diff --git a/package.json b/package.json
index ee9bb861..29c50f93 100644
--- a/package.json
+++ b/package.json
@@ -52,6 +52,7 @@
     "@hookform/resolvers": "^5.4.0",
     "@mui/icons-material": "^9.2.0",
     "@mui/material": "^9.2.0",
+    "@tanstack/query-broadcast-client-experimental": "^5.101.2",
     "@tanstack/react-query": "^5.101.2",
     "@tanstack/react-query-devtools": "^5.101.2",
     "@testing-library/dom": "^10.4.1",
diff --git a/src/lib/query/query-client.ts b/src/lib/query/query-client.ts
index 1223fc2b..bef74cd1 100644
--- a/src/lib/query/query-client.ts
+++ b/src/lib/query/query-client.ts
@@ -1,5 +1,6 @@
 import { MutationCache, QueryCache, QueryClient } from "@tanstack/react-query";
 import { toast } from "react-toastify";
+import { setupQueryBroadcast } from "@/lib/query/setup-query-broadcast";
 
 export const queryClient = new QueryClient({
   queryCache: new QueryCache({
@@ -30,4 +31,6 @@ export const queryClient = new QueryClient({
   },
 });
 
+setupQueryBroadcast(queryClient);
+
 export default queryClient;
diff --git a/src/lib/query/setup-query-broadcast.ts b/src/lib/query/setup-query-broadcast.ts
new file mode 100644
index 00000000..3b5f24d6
--- /dev/null
+++ b/src/lib/query/setup-query-broadcast.ts
@@ -0,0 +1,19 @@
+import { broadcastQueryClient } from "@tanstack/query-broadcast-client-experimental";
+import type { QueryClient } from "@tanstack/react-query";
+
+/** Same-origin channel name for cross-tab QueryClient sync. */
+export const QUERY_BROADCAST_CHANNEL = "seahaven-admin-query";
+
+/**
+ * Syncs QueryClient cache (invalidations + data) across browser tabs via BroadcastChannel.
+ * No-op outside the browser and during Vitest runs.
+ */
+export function setupQueryBroadcast(queryClient: QueryClient): void {
+  if (typeof window === "undefined") return;
+  if (import.meta.env.MODE === "test") return;
+
+  broadcastQueryClient({
+    queryClient,
+    broadcastChannel: QUERY_BROADCAST_CHANNEL,
+  });
+}
diff --git a/src/test/lib/query/setup-query-broadcast.test.ts b/src/test/lib/query/setup-query-broadcast.test.ts
new file mode 100644
index 00000000..bc203093
--- /dev/null
+++ b/src/test/lib/query/setup-query-broadcast.test.ts
@@ -0,0 +1,47 @@
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+import { QueryClient } from "@tanstack/react-query";
+
+const broadcastQueryClient = vi.fn();
+
+vi.mock("@tanstack/query-broadcast-client-experimental", () => ({
+  broadcastQueryClient: (...args: unknown[]) => broadcastQueryClient(...args),
+}));
+
+describe("setupQueryBroadcast", () => {
+  beforeEach(() => {
+    broadcastQueryClient.mockClear();
+    vi.unstubAllEnvs();
+  });
+
+  afterEach(() => {
+    vi.unstubAllEnvs();
+  });
+
+  it("does not call broadcastQueryClient when MODE is test", async () => {
+    vi.stubEnv("MODE", "test");
+    const { setupQueryBroadcast } = await import("@/lib/query/setup-query-broadcast");
+    const client = new QueryClient();
+
+    setupQueryBroadcast(client);
+
+    expect(broadcastQueryClient).not.toHaveBeenCalled();
+  });
+
+  it("calls broadcastQueryClient with the app channel when MODE is not test", async () => {
+    vi.stubEnv("MODE", "development");
+    vi.resetModules();
+
+    const { setupQueryBroadcast, QUERY_BROADCAST_CHANNEL } =
+      await import("@/lib/query/setup-query-broadcast");
+    const client = new QueryClient();
+
+    setupQueryBroadcast(client);
+
+    expect(broadcastQueryClient).toHaveBeenCalledTimes(1);
+    expect(broadcastQueryClient).toHaveBeenCalledWith({
+      queryClient: client,
+      broadcastChannel: QUERY_BROADCAST_CHANNEL,
+    });
+    expect(QUERY_BROADCAST_CHANNEL).toBe("seahaven-admin-query");
+  });
+});