@@ -0,0 +1,24 @@
+---
+"@objectstack/spec": minor
+"@objectstack/lint": minor
+"@objectstack/objectql": minor
+"@objectstack/metadata-protocol": minor
+"@objectstack/runtime": minor
+"@objectstack/platform-objects": minor
+---
+
+feat(metadata-protocol,objectql): an object extension is an item of a writable package — saved, published and exported through the existing doors
+
+Clause-②: yes (widening)
+Metadata-protocol: added
+
+A package can now add fields and rules to an object another package defines by saving an `object_extension` item through `/api/v1/meta`, the same way it saves any other item. This is what Studio's add-field on a base object writes (ADR-0146 D10): an extension in the base's paired `<base>.custom` package, its fields suffixed `__c`, the base untouched.
+
+- **The type** (`@objectstack/spec/kernel`). `object_extension` joins `MetadataTypeSchema` and the metadata type registry. Its body is `ObjectExtensionSchema` as it stands: it carries `extend` and no `name`. The row is filed under the target object's name, one extension per package per target. `OVERRIDE_TYPE_CLASSIFICATION` lists it as `excluded`: an extension adds to its target, it never replaces a base item, so it never carries `overrides`. That disposition does not close the type to a `.custom` pair.
+- **The doors.** Draft, publish, history restore and delete are the doors every item uses. The save, publish and restore doors refuse, with `VALIDATION_ERROR` / 400 and an issue path: a write bound to no package; `defaultRowScope` or `membershipKeys` in the body (the registry's own sentences); a row filed under a name other than its `extend`; an unregistered target; a target outside the package's dependency closure (with the closure prescription the other save doors print); and a `.custom` field without the `__c` suffix (the existing `__c` judge). A body the schema refuses answers 422 `INVALID_METADATA` with the schema's issues. No error code is added.
+- **The registry** (`@objectstack/objectql`). A published row registers its whole parsed body as the package's `'extend'` contributor (`SchemaRegistry.registerObjectExtension`), so its fields, validations and indexes appear on every read of the object, and a publish adds the column to the target's table. Every stored-row sync re-registers it (boot, write-through, read hydration), and deleting the row or uninstalling the package withdraws it (`SchemaRegistry.withdrawObjectExtension`). What a manifest's `objectExtensions` register is unchanged.
+- **The export** (`@objectstack/runtime`). `GET /packages/:id/export` carries each of the package's extension items as an `objectExtensions` entry that re-installs. A package's `objects` are now only the objects it owns: an object the package extends is no longer exported as an `'own'` object, which re-install refused with `ObjectOwnershipConflictError`.
+- **`@objectstack/platform-objects`**: the metadata-forms translation bundles gain the type's `label` and `description`, authored in zh-CN, ja-JP and es-ES.
+- **`@objectstack/lint/runtime`** exports `closureDependencyRemedy`, the closure prescription the save doors print.
+
+Two costs, both on purpose: `/meta/types` serves no JSON schema for `object_extension`, and a draft carries no `_diagnostics` badge, because the type is not bound in `BUILTIN_METADATA_TYPE_SCHEMAS` (that map requires the ADR-0010 envelope, which would change the schema a manifest's `objectExtensions` parse with).
@@ -642,7 +642,7 @@ Metadata query with filtering, sorting, and pagination
 
 | Property | Type | Required | Description |
 | :--- | :--- | :--- | :--- |
-| **types** | `Enum<'object' \| 'field' \| 'hook' \| 'seed' \| 'mapping' \| 'picklist' \| 'view' \| 'page' \| 'dashboard' \| 'app' \| 'action' \| 'report' \| 'dataset' \| 'flow' \| 'job' \| 'datasource' \| 'external_catalog' \| 'translation' \| 'api' \| 'email_template' \| 'doc' \| 'book' \| 'permission' \| 'position' \| 'capability' \| 'sharing_rule' \| 'queue' \| 'assignment_rule' \| 'agent' \| 'tool' \| 'skill'>[]` | optional | Filter by metadata types |
+| **types** | `Enum<'object' \| 'object_extension' \| 'field' \| 'hook' \| 'seed' \| 'mapping' \| 'picklist' \| 'view' \| 'page' \| 'dashboard' \| 'app' \| 'action' \| 'report' \| 'dataset' \| 'flow' \| 'job' \| 'datasource' \| 'external_catalog' \| 'translation' \| 'api' \| 'email_template' \| 'doc' \| 'book' \| 'permission' \| 'position' \| 'capability' \| 'sharing_rule' \| 'queue' \| 'assignment_rule' \| 'agent' \| 'tool' \| 'skill'>[]` | optional | Filter by metadata types |
 | **namespaces** | `string[]` | optional | Filter by namespaces |
 | **packageId** | `string` | optional | Filter by owning package |
 | **search** | `string` | optional | Full-text search query |
@@ -709,14 +709,15 @@ Metadata query with filtering, sorting, and pagination
 
