diff --git a/apps/api/src/isms/isms-document-control.service.spec.ts b/apps/api/src/isms/isms-document-control.service.spec.ts
index b20919204..7ffa17c4d 100644
--- a/apps/api/src/isms/isms-document-control.service.spec.ts
+++ b/apps/api/src/isms/isms-document-control.service.spec.ts
@@ -90,6 +90,20 @@ describe('IsmsDocumentControlService', () => {
         skipDuplicates: true,
       });
     });
+
+    it('takes the per-document lock BEFORE writing links (serializes vs approve)', async () => {
+      await service.addControls({
+        documentId: 'doc_1',
+        organizationId: 'org_1',
+        controlIds: ['ctl_1', 'ctl_2'],
+      });
+
+      const lockOrder = (mockDb.$executeRaw as jest.Mock).mock
+        .invocationCallOrder[0];
+      const writeOrder = (mockDb.ismsDocumentControlLink.createMany as jest.Mock)
+        .mock.invocationCallOrder[0];
+      expect(lockOrder).toBeLessThan(writeOrder);
+    });
   });
 
   describe('removeControl', () => {
diff --git a/apps/api/src/isms/isms-document-control.service.ts b/apps/api/src/isms/isms-document-control.service.ts
index c3454f5c9..1d5a715c0 100644
--- a/apps/api/src/isms/isms-document-control.service.ts
+++ b/apps/api/src/isms/isms-document-control.service.ts
@@ -5,6 +5,7 @@ import {
 } from '@nestjs/common';
 import { db } from '@db';
 import { invalidateApprovalIfNeeded } from './utils/approval';
+import { lockDocument } from './utils/document-lock';
 
 /**
  * Org-level mapping between an ISMS document and the organization's Controls
@@ -41,6 +42,11 @@ export class IsmsDocumentControlService {
     // register/narrative edits). Only a REAL change invalidates — an idempotent
     // re-link that inserts nothing must not downgrade an approved document.
     await db.$transaction(async (tx) => {
+      // Take the per-document lock BEFORE writing links so the mutation is fully
+      // serialized against approve() (which holds the same lock). invalidate is
+      // only called on a real change, and the lock is re-entrant, so re-taking it
+      // there is a no-op.
+      await lockDocument(tx, documentId);
       const { count } = await tx.ismsDocumentControlLink.createMany({
         data: uniqueControlIds.map((controlId) => ({
           ismsDocumentId: documentId,
@@ -68,6 +74,9 @@ export class IsmsDocumentControlService {
     // Only a real unlink (a row actually deleted) invalidates sign-off; removing
     // a control that wasn't linked must not downgrade an approved document.
     await db.$transaction(async (tx) => {
+      // Lock before the delete so the mutation serializes against approve()
+      // (same lock); invalidate only on a real unlink, re-taking the lock no-ops.
+      await lockDocument(tx, documentId);
       const { count } = await tx.ismsDocumentControlLink.deleteMany({
         where: { ismsDocumentId: documentId, controlId },
       });