diff --git a/.changeset/sys-secret-store-platform-infra.md b/.changeset/sys-secret-store-platform-infra.md
new file mode 100644
index 0000000000..08b5b86943
--- /dev/null
+++ b/.changeset/sys-secret-store-platform-infra.md
@@ -0,0 +1,42 @@
+---
+"@objectstack/platform-objects": minor
+"@objectstack/service-settings": patch
+"@objectstack/verify": patch
+---
+
+feat(platform-objects,service-settings,verify): `sys_secret` is platform infrastructure — registered by `PlatformObjectsPlugin`, not by the settings service (#4270)
+
+The environment's encrypted-secret store (`sys_secret`, ADR-0066 D2/④) was
+registered by `@objectstack/service-settings`, but it has three producer
+classes and only one of them is settings: the settings service's encrypted
+specifiers, the ObjectQL engine's own `secret`-field encryption
+(`encryptSecretFields`/`resolveSecret` — the generic write path of ANY
+business object carrying a `Field.secret()`), and the datasource credential
+binder. Unlike the `sys_migration` precedent (#4243), the failure posture is
+fail-CLOSED: on a kernel composed without settings, every insert/update of an
+object with a secret field threw — with an error message that told the
+operator to "Ensure the platform-objects (sys_secret) are registered", naming
+a package that did not register it.
+
+The registration now lives in `PlatformObjectsPlugin`
+(`@objectstack/platform-objects/plugin`) — the plugin `os serve` already
+auto-injects into every served kernel — so the store exists with the
+platform, independent of which optional services are composed, and the
+engine's fail-closed error message is true. Definition ownership is unchanged
+(`sys_secret` stays in `@objectstack/platform-objects` and in
+`PLATFORM_OBJECTS_BY_PACKAGE`); the settings service remains a producer and
+consumer through its `sys_secret`-backed secret store.
+
+Consequences:
+
+- `@objectstack/service-settings` no longer contributes `sys_secret` to the
+  manifest (`settingsObjects` is now `[SysSetting, SysSettingAudit]`). An
+  embedder composing `SettingsServicePlugin` on a hand-built kernel that
+  relied on it for the `sys_secret` table must compose
+  `PlatformObjectsPlugin` (the plugin every supported assembly path already
+  includes). The move REPLACES the registration — nothing registers the
+  object twice.
+- `@objectstack/verify`'s boot harness now composes `PlatformObjectsPlugin`,
+  mirroring `os serve`'s auto-inject — which also means harness kernels now
+  carry the `sys_migration` ledger + fresh-datastore attestation (#4243) the
+  served assembly always had.
diff --git a/packages/cli/src/commands/serve.ts b/packages/cli/src/commands/serve.ts
index 337da3f518..5ecb472a7f 100644
--- a/packages/cli/src/commands/serve.ts
+++ b/packages/cli/src/commands/serve.ts
@@ -1494,7 +1494,10 @@ export default class Serve extends Command {
       // infrastructure every served kernel needs: the `sys_migration`
       // data-migration flag ledger + fresh-datastore attestation (#4243 —
       // without it the engine's deployment gates read "no ledger" and fall
-      // back to the lax legacy posture), and the platform-default
+      // back to the lax legacy posture), the `sys_secret` cipher store
+      // (#4270 — the engine's secret-field write path and the datasource
+      // credential binder fail CLOSED without it; the crypto wiring below
+      // assumes the table exists), and the platform-default
       // translation bundles (Setup App + metadata-type configuration
       // forms). Without the latter, Setup nav labels and metadata-admin
       // form labels fall back to English literals even when
diff --git a/packages/platform-objects/src/plugin.test.ts b/packages/platform-objects/src/plugin.test.ts
index 892d94f025..0fb409f0da 100644
--- a/packages/platform-objects/src/plugin.test.ts
+++ b/packages/platform-objects/src/plugin.test.ts
@@ -2,7 +2,7 @@
 
 import { describe, it, expect } from 'vitest';
 import { PlatformObjectsPlugin } from './plugin.js';
-import { SysMigration } from './system/index.js';
+import { SysMigration, SysSecret } from './system/index.js';
 
