diff --git a/projects/kit/README.md b/projects/kit/README.md
index c6ea1a8..9f356c6 100644
--- a/projects/kit/README.md
+++ b/projects/kit/README.md
@@ -398,7 +398,7 @@ declare dummy adapters.
 Mutations may still call `OfflineSyncService.enqueue` explicitly. A product that must keep ordinary HTTP services
 offline-unaware can instead register `mutationPolicies`; a matched `POST` / `PUT` / `PATCH` / `DELETE` is prepared
 locally before transport and returns an optimistic response while the Outbox owns remote replay.
-Web storage uses Ionic Storage; iOS and Android use encrypted `@capacitor-community/sqlite`. Importing either the
+Web storage uses Ionic Storage and supports `readCacheOnly` only. `synchronized` fails fast on Web because the current repository has no cross-tab lock; iOS and Android use encrypted `@capacitor-community/sqlite`. Importing either the
 primary entry point or `/offline` does not pull the optional native SQLite plugin into web-only applications.
 
 For cold-start offline route access, `OfflineCoordinatorService.activateOfflineSession()` restores only a manifest
diff --git a/projects/kit/offline/src/lib/offline-contract.spec.ts b/projects/kit/offline/src/lib/offline-contract.spec.ts
index 20cff07..80fde12 100644
--- a/projects/kit/offline/src/lib/offline-contract.spec.ts
+++ b/projects/kit/offline/src/lib/offline-contract.spec.ts
@@ -1,6 +1,6 @@
 import { describe, expect, it } from 'vitest';
 import { requirePositiveOfflineInteger } from './offline-identity';
-import { provideOffline } from './offline-provider';
+import { assertSupportedOfflineMode, provideOffline } from './offline-provider';
 import { normalizeOfflineReplicaPullPage } from './offline-replica-puller';
 import { defineOfflineReplicaSchema } from './offline-replica-schema';
 import { offlineSessionManifestAllows } from './offline-session.service';
@@ -75,6 +75,18 @@ describe('shared offline boundary contracts', () => {
     void compileOnly;
   });
 
+  it('fails closed before enabling synchronized Outbox writes on Web', () => {
+    expect(() => assertSupportedOfflineMode('web', 'synchronized')).toThrow(
+      'Offline synchronized mode is supported only by native repositories.',
+    );
+    expect(() => assertSupportedOfflineMode('web', 'readCacheOnly')).not.toThrow();
+    expect(() => assertSupportedOfflineMode('electron', 'synchronized')).toThrow(
+      'Offline synchronized mode is supported only by native repositories.',
+    );
+    expect(() => assertSupportedOfflineMode('ios', 'synchronized')).not.toThrow();
+    expect(() => assertSupportedOfflineMode('android', 'synchronized')).not.toThrow();
+  });
+
   it('requires an encryption key factory for synchronized providers at compile time', () => {
     type Options = import('./offline-provider').ProvideSynchronizedOfflineOptions;
     type IsRequired = Record<string, never> extends Pick<Options, 'createEncryptionKey'> ? false : true;
diff --git a/projects/kit/offline/src/lib/offline-provider.ts b/projects/kit/offline/src/lib/offline-provider.ts
index fb04112..c3c998c 100644
--- a/projects/kit/offline/src/lib/offline-provider.ts
+++ b/projects/kit/offline/src/lib/offline-provider.ts
@@ -8,7 +8,12 @@ import { OFFLINE_COMMAND_HOOKS } from './offline-command-hooks';
 import type { OfflineKitOptions } from './offline-kit-options';
 import { OFFLINE_KIT_OPTIONS } from './offline-kit-options';
 import { OfflineCoordinatorService } from './offline-coordinator.service';
-import { IonicOfflineRepository, OFFLINE_REPOSITORY, selectOfflineRepository } from './offline-repository';
+import {
+  IonicOfflineRepository,
+  OFFLINE_REPOSITORY,
+  selectOfflineRepository,
+  supportsSynchronizedOfflineRepository,
+} from './offline-repository';
 import type { OfflineMutationRequestPolicy, OfflineRequestPolicy } from './offline-request-policy';
 import { provideOfflineMutationRequestPolicy, provideOfflineRequestPolicy } from './offline-request-policy';
 import type { OfflineReplicaProjector, OfflineReplicaPuller } from './offline-replica-puller';
@@ -121,6 +126,21 @@ export function provideOffline(options: ProvideOfflineOptions): EnvironmentProvi
     ...options.requestPolicies.flatMap((policy) => provideOfflineRequestPolicy(policy)),
     ...(options.mutationPolicies ?? []).flatMap((policy) => provideOfflineMutationRequestPolicy(policy)),
     ...(options.providers ?? []),
-    provideAppInitializer(() => inject(OfflineCoordinatorService).initialize()),
+    provideAppInitializer(() => {
+      assertSupportedOfflineMode(Capacitor.getPlatform(), options.mode ?? 'synchronized');
+      return inject(OfflineCoordinatorService).initialize();
+    }),
   ]);
 }
+
+/** Prevents unsupported multi-tab Web writes from silently losing Outbox state. */
+export function assertSupportedOfflineMode(
+  platform: string,
+  mode: 'synchronized' | 'readCacheOnly',
+): void {
+  if (!supportsSynchronizedOfflineRepository(platform) && mode === 'synchronized') {
+    throw new Error(
+      'Offline synchronized mode is supported only by native repositories. Use readCacheOnly until the selected repository provides cross-context locking.',
+    );
+  }
+}
diff --git a/projects/kit/offline/src/lib/offline-repository.ts b/projects/kit/offline/src/lib/offline-repository.ts
index 8634059..71d4762 100644
--- a/projects/kit/offline/src/lib/offline-repository.ts
+++ b/projects/kit/offline/src/lib/offline-repository.ts
@@ -202,13 +202,18 @@ export interface OfflineRepository {
 /** DI token for the selected platform repository. */
 export const OFFLINE_REPOSITORY = new InjectionToken<OfflineRepository>('OFFLINE_REPOSITORY');
 
-/** Selects encrypted SQLite on native platforms and Ionic Storage on web. */
+/** Returns whether the platform uses the cross-process-safe native repository. */
+export function supportsSynchronizedOfflineRepository(platform: string): boolean {
+  return platform === 'ios' || platform === 'android';
+}
+
+/** Selects encrypted SQLite on native platforms and Ionic Storage elsewhere. */
 export function selectOfflineRepository(
   platform: string,
   webRepository: OfflineRepository,
   nativeRepository: OfflineRepository,
 ): OfflineRepository {
-  return platform === 'ios' || platform === 'android' ? nativeRepository : webRepository;
+  return supportsSynchronizedOfflineRepository(platform) ? nativeRepository : webRepository;
 }
 
 interface OfflineMetadata {