@@ -0,0 +1,23 @@
+---
+'@objectstack/spec': minor
+---
+
+fix(spec)!: an object's `external` binding on a managed datasource is refused at the parse and by `defineStack`, as ADR-0015 §4.2 declares
+
+Clause-②: no (narrowing)
+
+Metadata-protocol: narrowed
+
+<!-- adr-0087: registered object-external-binding-managed-datasource-refused -->
+
+**BREAKING** (what an author can declare narrows), shipped as `minor` under the launch-window convention for breaking changes.
+
+ADR-0015 §4.2 allows `object.external` only on a datasource whose `schemaMode` is not `'managed'`, and declares the rule "enforced at metadata-load time". No load door enforced it: `defineStack`, `os validate` and the object save door (`PUT /api/v1/meta/object/NAME`) all accepted an `external` binding on a managed datasource, the primary `default` included. The runtime then disagreed with itself about such an object: its schema sync treated it as federated and never created its table, while the boot validation skipped it and the write gate exempted it because its datasource is managed.
+
+- **FROM:** an object declaring `external` was accepted whatever its datasource: `datasource` omitted or `'default'`, or a datasource declared with `schemaMode: 'managed'` or with no `schemaMode`.
+- **TO:** the rule is judged where each half is visible.
+  - An `external` binding whose object `datasource` is `'default'` (also the value when none is written) is refused by `ObjectSchema`'s parse at `external`: `ObjectSchema.create` throws at module load, `defineStack` and `os validate` refuse with `STACK_SCHEMA_INVALID` (422), and the object save door answers `422 INVALID_METADATA` and stores nothing. The platform's primary datasource is managed by construction.
+  - An `external` binding on a datasource the same stack declares with `schemaMode: 'managed'`, or with no `schemaMode`, is refused by `defineStack` (and so `os validate` and `os build`) with `STACK_CROSS_REFERENCE_INVALID` (422), one finding per object.
+  - A package `defaultDatasource` or a `datasourceMapping` rule does not make an object federated: they route its queries, while the boot validation and the write gate read the object's own `datasource`.
+- **Unchanged:** a binding on a datasource declared `schemaMode: 'external'` or `'validate-only'` is accepted. A binding on a datasource the stack does not declare (registered by the host or another package, or stored at runtime) gets no verdict at these doors, because they cannot see its `schemaMode`. `defineStack(config, { strict: false })` skips the stack-level judgement by contract. A stored object row in the refused shape still registers, under a `[metadata_spec_invalid]` diagnostic, and its next save is refused; `os migrate meta --stored` lists each such row under `object-external-binding-managed-datasource-refused`.
+- **The fix for an author:** set the object's `datasource` to the datasource that holds the remote table and declare that datasource with `schemaMode: 'external'` (or `'validate-only'`) and its `external` settings, for example `datasource: 'warehouse'` beside `defineDatasource({ name: 'warehouse', schemaMode: 'external', external: { … } })`. An object whose rows ObjectStack should own drops `external` instead.
@@ -0,0 +1,96 @@
+// Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
+
+/**
+ * [ADR-0015 §4.2 · #25168] The object save door refuses an `external` binding
+ * on the platform's primary datasource.
+ *
+ * ADR-0015 §4.2 allows `object.external` only on a datasource whose
+ * `schemaMode` is not `'managed'`, declared "enforced at metadata-load time".
+ * `saveMetaItem` used to store such an object: `datasource` omitted or
+ * `'default'`, `external` present. `ObjectSchema`'s refinement now refuses the
+ * binding at `external` (the primary datasource is managed by construction),
+ * and this door inherits it through its spec-conformance parse: `422
+ * INVALID_METADATA`, the finding at `external`, nothing stored.
+ *
+ * The half that needs a named datasource's definition is not this door's
+ * parse: it cannot see the datasource, so a binding that names one saves here
+ * whatever that datasource declares.
