diff --git a/‎.changeset/21842-validate-reads-live-registry.md‎
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diff --git a/‎packages/qa/dogfood/test/external-validate-sees-runtime-save.dogfood.test.ts‎
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diff --git a/‎packages/services/service-datasource/src/__tests__/external-metadata-read-at-use.test.ts‎
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@@ -0,0 +1,11 @@
+---
+'@objectstack/service-datasource': patch
+---
+
+fix(service-datasource): `POST /api/v1/datasources/:name/external/validate` sees a federated object saved at runtime, with no restart (#21842)
+
+Clause-②: no
+
+- **What was wrong.** The federation service read its objects from the `metadata` service. That service holds a copy of the engine's object registry taken once at boot. `PUT /api/v1/meta/object/:name`, and the external-table import that saves through it, write `sys_metadata` and the engine registry, but never that copy. So after a federated object was saved at runtime, validate answered the code-defined objects only, and listed the saved one after a restart. An object re-saved at runtime was judged on its definition as it stood at boot.
+- **What it reads now.** `ExternalDatasourceServicePlugin` reads objects (`listObjects` and `getObject`) from the engine's object registry on the `objectql` service, which is the registry the save writes through to. The registry is looked up when validation runs, not when the plugin starts. A saved or imported object is listed and judged on what was saved, the moment the save answers.
+- **What does not move.** The comparison is unchanged: the same federation predicate, the same column and type checks, and datasource definitions read from the same place. On `objectstack dev` the boot validation gate sweeps the same objects with the same verdicts as before. No route's request or response shape changes, and no export is added.
@@ -0,0 +1,150 @@
+// Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
+//
+// [#21842, ADR-0015 §6] `POST /datasources/:name/external/validate` lists a
+// federated object the moment it is saved at runtime, with no restart, over
+// the real showcase composition.
+//
+// ## What was broken
+//
+// The federation service read its objects from the `metadata` service, which
+// holds a copy of the engine's object registry taken once at boot.
+// `PUT /meta/object/:name`, and the external-table import that saves through
+// it, write `sys_metadata` and the engine registry, never that copy. Measured
+// on the showcase (`objectstack dev`): after a federated object was saved on
+// `showcase_external`, validate still answered the two code-defined objects
+// only, and listed the saved one after a restart. The service now reads the
+// engine registry, resolved when validation runs.
+//
+// ## What this file pins
+//
+// Through the doors an operator uses: the code-defined objects are listed;
+// an object saved through `PUT /meta/object/:name` is listed with a real
+// verdict (it declares a column the remote lacks, and exactly that column is
+// reported); an object imported through `…/external/tables/:remote/import` is
+// listed beside them. The verify harness composes no metadata plugin, so its
+// `metadata` service is the kernel's fallback registered after the
+// federation service's `init()`, as on `objectstack start`; the service reads
+// it when validation runs ([#21876]), so each row here is a real comparison.
+// The seam pins sit beside the plugin
+// (`packages/services/service-datasource/src/__tests__/external-validate-reads-live-registry.test.ts`).
+//
+// The working directory is a temporary one because the showcase's external
+// datasource and its fixture both name a cwd-relative SQLite file: this
+// file's remote database is its own, not one a parallel file writes.
+
+import { describe, it, expect, beforeAll, afterAll } from 'vitest';
+import showcaseStack, { onEnable } from '@objectstack/example-showcase';
+import { ExternalDatasourceServicePlugin } from '@objectstack/service-datasource';
+import { bootStack, type VerifyStack } from '@objectstack/verify';
+import { mkdtempSync, rmSync } from 'node:fs';
+import { tmpdir } from 'node:os';
+import { join } from 'node:path';
+
+/** The showcase's external datasource (`showcase-external.datasource.ts`). */
+const DATASOURCE = 'showcase_external';
+/** The two objects the showcase binds to it in code (`data/objects/external/`). */
