diff --git a/web/docs/codebase-summary.md b/web/docs/codebase-summary.md
index 945854f..ef60dd0 100644
--- a/web/docs/codebase-summary.md
+++ b/web/docs/codebase-summary.md
@@ -23,6 +23,7 @@
 |------|---------|
 | `src/lib/game-logic.js` | Stateless utilities: generateGrid (constraint-aware picker — exact 5 per row & per col, ascending-sorted columns, soft "no 3 consecutive filled cols per row" via rejection sampling), saveGrid, loadGrid, saveCrossedState, loadCrossedState, isRowComplete, getWaitingNumber. |
 | `src/lib/master-store.svelte.js` | Shared reactive `{called, remaining}` $state for the master deck, persisted to `loto_master`. Exports `masterState`, `loadMaster`, `saveMaster`, `startNewGame`, `drawNext`, `resetMaster`. Hydrated once via `+layout.svelte`'s `onMount` so player-side reads see the full history regardless of mount order. Replaced the single-slot `call-bus` to fix history-loss bugs (regen, reload, multi-tab) — see `plans/reports/code-reviewer-260430-2024-both-mode-consistency.md`. |
+| `src/lib/tab-lock.svelte.js` | Single-tab guard via `BroadcastChannel`. New tab broadcasts `claim`; old tab sets `tabLock.frozen = true`. `+layout.svelte` mounts it on boot and renders a fullscreen overlay banner ("Loto đã mở ở tab khác. Tap để chuyển về tab này.") when frozen — tap calls `reclaimTab` which broadcasts back, freezing the other tab in turn. No-op in browsers without `BroadcastChannel` (legacy iOS Safari ≤15.4). Prevents double auto-call intervals, double localStorage writers, and overlapping audio across tabs. |
 | `src/lib/player-auto-cross.js` | Pure `applyMasterCalls({grid, crossed, called, lastHandledIndex, manualUnticks, mode})`. Cursor-by-index dedup (vs the retired `at`-timestamp model), so any caller can pass `lastHandledIndex: 0` to replay master's full history (used by player regen + "Xoá đánh dấu" in both mode). Manual unticks suppress re-cross on replay. |
 | `src/lib/vietnamese-number.js` | `numberToVietnamese(n)` — pure utility mapping 0..90 to spoken Vietnamese, with tonal exceptions (15 → "mười lăm", 21 → "hai mươi mốt", 25 → "hai mươi lăm"). Out-of-range falls back to `String(n)`. |
 | `src/lib/voice.js` | Bundled-MP3 playback. Exports `playNumber(n)`, `playWaiting(n)` (sequences cho + N), `playBingo()`, `cancelPlayback()`. Lazy `<audio>` cache, cancel-then-play, token-based cancel ensures stale promises can't resume after a new event. Reads active voice from `settings.voice`; URLs go through `import { base } from "$app/paths"` for basePath safety. |
diff --git a/web/src/lib/tab-lock.svelte.js b/web/src/lib/tab-lock.svelte.js
new file mode 100644
index 0000000..548eaf1
--- /dev/null
+++ b/web/src/lib/tab-lock.svelte.js
@@ -0,0 +1,58 @@
+/**
+ * Single-tab guard for the Lô tô app. When the user opens the app in a
+ * second tab, the second tab broadcasts a claim and the first tab
+ * freezes itself until the user explicitly takes it back. Prevents
+ * double auto-call intervals, double localStorage writers, and
+ * overlapping audio across tabs of the same origin.
+ *
+ * No-op in browsers without `BroadcastChannel` (legacy iOS Safari).
+ *
+ * @module lib/tab-lock
+ */
+
+const CHANNEL = "loto_tab_lock";
+
+const TAB_ID =
+  typeof crypto !== "undefined" && typeof crypto.randomUUID === "function"
+    ? crypto.randomUUID()
+    : `tab-${Math.random().toString(36).slice(2)}-${Date.now()}`;
+
+export const tabLock = $state({
+  /** True when another tab has taken over and this tab should pause. */
+  frozen: false,
+});
+
+/** @type {BroadcastChannel | null} */
+let bc = null;
+
+/**
+ * Start listening for tab-claim messages and immediately announce that
+ * THIS tab is now active. Returns a cleanup that closes the channel.
+ * Calling more than once is a no-op so layout HMR doesn't double-bind.
+ */
+export function startTabLock() {
+  if (typeof BroadcastChannel === "undefined") return () => {};
+  if (bc) return () => {};
+  bc = new BroadcastChannel(CHANNEL);
+  bc.onmessage = (e) => {
+    if (e.data?.type === "claim" && e.data.id !== TAB_ID) {
+      tabLock.frozen = true;
+    }
+  };
+  bc.postMessage({ type: "claim", id: TAB_ID });
+  return () => {
+    if (bc) {
+      bc.close();
+      bc = null;
+    }
+  };
+}
+
+/**
+ * Re-claim the lock from the frozen banner. Unfreezes this tab and
+ * broadcasts a new claim — any other live tab freezes itself in turn.
+ */
+export function reclaimTab() {
+  tabLock.frozen = false;