+ */
+import { describe, expect, it } from 'vitest';
+import { assertEngineDeleteDispatch, assertEngineUpdateDispatch, assertEngineFindOnePredicate, type EngineFindOneQueryInput } from '@objectstack/metadata-core';
+import { ObjectStackProtocolImplementation } from './protocol.js';
+
+/** The engine surface the repository write path touches (as the select-shorthand harness). */
+function makeProtocol() {
+    const rows = new Map<string, Record<string, unknown>>();
+    let nextId = 0;
+    const engine: any = {
+        async findOne(object: string, query?: EngineFindOneQueryInput) {
+            assertEngineFindOnePredicate(object, query); return null; },
+        async find() { return []; },
+        async insert(table: string, data: Record<string, unknown>) {
+            nextId += 1;
+            if (table === 'sys_metadata') rows.set(`${data.type}|${data.name}`, data);
+            return { id: `r_${nextId}` };
+        },
+        async update(_t: string, data: Record<string, unknown>, opts?: Record<string, unknown>) {
+            assertEngineUpdateDispatch(data, opts);
+            return { id: null };
+        },
+        async delete(_t: string, opts?: Record<string, unknown>) {
+            assertEngineDeleteDispatch(opts);
+            return { deleted: 0 };
+        },
+        registry: { registerItem: () => {}, registerObject: () => {} },
+    };
+    const protocol: any = new ObjectStackProtocolImplementation(engine, () => new Map());
+    return { protocol, rows };
+}
+
+async function rejection(promise: Promise<unknown>): Promise<any> {
+    try {
+        await promise;
+    } catch (err) {
+        return err;
+    }
+    throw new Error('expected the save to be rejected, but it resolved');
+}
+
+const order = (extra: Record<string, unknown>) => ({
+    name: 'wh_order',
+    label: 'Order',
+    sharingModel: 'private',
+    fields: { title: { name: 'title', label: 'Title', type: 'text' } },
+    ...extra,
+});
+
+describe('#25168 the object save door refuses an `external` binding on the primary datasource', () => {
+    it.each([
+        ['`datasource` omitted', { external: { remoteName: 'orders' } }],
+        ["`datasource: 'default'`", { datasource: 'default', external: { remoteName: 'orders' } }],
+    ])('%s: 422 INVALID_METADATA at `external`, and nothing is stored', async (_label, extra) => {
+        const { protocol, rows } = makeProtocol();
+
+        const err = await rejection(protocol.saveMetaItem({ type: 'object', name: 'wh_order', item: order(extra) }));
+
+        expect(err.code).toBe('INVALID_METADATA');
+        expect(err.status).toBe(422);
+        expect(err.issues).toEqual([
+            expect.objectContaining({ path: 'external', code: 'custom' }),
+        ]);
+        expect(err.issues[0].message).toMatch(/^Object 'wh_order' declares `external`, but its `datasource` is 'default'/);
+        expect(rows.size).toBe(0);
+    });
+
+    it.each([
+        ['a binding naming a datasource (this half needs only the object)', { datasource: 'lake', external: { remoteName: 'orders' } }],
+        ['no binding, on the primary datasource', { datasource: 'default' }],
+    ])('control — %s saves', async (_label, extra) => {
+        const { protocol, rows } = makeProtocol();
+
+        const result = await protocol.saveMetaItem({ type: 'object', name: 'wh_order', item: order(extra) });
+
+        expect(result.success).toBe(true);
+        expect(rows.size).toBe(1);
+    });
+});
@@ -61,6 +61,7 @@ describe('the refused-shape list — registered against the migration chain', ()
       'cbp-master-relation-tiers-retired',
       'view-data-api-provider-retired',
       'flow-value-slot-template-dialect-refused',
+      'object-external-binding-managed-datasource-refused',
     ]);
   });
 
@@ -470,3 +471,66 @@ describe('#23317 — a stored page whose component binds the retired API source
     }
   });
 });
+
+// ── #25168 ────────────────────────────────────────────────────────────────
+
+const federatedRow = (extra: Record<string, unknown>) => ({
+  name: 'wh_order',
+  label: 'Order',
+  sharingModel: 'private',
+  fields: { title: { type: 'text', label: 'Title' } },
+  external: { remoteName: 'orders' },
+  ...extra,
+});
+
+const externalIssue = (row: unknown) => {
+  const parsed = ObjectSchema.safeParse(row);
+  return parsed.success ? undefined : parsed.error.issues.find((i) => i.path.join('.') === 'external');
+};
+
+describe('#25168 — a stored object whose `external` binding is on a managed datasource is listed by object', () => {
+  it.each([
+    ['`datasource` omitted', {}],
+    ["`datasource: 'default'`", { datasource: 'default' }],
+  ])('%s: one TODO naming the object, the rule and the owner\'s choice; the row is not rewritten', (_label, extra) => {
+    const row = federatedRow(extra);
+    const { item, todos } = storedTodos('object', row);
+    expect(item).toEqual(row);
+    expect(todos).toHaveLength(1);
+    const todo = todos[0]!;
+    expect(todo.conversionId).toBe('object-external-binding-managed-datasource-refused');
+    expect(todo.path).toBe('objects[0].external');
+    expect(todo.reason).toMatch(/^No lossless rewrite: dropping `external` turns the remote table into one ObjectStack creates/);
+    expect(todo.reason).toContain("Object 'wh_order' declares `external`, but its `datasource` is 'default'");
+    expect(JSON.parse(todo.from)).toEqual({ datasource: (extra as { datasource?: string }).datasource ?? null, external: { remoteName: 'orders' } });
+    // The listing and the parse refuse ONE set: the stored row still fails the parse, at the same key.