 | Property | Type | Required | Description |
 | :--- | :--- | :--- | :--- |
-| **type** | `Enum<'object' \| 'field' \| 'hook' \| 'seed' \| 'mapping' \| 'picklist' \| 'view' \| 'page' \| 'dashboard' \| 'app' \| 'action' \| 'report' \| 'dataset' \| 'flow' \| 'job' \| … +16 more>` | ✅ | Metadata type |
+| **type** | `Enum<'object' \| 'object_extension' \| 'field' \| 'hook' \| 'seed' \| 'mapping' \| 'picklist' \| 'view' \| 'page' \| 'dashboard' \| 'app' \| 'action' \| 'report' \| 'dataset' \| … +18 more>` | ✅ | Metadata type |
 | **name** | `string` | ✅ | Item name (snake_case) |
 | **data** | `Record<string, any>` | ✅ | Metadata payload |
 | **namespace** | `string` | optional | Optional namespace |
 
 ### Allowed Values: `MetadataRegisterRequest.type`
 
 * `object`
+* `object_extension`
 * `field`
 * `hook`
 * `seed`
 
@@ -179,7 +179,7 @@ const result = MetadataBulkResultSchema.parse(data);
 
 | Property | Type | Required | Description |
 | :--- | :--- | :--- | :--- |
-| **types** | `Enum<'object' \| 'field' \| 'hook' \| 'seed' \| 'mapping' \| 'picklist' \| 'view' \| 'page' \| 'dashboard' \| 'app' \| 'action' \| 'report' \| 'dataset' \| 'flow' \| 'job' \| 'datasource' \| 'external_catalog' \| 'translation' \| 'api' \| 'email_template' \| 'doc' \| 'book' \| 'permission' \| 'position' \| 'capability' \| 'sharing_rule' \| 'queue' \| 'assignment_rule' \| 'agent' \| 'tool' \| 'skill'>[]` | optional | Filter by metadata types |
+| **types** | `Enum<'object' \| 'object_extension' \| 'field' \| 'hook' \| 'seed' \| 'mapping' \| 'picklist' \| 'view' \| 'page' \| 'dashboard' \| 'app' \| 'action' \| 'report' \| 'dataset' \| 'flow' \| 'job' \| 'datasource' \| 'external_catalog' \| 'translation' \| 'api' \| 'email_template' \| 'doc' \| 'book' \| 'permission' \| 'position' \| 'capability' \| 'sharing_rule' \| 'queue' \| 'assignment_rule' \| 'agent' \| 'tool' \| 'skill'>[]` | optional | Filter by metadata types |
 | **namespaces** | `string[]` | optional | Filter by namespaces |
 | **packageId** | `string` | optional | Filter by owning package |
 | **search** | `string` | optional | Full-text search query |
@@ -226,6 +226,7 @@ const result = MetadataBulkResultSchema.parse(data);
 ### Allowed Values
 
 * `object`
+* `object_extension`
 * `field`
 * `hook`
 * `seed`
@@ -266,7 +267,7 @@ const result = MetadataBulkResultSchema.parse(data);
 
 | Property | Type | Required | Description |
 | :--- | :--- | :--- | :--- |
-| **type** | `Enum<'object' \| 'field' \| 'hook' \| 'seed' \| 'mapping' \| 'picklist' \| 'view' \| 'page' \| 'dashboard' \| 'app' \| 'action' \| 'report' \| 'dataset' \| 'flow' \| 'job' \| … +16 more>` | ✅ | Metadata type identifier |
+| **type** | `Enum<'object' \| 'object_extension' \| 'field' \| 'hook' \| 'seed' \| 'mapping' \| 'picklist' \| 'view' \| 'page' \| 'dashboard' \| 'app' \| 'action' \| 'report' \| 'dataset' \| … +18 more>` | ✅ | Metadata type identifier |
 | **label** | `string` | ✅ | Display label for the metadata type |
 | **description** | `string` | optional | Description of the metadata type |
 | **filePatterns** | `string[]` | ✅ | Glob patterns to discover files of this type |
@@ -283,6 +284,7 @@ const result = MetadataBulkResultSchema.parse(data);
 ### Allowed Values: `MetadataTypeRegistryEntry.type`
 
