)
+
+export type StorageAdapter = {
+  getItem(namespace: string, key: string): Promise<StorageGetResult>;
+  setItem(namespace: string, key: string, value: Json): Promise<void>;
+  removeItem(namespace: string, key: string): Promise<void>;
+  getAllKeys(namespace: string): Promise<string[]>;
+  clear(namespace: string): Promise<void>;
+};
+```
+
+Adapters are responsible for:
+
+- Building the full storage key (e.g., `storageService:namespace:key`)
+- Serializing/deserializing JSON data
+- Returning the correct response format for getItem
+
+## Contributing
+
+This package is part of a monorepo. Instructions for contributing can be found in the [monorepo README](https://github.com/MetaMask/core#readme).