 /**
  * Hand-rolled fake PluginContext (mirrors the service-plugin tests) — this
@@ -33,8 +33,8 @@ function makeCtx() {
   return ctx;
 }
 
-describe('PlatformObjectsPlugin: sys_migration ledger registration (#4243)', () => {
-  it('registers SysMigration through the manifest service', async () => {
+describe('PlatformObjectsPlugin: platform-infrastructure object registration (#4243, #4270)', () => {
+  it('registers SysMigration and SysSecret through the manifest service', async () => {
     const ctx = makeCtx();
     const manifests: any[] = [];
     ctx.registerService('manifest', { register: (m: any) => manifests.push(m) });
@@ -44,8 +44,27 @@ describe('PlatformObjectsPlugin: sys_migration ledger registration (#4243)', ()
     expect(manifests).toHaveLength(1);
     expect(manifests[0].id).toBe('com.objectstack.platform-objects');
     expect(manifests[0].scope).toBe('system');
-    expect(manifests[0].objects).toEqual([SysMigration]);
-    expect(manifests[0].objects[0].name).toBe('sys_migration');
+    expect(manifests[0].objects).toEqual([SysMigration, SysSecret]);
+    expect(manifests[0].objects.map((o: any) => o.name)).toEqual(['sys_migration', 'sys_secret']);
+  });
+
+  /**
+   * #4270 pin: the engine's secret-field write path is fail-CLOSED — with no
+   * `sys_secret` store it throws, and its error message tells the operator to
+   * "Ensure the platform-objects (sys_secret) are registered". This plugin is
+   * what makes that sentence true; dropping SysSecret from the registration
+   * would fail any kernel composed without service-settings (and even with
+   * it — settings no longer registers the object either).
+   */
+  it('sys_secret is registered HERE, so the engine fail-closed error names the right package', async () => {
+    const ctx = makeCtx();
+    const manifests: any[] = [];
+    ctx.registerService('manifest', { register: (m: any) => manifests.push(m) });
+
+    await new PlatformObjectsPlugin().init(ctx);
+
+    const names = manifests.flatMap((m: any) => m.objects.map((o: any) => o.name));
+    expect(names).toContain('sys_secret');
   });
 
   it('degrades silently without a manifest service (lean/i18n-only kernels)', async () => {
diff --git a/packages/platform-objects/src/plugin.ts b/packages/platform-objects/src/plugin.ts
index 822f90f719..698f04eccf 100644
--- a/packages/platform-objects/src/plugin.ts
+++ b/packages/platform-objects/src/plugin.ts
@@ -3,6 +3,7 @@
 import { SetupAppTranslations } from './apps/translations/index.js';
 import { MetadataFormsTranslations } from './metadata-translations/index.js';
 import { SysMigration } from './system/sys-migration.object.js';
+import { SysSecret } from './system/sys-secret.object.js';
 import { attestFreshDatastore } from './system/migration-flag.js';
 