+const CODE_DEFINED = ['showcase_ext_customer', 'showcase_ext_order'];
+/** Saved at runtime through the metadata door, bound to the remote `customers` table. */
+const SAVED = 'dogfood_ext_cust_21842';
+/** A column the saved object declares and the remote `customers` table does not have. */
+const MISSING = 'loyalty_tier';
+/** Imported at runtime from the remote `orders` table. */
+const IMPORTED = 'dogfood_ext_ord_21842';
+
+const validatePath = `/datasources/${DATASOURCE}/external/validate`;
+
+interface Row {
+  object: string;
+  ok: boolean;
+  diffs: Array<{ kind: string; column?: string; severity: string }>;
+}
+
+function rowsOf(json: unknown): Row[] {
+  const body = json as { data?: { results?: Row[] } } | undefined;
+  return body?.data?.results ?? [];
+}
+
+function listedObjects(json: unknown): string[] {
+  return rowsOf(json).map((r) => String(r.object)).sort();
+}
+
+const rowOf = (json: unknown, object: string) => rowsOf(json).find((r) => r.object === object);
+
+describe('external validate lists a federated object saved at runtime, with no restart (showcase)', () => {
+  let stack: VerifyStack;
+  let token: string;
+  let prevCwd: string;
+  let dir: string;
+
+  const call = async (method: string, path: string, body?: unknown) => {
+    const res = await stack.apiAs(token, method, path, body);
+    const json: unknown = await res.json().catch(() => ({}));
+    return { status: res.status, json };
+  };
+
+  beforeAll(async () => {
+    prevCwd = process.cwd();
+    dir = mkdtempSync(join(tmpdir(), 'dogfood-21842-'));
+    process.chdir(dir);
+    // Provision the remote tables, exactly as `os dev` does at boot (the
+    // harness imports only the stack's default export, so `onEnable` never
+    // runs on its own).
+    await onEnable({ logger: { info() {}, warn() {} } } as never);
+    stack = await bootStack(showcaseStack, {
+      databaseFile: join(dir, 'showcase.db'),
+      extraPlugins: [new ExternalDatasourceServicePlugin()],
+    });
+    token = await stack.signIn();
+  }, 180_000);
+
+  afterAll(async () => {
+    await stack?.stop();
+    if (prevCwd) process.chdir(prevCwd);
+    if (dir) rmSync(dir, { recursive: true, force: true });
+  });
+
+  it('lists the code-defined objects bound to the datasource', async () => {
+    const validated = await call('POST', validatePath);
+    expect(validated.status, JSON.stringify(validated.json)).toBe(200);
+    expect(listedObjects(validated.json)).toEqual(CODE_DEFINED);
+  });
+
+  it('lists an object saved through PUT /meta/object/:name with a real verdict, and still the code-defined ones', async () => {
+    const saved = await call('PUT', `/meta/object/${SAVED}`, {
+      name: SAVED,
+      label: 'Dogfood External Customer',
+      sharingModel: 'public_read_write',
+      datasource: DATASOURCE,
+      external: { remoteName: 'customers' },
+      fields: {
+        name: { type: 'text', label: 'Name' },
+        email: { type: 'text', label: 'Email' },
+        [MISSING]: { type: 'text', label: 'Loyalty Tier' },
+      },
+    });
+    expect(saved.status, JSON.stringify(saved.json)).toBe(200);
+
+    const validated = await call('POST', validatePath);
+    expect(validated.status, JSON.stringify(validated.json)).toBe(200);
+    expect(listedObjects(validated.json)).toEqual([...CODE_DEFINED, SAVED].sort());
+    // Compared, not waved through: exactly the declared column the remote lacks.
+    expect(rowOf(validated.json, SAVED)).toMatchObject({
+      ok: false,
+      diffs: [{ kind: 'missing_column', remoteName: 'customers', column: MISSING, severity: 'error' }],
+    });
+    for (const name of CODE_DEFINED) expect(rowOf(validated.json, name)?.ok, name).toBe(true);
+  });
+
+  it('lists an object imported from a remote table, beside the saved and code-defined ones', async () => {
+    const imported = await call('POST', `/datasources/${DATASOURCE}/external/tables/orders/import`, { name: IMPORTED });
+    expect(imported.status, JSON.stringify(imported.json)).toBe(201);
+
+    const validated = await call('POST', validatePath);
+    expect(validated.status, JSON.stringify(validated.json)).toBe(200);
+    expect(listedObjects(validated.json)).toEqual([...CODE_DEFINED, IMPORTED, SAVED].sort());
+    expect(rowOf(validated.json, IMPORTED), JSON.stringify(rowOf(validated.json, IMPORTED))).toMatchObject({
+      ok: true,
+      diffs: [],
+    });
+  });
+});
@@ -136,10 +136,24 @@ interface Harness {
   services: Map<string, unknown>;
 }
 