 * `object`
+* `object_extension`
 * `field`
 * `hook`
 * `seed`
 
@@ -57,6 +57,15 @@ export const KIND_COVERAGE: Record<MetadataType, KindCoverage> = {
     notes:
       'Also carries the validation-rule surface: rules are authored inline via object `validations` (the `validation` KIND was retired in #4509 — ADR-0088 — because a standalone rule had no way to name the object it validated). Every declared rule type is write-path enforced (rule-validator dispatches all of state_machine/script/cross_field/format/json_schema/conditional — ADR-0020 "no silent no-ops", closing the #1475 gap) and each is demonstrated: state_machine (task/project), script+cross_field (project), format/json_schema/conditional (account). Field-level requiredWhen/readonlyWhen are likewise enforced and demonstrated on invoice.',
   },
+  // [#28574] A registry kind since an extension became an authorable item of a
+  // writable package. The showcase's own extension is the code half (the stack's
+  // `objectExtensions`, also tracked in STACK_COLLECTION_COVERAGE); the runtime
+  // half is `PUT /api/v1/meta/object_extension/<target>?package=<pkg>`.
+  object_extension: {
+    status: 'demonstrated',
+    files: ['src/data/extensions/account.extension.ts'],
+    notes: 'Authored in code as an `objectExtensions` entry; the same body is an `object_extension` item when a writable package saves it through `/api/v1/meta`.',
+  },
   field: {
     status: 'demonstrated',
     files: ['src/data/objects/field-zoo.object.ts'],
 
@@ -70,6 +70,11 @@ export {
   runtimeGatedTypes,
   stackKeyForType,
 } from './runtime-gate.js';
+// [#28574] The closure prescription the save doors print, for a door outside
+// this package that judges an object name under a package closure (the
+// `object_extension` item gate in `@objectstack/metadata-protocol`), so the
+// sentence is not spelled a second time.
+export { closureDependencyRemedy } from './object-graph.js';
 export type {
   RuntimeGateResult,
   RuntimePackageScope,
 
@@ -79,6 +79,7 @@ export interface AuthoringGateHost {
     objectNamespacePrefixViolation: ObjectStackProtocolImplementation['objectNamespacePrefixViolation'];
     hierarchyResolverMounted: ObjectStackProtocolImplementation['hierarchyResolverMounted'];
     isArtifactBacked: ObjectStackProtocolImplementation['isArtifactBacked'];
+    objectExtensionItemRefusal: ObjectStackProtocolImplementation['objectExtensionItemRefusal'];
     orgWallEnforced: ObjectStackProtocolImplementation['orgWallEnforced'];
     permissionSetNameCollisionRefusal: ObjectStackProtocolImplementation['permissionSetNameCollisionRefusal'];
     refuseRestoredBodyTheSaveDoorRefuses: ObjectStackProtocolImplementation['refuseRestoredBodyTheSaveDoorRefuses'];
@@ -101,7 +102,7 @@ export function restoredBodyDerivation(
 
 /** The body of `ObjectStackProtocolImplementation#restoredBodyWriter`; the method's docblock there is its contract. */
 export function restoredBodyWriter(
-    this: Pick<AuthoringGateHost, 'refuseRestoredBodyTheSaveDoorRefuses' | 'restoredBodyDerivation'>,
+    this: Pick<AuthoringGateHost, 'objectExtensionItemRefusal' | 'refuseRestoredBodyTheSaveDoorRefuses' | 'restoredBodyDerivation'>,
     type: string, name: string, binding: string | null, source: string): (historyBody: unknown) => Promise<unknown> {
     const derive = this.restoredBodyDerivation(type);
     return async (historyBody) => {
@@ -112,6 +113,9 @@ export function restoredBodyWriter(
         // [ADR-0146 D10, #28564] The `__c` reservation, on the same binding.
         const suffixRefusal = objectBodyExtensionFieldSuffixRefusal(type, historyBody, name, binding);
         if (suffixRefusal) throw suffixRefusal;
+        // [#28574] An `object_extension` version against its address, on the same binding.
+        const extensionRefusal = this.objectExtensionItemRefusal(type, historyBody, name, binding);
+        if (extensionRefusal) throw extensionRefusal;
         await this.refuseRestoredBodyTheSaveDoorRefuses(type, name, historyBody, binding, source);
         return derive ? derive(historyBody) : historyBody;
     };
 
@@ -13,8 +13,14 @@
  *     protocol` (`saveMetaItem`, `rollbackMetaItem`, and the promotion
  *     `publishMetaItem` / `publishPackageDrafts` share), for an `object` body,
  *     through {@link objectBodyExtensionFieldSuffixRefusal};
+ *   - the same three doors for an `object_extension` item, as the package's one
+ *     `objectExtensions` entry, through {@link judgeExtensionFieldSuffix}
+ *     (`objectExtensionItemRefusal`, `protocol-object-extension-item.ts`);
  *   - `duplicatePackage`, for every object the copy would write, judged against
  *     the TARGET id before the copy mints anything;
+ *   - `reassignOrphanedMetadata` (the adopt-orphans door), for every active
+ *     `object` row the adoption would rebind, judged against the TARGET id
+ *     before the first storage update;
  *   - `os validate` (`@objectstack/cli`) and the marketplace install
  *     (`@objectstack/cloud-connection`), for a whole package body, through
  *     {@link judgeExtensionFieldSuffix}.