+  if (bc) bc.postMessage({ type: "claim", id: TAB_ID });
+}
diff --git a/web/src/lib/tab-lock.test.js b/web/src/lib/tab-lock.test.js
new file mode 100644
index 0000000..646b240
--- /dev/null
+++ b/web/src/lib/tab-lock.test.js
@@ -0,0 +1,87 @@
+// @vitest-environment happy-dom
+
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+
+/** Minimal stub matching the BroadcastChannel surface we use. */
+class FakeBC {
+  /** @type {Map<string, FakeBC[]>} */
+  static channels = new Map();
+  /** @param {string} name */
+  constructor(name) {
+    this.name = name;
+    this.onmessage = null;
+    this.closed = false;
+    const peers = FakeBC.channels.get(name) ?? [];
+    peers.push(this);
+    FakeBC.channels.set(name, peers);
+  }
+  /** @param {any} data */
+  postMessage(data) {
+    if (this.closed) return;
+    const peers = FakeBC.channels.get(this.name) ?? [];
+    for (const p of peers) {
+      if (p === this || p.closed || !p.onmessage) continue;
+      p.onmessage({ data });
+    }
+  }
+  close() {
+    this.closed = true;
+    const peers = FakeBC.channels.get(this.name) ?? [];
+    FakeBC.channels.set(
+      this.name,
+      peers.filter((p) => p !== this),
+    );
+  }
+}
+
+beforeEach(() => {
+  FakeBC.channels.clear();
+  vi.stubGlobal("BroadcastChannel", /** @type {any} */ (FakeBC));
+  vi.resetModules();
+});
+
+afterEach(() => {
+  vi.unstubAllGlobals();
+});
+
+describe("tab-lock", () => {
+  it("startTabLock returns no-op cleanup when BroadcastChannel is missing", async () => {
+    vi.stubGlobal("BroadcastChannel", undefined);
+    const mod = await import("./tab-lock.svelte.js?nobc");
+    const cleanup = mod.startTabLock();
+    expect(typeof cleanup).toBe("function");
+    expect(mod.tabLock.frozen).toBe(false);
+    cleanup();
+  });
+
+  it("second tab's claim freezes the first tab", async () => {
+    const tabA = await import("./tab-lock.svelte.js?a");
+    const tabB = await import("./tab-lock.svelte.js?b");
+    tabA.startTabLock();
+    expect(tabA.tabLock.frozen).toBe(false);
+    tabB.startTabLock();
+    expect(tabA.tabLock.frozen).toBe(true);
+    expect(tabB.tabLock.frozen).toBe(false);
+  });
+
+  it("reclaimTab unfreezes self and freezes the other", async () => {
+    const tabA = await import("./tab-lock.svelte.js?ra-a");
+    const tabB = await import("./tab-lock.svelte.js?ra-b");
+    tabA.startTabLock();
+    tabB.startTabLock();
+    expect(tabA.tabLock.frozen).toBe(true);
+    tabA.reclaimTab();
+    expect(tabA.tabLock.frozen).toBe(false);
+    expect(tabB.tabLock.frozen).toBe(true);
+  });
+
+  it("startTabLock is idempotent within a single module", async () => {
+    const tab = await import("./tab-lock.svelte.js?idem");
+    const c1 = tab.startTabLock();
+    const c2 = tab.startTabLock();
+    expect(typeof c1).toBe("function");
+    expect(typeof c2).toBe("function");
+    c1();
+    c2();
+  });
+});
diff --git a/web/src/routes/+layout.svelte b/web/src/routes/+layout.svelte
index ea4f208..e244f97 100644
--- a/web/src/routes/+layout.svelte
+++ b/web/src/routes/+layout.svelte
@@ -3,16 +3,42 @@
   import "../app.css";
   import { loadMaster } from "$lib/master-store.svelte.js";
   import { loadSettings } from "$lib/settings-store.svelte.js";
+  import {
+    reclaimTab,
+    startTabLock,
+    tabLock,
+  } from "$lib/tab-lock.svelte.js";
 
   let { children } = $props();
 
   // Hydrate global stores once so children mount with consistent state.
   // Master state must hydrate before PlayerBoard reads `masterState.called`
   // length — see the mode-mount race fix in the consistency refactor.
+  // The tab-lock cleanup is returned so HMR / route changes close the
+  // BroadcastChannel cleanly.
   onMount(() => {
     loadSettings();
     loadMaster();
+    return startTabLock();
   });
 </script>
 
-{@render children()}
+{#if tabLock.frozen}
+  <button
+    type="button"
+    onclick={reclaimTab}
+    class="fixed inset-0 z-50 flex flex-col items-center justify-center
+           bg-slate-900/95 text-white text-center px-6
+           cursor-pointer focus:outline-none"
+    aria-label="Loto đã mở ở tab khác. Nhấn để chuyển về tab này."
+  >
+    <p class="text-2xl sm:text-3xl font-bold mb-3">
+      🏮 Loto đã mở ở tab khác
+    </p>
+    <p class="text-base text-slate-300 max-w-sm">
+      Tap để chuyển về tab này. Tab kia sẽ tự dừng.
+    </p>
+  </button>
+{:else}
+  {@render children()}
+{/if}