-/** A kernel context whose `getService` throws on an unregistered name, as the kernel's does. */
-function harness(): Harness {
+/**
+ * A kernel context whose `getService` throws on an unregistered name, as the kernel's does.
+ *
+ * [#21842] The federated objects live in the engine's object registry on the
+ * `objectql` service, which is where the federation service reads objects
+ * (the registry a runtime save writes through to). The `metadata` fake still
+ * holds its own copy, as the kernel's fallback does after the startup bridge;
+ * the readers this file pins are the ones that stay on the metadata service.
+ */
+function harness(opts: { registry?: boolean } = {}): Harness {
   const services = new Map<string, unknown>();
   services.set('data', { introspectDatasource: vi.fn(async () => remoteSchema()) });
+  if (opts.registry !== false) {
+    const objects = new Map<string, unknown>([[CUSTOMER.name, CUSTOMER], [ORDER.name, ORDER]]);
+    services.set('objectql', {
+      registry: { getObject: (n: string) => objects.get(n), getAllObjects: () => [...objects.values()] },
+    });
+  }
   const ctx = {
     getService: (name: string) => {
       if (!services.has(name)) throw new Error(`Service '${name}' not found`);
@@ -257,18 +271,16 @@ describe('the federation service reads a metadata service registered after init
 
   it('asks for the service again at each use, never remembering the first answer', async () => {
     const { h, service } = await startOrdering();
-    expect(objectsOf(await service.validateAll())).toEqual(['wh_customer', 'wh_order']);
-
-    const replacement = metadataFake();
-    await replacement.register('object', 'wh_extra', {
-      name: 'wh_extra',
-      datasource: 'warehouse',
-      external: { remoteName: 'orders' },
-      fields: {},
-    });
-    h.services.set('metadata', replacement.service);
+    const customerDiffs = async () =>
+      (await service.validateAll()).results.find((r) => r.object === 'wh_customer')?.diffs;
+    expect(await customerDiffs()).toEqual([expect.objectContaining({ kind: 'missing_column', column: 'email' })]);
+
+    // [#21842] Objects come from the engine registry, so the replacement is
+    // told apart by the datasource definition it answers: a `managed`
+    // datasource is not compared against its remote at all.
+    h.services.set('metadata', metadataFake({ name: 'warehouse', schemaMode: 'managed' }).service);
 
-    expect(objectsOf(await service.validateAll())).toEqual(['wh_customer', 'wh_extra', 'wh_order']);
+    expect(await customerDiffs()).toEqual([]);
   });
 });
 
@@ -292,7 +304,9 @@ describe('control: a metadata service registered before init (the dev ordering)
 
 describe('with no metadata service at all, every reader keeps its fallback', () => {
   it('validate and the sweep answer an empty report, the draft a bare name, the catalog an unpersisted snapshot', async () => {
-    const service = await federation(harness());
+    // [#21842] No engine registry either: that is where validate's objects
+    // come from, so with neither service the report has nothing to list.
+    const service = await federation(harness({ registry: false }));
 
     expect(await service.validateAll()).toEqual({ ok: true, results: [] });
     expect((await service.generateObjectDraft('warehouse', 'customers')).name).toBe('customers');