+    expect(externalIssue(item)).toBeDefined();
+  });
+
+  it('a stack that declares the named datasource managed is listed by the stack-level half', () => {
+    const stack = {
+      datasources: [{ name: 'warehouse', driver: 'sqlite', config: {} }],
+      objects: [federatedRow({ datasource: 'warehouse' })],
+    };
+    const { todos } = todosOf(stack);
+    const listed = todos.filter((t) => t.conversionId === 'object-external-binding-managed-datasource-refused');
+    expect(listed).toHaveLength(1);
+    expect(listed[0]!.reason).toContain("declares `external` on datasource 'warehouse', whose `schemaMode` is 'managed'");
+  });
+
+  it.each([
+    ['a named datasource the row cannot see', federatedRow({ datasource: 'warehouse' })],
+    ['an object with no `external`', { ...federatedRow({}), external: undefined }],
+  ])('%s is not listed — and the parse accepts it', (_label, row) => {
+    expect(storedTodos('object', row).todos).toEqual([]);
+    expect(externalIssue(row)).toBeUndefined();
+  });
+
+  it('a datasource the stack declares external is not listed', () => {
+    const stack = {
+      datasources: [{ name: 'warehouse', driver: 'sqlite', config: {}, schemaMode: 'external', external: {} }],
+      objects: [federatedRow({ datasource: 'warehouse' })],
+    };
+    expect(todosOf(stack).todos.filter((t) => t.conversionId === 'object-external-binding-managed-datasource-refused')).toEqual([]);
+  });
+});
@@ -38,7 +38,9 @@
  *   component-props gate parses at the CLI doors and at the runtime save
  *   door — and the value-slot judge
  *   `AutomationEngine.registerFlow` and `objectstack validate` run
- *   ({@link flowNodeValueTemplateRefusals}). A site is listed here exactly
+ *   ({@link flowNodeValueTemplateRefusals}), and the `external` binding judge
+ *   the object refinement and `defineStack` share
+ *   ({@link externalBindingDatasourceRefusal}). A site is listed here exactly
  *   when that door refuses it.
  * - It never prints a credential. A view-level API source could carry request
  *   `headers`. The listing names the URL and the method, and withholds the rest.
@@ -53,6 +55,7 @@ import {
   isControlledByParentMasterField,
   type ControlledByParentMasterRefusal,
 } from '../data/controlled-by-parent-master.js';
+import { externalBindingDatasourceRefusal } from '../data/external-binding-datasource.js';
 import { ComponentPropsMap } from '../ui/component.zod.js';
 import { ViewDataSchema } from '../ui/view.zod.js';
 import type { ConversionTodoDetail } from './types.js';
@@ -175,6 +178,43 @@ const controlledByParentWithoutOneMaster: RefusedStoredShape = {
   },
 };
 
+// ── #25168: an object's `external` binding on a managed datasource ──
+
+const externalBindingOnManagedDatasource: RefusedStoredShape = {
+  id: 'object-external-binding-managed-datasource-refused',
+  toMajor: 18,
+  surface: 'object.external — a federated binding on the primary datasource or on a managed one the stack declares',
+  detect(stack, report) {
+    // The datasources this stack declares, as the stack-level half of the rule
+    // reads them. A stored object row arrives alone, so for it only the
+    // primary half (`datasource: 'default'`, written or omitted) can answer.
+    const declared = new Map<string, Dict>();
+    for (const ds of Array.isArray(stack.datasources) ? stack.datasources : []) {
+      if (isDict(ds) && typeof ds.name === 'string') declared.set(ds.name, ds);
+    }
+    mapCollection(stack, 'objects', (obj, path) => {
+      const datasource = typeof obj.datasource === 'string' ? obj.datasource : undefined;
+      const refusal = externalBindingDatasourceRefusal(
+        obj.name,
+        obj.external,
+        datasource,
+        datasource === undefined ? undefined : declared.get(datasource),
+      );
+      if (refusal === undefined) return obj;
+      report({
+        path: `${path}.${refusal.path}`,
+        from: JSON.stringify({ datasource: datasource ?? null, external: obj.external }),
+        reason: 'No lossless rewrite: dropping `external` turns the remote table into one ObjectStack creates, '
+          + 'and naming another datasource or re-declaring this one external says where the rows live, which '
+          + 'only the owner knows. Left as stored, the object registers under `[metadata_spec_invalid]` and its '
+          + 'next save is refused with 422 INVALID_METADATA. '
+          + refusal.message,
+      });
+      return obj;
+    });
+  },
+};
+
 // ── #23271: a view whose data source is the retired view-level API binding ──
 
 /** The request keys the listing names; every other key of a request block is withheld. */
@@ -361,6 +401,7 @@ export const REFUSED_STORED_SHAPES: readonly RefusedStoredShape[] = [
   controlledByParentWithoutOneMaster,
   viewDataApiProvider,
   flowValueSlotTemplateToken,
+  externalBindingOnManagedDatasource,
 ];
 
 /**
 
@@ -0,0 +1,92 @@
+// Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
+
+/**
+ * [ADR-0015 §4.2 · #25168] The object half of "`object.external` only when the
+ * object's datasource has `schemaMode !== 'managed'`": an `external` binding
+ * whose object `datasource` is `'default'` — written, or the parsed value of an
+ * omitted one — is refused by `ObjectSchema`'s parse, at `external`, because
+ * the platform's primary datasource is managed by construction.
