diff --git a/web/src/services/dashboardService.test.ts b/web/src/services/dashboardService.test.ts
new file mode 100644
index 00000000..f2bfc39b
--- /dev/null
+++ b/web/src/services/dashboardService.test.ts
@@ -0,0 +1,48 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import { describe, expect, it, vi } from 'vitest';
+
+vi.mock('../config', () => ({
+  USE_MOCK: true,
+  API_BASE_URL: '/api',
+}));
+
+import { getDashboard } from './dashboardService';
+
+describe('dashboardService mock dashboard', () => {
+  it('returns defensive copies for overview stats and cluster throughput', async () => {
+    const dashboard = await getDashboard();
+    const originalTotalClusters = dashboard.stats.totalClusters;
+    const originalClusterName = dashboard.clusters[0].name;
+    const originalThroughput = dashboard.clusters[0].throughput[0];
+
+    dashboard.stats.totalClusters = 0;
+    dashboard.clusters[0].name = 'mutated-cluster';
+    dashboard.clusters[0].throughput[0] = 0;
+
+    const fresh = await getDashboard();
+
+    expect(fresh.stats.totalClusters).toBe(originalTotalClusters);
+    expect(fresh.clusters[0].name).toBe(originalClusterName);
+    expect(fresh.clusters[0].throughput[0]).toBe(originalThroughput);
+    expect(fresh.stats).not.toBe(dashboard.stats);
+    expect(fresh.clusters).not.toBe(dashboard.clusters);
+    expect(fresh.clusters[0]).not.toBe(dashboard.clusters[0]);
+    expect(fresh.clusters[0].throughput).not.toBe(dashboard.clusters[0].throughput);
+  });
+});
diff --git a/web/src/services/dashboardService.ts b/web/src/services/dashboardService.ts
index ad9ce343..0d396eff 100644
--- a/web/src/services/dashboardService.ts
+++ b/web/src/services/dashboardService.ts
@@ -3,11 +3,18 @@ import * as metricsApi from '../api/metrics';
 import { dashboardStats, clusterOverview } from '../mock/dashboard';
 import type { DashboardData } from '../api/metrics';
 
+function copyClusterOverview(cluster: DashboardData['clusters'][number]) {
+  return {
+    ...cluster,
+    throughput: [...cluster.throughput],
+  };
+}
+
 export async function getDashboard(): Promise<DashboardData> {
   if (USE_MOCK) {
     return {
-      stats: dashboardStats,
-      clusters: clusterOverview as unknown as DashboardData['clusters'],
+      stats: { ...dashboardStats },
+      clusters: (clusterOverview as DashboardData['clusters']).map(copyClusterOverview),
     };
   }
   return metricsApi.getDashboard();