 /**
@@ -18,6 +19,17 @@ import { attestFreshDatastore } from './system/migration-flag.js';
  *    file collection, the engine's strict value-shape gates (#3438/#4235)
  *    and the migration CLI all read the same ledger, and none of them
  *    should have to drag an unrelated service into the boot to find it.
+ *  - **`sys_secret`** — the environment's encrypted-secret store
+ *    (ADR-0066 D2/④). Same shape as the ledger, sharper stakes (#4270):
+ *    its producers span domains — the settings service's encrypted
+ *    specifiers, the engine's own `secret`-field encryption
+ *    (`encryptSecretFields`/`resolveSecret`, a generic write path any
+ *    business object with a `Field.secret()` hits), and the datasource
+ *    credential binder — and the engine is fail-CLOSED about it: writing
+ *    a secret field with no reachable `sys_secret` store throws. While
+ *    service-settings registered it, a kernel composed without settings
+ *    hard-failed every secret-field write with an error that blamed
+ *    platform-objects. Registered here, that error message is true.
  *  - **Fresh-datastore attestation** (#3438, ADR-0104 2026-07-30) —
  *    travels with the ledger registration: a store this boot created from
  *    empty is attested at `kernel:ready`, whichever services are composed.
@@ -31,8 +43,9 @@ import { attestFreshDatastore } from './system/migration-flag.js';
  *
  * Gracefully degrades on lean kernels: without a manifest service the
  * ledger is simply absent — every flag reader answers "not verified", the
- * safe posture — and without an i18n service the UI falls back to the
- * inline literals carried on each App / Dashboard / `*.form.ts` schema.
+ * safe posture — secret-field writes fail closed (throw, never cleartext),
+ * and without an i18n service the UI falls back to the inline literals
+ * carried on each App / Dashboard / `*.form.ts` schema.
  *
  * Structurally typed against `@objectstack/core`'s `Plugin` contract so
  * this package does not need to depend on the kernel at compile time.
@@ -58,11 +71,12 @@ export class PlatformObjectsPlugin {
         version: this.version,
         type: 'plugin',
         scope: 'system',
-        objects: [SysMigration],
+        objects: [SysMigration, SysSecret],
       });
     } catch {
       // No manifest service (lean / i18n-only kernels) — the ledger stays
-      // unregistered and every flag reader answers "not verified".
+      // unregistered (every flag reader answers "not verified") and the
+      // secret store stays unregistered (secret-field writes fail closed).
     }
   }
 
diff --git a/packages/platform-objects/src/system/sys-secret.object.ts b/packages/platform-objects/src/system/sys-secret.object.ts
index 27ff164e9b..9f61348f5d 100644
--- a/packages/platform-objects/src/system/sys-secret.object.ts
+++ b/packages/platform-objects/src/system/sys-secret.object.ts
@@ -25,7 +25,13 @@ import { ObjectSchema, Field } from '@objectstack/spec/data';
  *     reads of e.g. feature flags.
  *
  * managedBy: 'engine-owned' — never edited from a generic Object grid. All
- * writes flow through `SettingsService` and an `ICryptoProvider`.
+ * writes flow through an `ICryptoProvider` on one of three privileged
+ * producer paths (#4270): `SettingsService` (encrypted settings specifiers),
+ * the engine's own `secret`-field encryption (`encryptSecretFields` — the
+ * generic write path of any object carrying a `Field.secret()`), and the
+ * datasource credential binder. Because the producers span domains and the
+ * engine fails CLOSED without this store, it is registered by
+ * `PlatformObjectsPlugin` (platform infrastructure), not by service-settings.
  *
  * @namespace sys
  */
diff --git a/packages/services/service-settings/src/manifest.test.ts b/packages/services/service-settings/src/manifest.test.ts
new file mode 100644
index 0000000000..920a47ca0c
--- /dev/null
+++ b/packages/services/service-settings/src/manifest.test.ts
@@ -0,0 +1,23 @@
+// Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
+
+import { describe, it, expect } from 'vitest';
+import { settingsObjects } from './manifest.js';
+
+describe('service-settings manifest objects (#4270)', () => {
+  it('owns exactly the K/V store and its audit trail', () => {
+    expect(settingsObjects.map((o: any) => o.name)).toEqual(['sys_setting', 'sys_setting_audit']);
+  });
+
+  /**
+   * #4270 pin against double registration: `sys_secret` moved to
+   * `PlatformObjectsPlugin` (its producers span settings, the engine's
+   * `secret`-field encryption, and the datasource credential binder — and the
+   * engine fails closed without the store). #4236 flagged "same object
+   * registered twice" as behaviour to avoid; the move REPLACED this
+   * registration, it did not add a second one. Re-adding SysSecret here would
+   * recreate exactly that conflict.
+   */
+  it('does not (re-)register sys_secret — that is PlatformObjectsPlugin material', () => {
+    expect(settingsObjects.map((o: any) => o.name)).not.toContain('sys_secret');
+  });
+});
diff --git a/packages/services/service-settings/src/manifest.ts b/packages/services/service-settings/src/manifest.ts
index 81550ab6e0..49651ab89e 100644
--- a/packages/services/service-settings/src/manifest.ts
+++ b/packages/services/service-settings/src/manifest.ts
@@ -1,12 +1,21 @@
 // Copyright (c) 2025 ObjectStack. Licensed under the Apache-2.0 license.
 