+ *
+ * Pinned: both spellings of the primary refused at the same path with the same
+ * message, by `safeParse` and by `ObjectSchema.create` at module load; the
+ * CONTROLS (a named datasource, which this half cannot judge, and an object
+ * with no `external`) accepted; and the predicate's other half, which needs
+ * the datasource's definition, answering from it alone.
+ */
+import { describe, expect, it } from 'vitest';
+import { ObjectSchema } from './object.zod';
+import { externalBindingDatasourceRefusal } from './external-binding-datasource';
+
+const object = (extra: Record<string, unknown>) => ({
+  name: 'wh_order',
+  label: 'Order',
+  sharingModel: 'private',
+  fields: { title: { type: 'text', label: 'Title' } },
+  ...extra,
+});
+
+const issuesOf = (value: unknown) => {
+  const parsed = ObjectSchema.safeParse(value);
+  return parsed.success ? [] : parsed.error.issues;
+};
+
+const PRIMARY = /^Object 'wh_order' declares `external`, but its `datasource` is 'default' \(also the value when none is written\)/;
+
+describe('ObjectSchema refuses an `external` binding on the primary datasource', () => {
+  it.each([
+    ['`datasource` omitted', { external: { remoteName: 'orders' } }],
+    ["`datasource: 'default'`", { datasource: 'default', external: { remoteName: 'orders' } }],
+    ['an empty binding', { external: {} }],
+  ])('%s: one issue at `external`, naming the object and its datasource', (_label, extra) => {
+    const issues = issuesOf(object(extra));
+    expect(issues).toHaveLength(1);
+    expect(issues[0]).toMatchObject({ code: 'custom', path: ['external'] });
+    expect(issues[0]!.message).toMatch(PRIMARY);
+  });
+
+  it('`ObjectSchema.create` refuses it at module load, with the same issue', () => {
+    let thrown: { issues?: Array<{ path: unknown[]; message: string }> } | undefined;
+    try {
+      ObjectSchema.create(object({ external: { remoteName: 'orders' } }) as never);
+    } catch (e) {
+      thrown = e as typeof thrown;
+    }
+    expect(thrown?.issues).toEqual([expect.objectContaining({ path: ['external'], message: expect.stringMatching(PRIMARY) })]);
+  });
+
+  it.each([
+    ['a named datasource', { datasource: 'warehouse', external: { remoteName: 'orders' } }],
+    ['no `external`, on the primary', { datasource: 'default' }],
+    ['no `external`, `datasource` omitted', {}],
+  ])('control — %s parses', (_label, extra) => {
+    expect(issuesOf(object(extra))).toEqual([]);
+  });
+});
+
+describe('externalBindingDatasourceRefusal — the one judgement both halves read', () => {
+  it('answers nothing without a binding, whatever the datasource', () => {
+    expect(externalBindingDatasourceRefusal('o', undefined, 'default')).toBeUndefined();
+    expect(externalBindingDatasourceRefusal('o', null, 'warehouse', { schemaMode: 'managed' })).toBeUndefined();
+  });
+
+  it('answers nothing for a named datasource it cannot see', () => {
+    expect(externalBindingDatasourceRefusal('o', {}, 'warehouse')).toBeUndefined();
+  });
+
+  it.each([
+    ["declared 'managed'", { schemaMode: 'managed' }],
+    ['declared with no `schemaMode` (managed by default)', {}],
+  ])('refuses a binding on a datasource %s', (_label, declared) => {
+    const refusal = externalBindingDatasourceRefusal('o', {}, 'warehouse', declared);
+    expect(refusal).toMatchObject({ path: 'external', shape: 'managed', datasource: 'warehouse' });
+    expect(refusal!.message).toMatch(/^Object 'o' declares `external` on datasource 'warehouse', whose `schemaMode` is 'managed'/);
+  });
+
+  it.each(['external', 'validate-only'])("accepts a binding on a datasource declared '%s'", (schemaMode) => {
+    expect(externalBindingDatasourceRefusal('o', {}, 'warehouse', { schemaMode })).toBeUndefined();
+  });
+
+  it('judges the primary by name alone: a definition passed for it changes nothing', () => {
+    expect(externalBindingDatasourceRefusal('o', {}, 'default', { schemaMode: 'external' })?.shape).toBe('primary');
+  });
+});