-import { SysSetting, SysSecret, SysSettingAudit } from '@objectstack/platform-objects/system';
+import { SysSetting, SysSettingAudit } from '@objectstack/platform-objects/system';
 
 export const SETTINGS_PLUGIN_ID = 'com.objectstack.service.settings';
 export const SETTINGS_PLUGIN_VERSION = '0.1.0';
 
-/** Objects owned by service-settings. Currently just the K/V store. */
-export const settingsObjects: any[] = [SysSetting, SysSecret, SysSettingAudit];
+/**
+ * Objects owned by service-settings: the K/V store and its audit trail.
+ *
+ * `sys_secret` is deliberately NOT here (#4270): its producers span domains
+ * (encrypted settings here, the engine's `secret`-field encryption, the
+ * datasource credential binder) and the engine fails closed when the store
+ * is missing — so it is registered by `PlatformObjectsPlugin` as platform
+ * infrastructure, present with or without this service (cf. `sys_migration`,
+ * #4243). This service remains a producer/consumer via its secret store.
+ */
+export const settingsObjects: any[] = [SysSetting, SysSettingAudit];
 
 /** Manifest header shared by compile-time config and runtime registration. */
 export const settingsPluginManifestHeader = {
diff --git a/packages/verify/package.json b/packages/verify/package.json
index 5effc7e6b4..cb8a987774 100644
--- a/packages/verify/package.json
+++ b/packages/verify/package.json
@@ -22,6 +22,7 @@
   "dependencies": {
     "@objectstack/core": "workspace:*",
     "@objectstack/objectql": "workspace:*",
+    "@objectstack/platform-objects": "workspace:*",
     "@objectstack/plugin-auth": "workspace:*",
     "@objectstack/plugin-hono-server": "workspace:*",
     "@objectstack/plugin-security": "workspace:*",
diff --git a/packages/verify/src/harness.ts b/packages/verify/src/harness.ts
index 2d74dc1e19..1a615da6ab 100644
--- a/packages/verify/src/harness.ts
+++ b/packages/verify/src/harness.ts
@@ -29,6 +29,7 @@ import { SecurityPlugin } from '@objectstack/plugin-security';
 import { SharingServicePlugin } from '@objectstack/plugin-sharing';
 import { SettingsServicePlugin, LocalCryptoProvider } from '@objectstack/service-settings';
 import { AnalyticsServicePlugin } from '@objectstack/service-analytics';
+import { PlatformObjectsPlugin } from '@objectstack/platform-objects/plugin';
 
 /** A Hono app exposes `.request(path, init)` returning a standard `Response`. */
 interface InjectableApp {
@@ -172,6 +173,12 @@ export async function bootStack(
   // The app under test (objects, datasets, cubes, flows, seed data).
   await kernel.use(new AppPlugin(config));
 
+  // Platform infrastructure `os serve` auto-injects into every served kernel:
+  // the `sys_migration` flag ledger (#4243) and the `sys_secret` cipher store
+  // (#4270) — the latter is what the LocalCryptoProvider wiring below writes
+  // into, and the engine fails CLOSED on secret-field writes without it.
+  await kernel.use(new PlatformObjectsPlugin());
+
   // Service plugins `objectstack dev` auto-loads for an app of this shape.
   await kernel.use(new SettingsServicePlugin());
   await kernel.use(opts.analytics ?? new AnalyticsServicePlugin());
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index e9cbf52dc4..7aa6816e3c 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -2352,6 +2352,9 @@ importers:
       '@objectstack/objectql':
         specifier: workspace:*
         version: link:../objectql
+      '@objectstack/platform-objects':
+        specifier: workspace:*
+        version: link:../platform-objects
       '@objectstack/plugin-auth':
         specifier: workspace:*
         version: link:../plugins/plugin-auth