diff --git a/web/docs/codebase-summary.md b/web/docs/codebase-summary.md
index 801d774..3a1ac49 100644
--- a/web/docs/codebase-summary.md
+++ b/web/docs/codebase-summary.md
@@ -22,6 +22,7 @@
 |------|---------|
 | `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/call-bus.svelte.js` | Pub/sub for master draws → player auto-tick. Reactive `bus.lastDrawn` slot (emits `{ num, at }`). Used in `mode: "both"` to auto-mark master-called numbers on player board. |
+| `src/lib/auto-tick.js` | Pure `processAutoTick({grid, crossed, lastDraw, lastHandledAt, mode})` extracted from PlayerBoard's bus-driven $effect. Owns the dedup-by-`at` invariant: `lastHandledAt` advances on every NEW timestamp (even no-op draws) so reactive re-runs from `crossed`/`grid` changes never re-fire a stale draw. |
 | `src/lib/vietnamese-number.js` | `numberToVietnamese(n)` — pure utility mapping 0..90 to spoken Vietnamese, with tonal exceptions (15 → "mười lăm", 21 → "hai mươi mốt", 25 → "hai mươi lăm"). Out-of-range falls back to `String(n)`. |
 | `src/lib/voice.js` | Bundled-MP3 playback. Exports `playNumber(n)`, `playWaiting(n)` (sequences cho + N), `playBingo()`, `cancelPlayback()`. Lazy `<audio>` cache, cancel-then-play, token-based cancel ensures stale promises can't resume after a new event. Reads active voice from `settings.voice`; URLs go through `import { base } from "$app/paths"` for basePath safety. |
 | `src/lib/audio-manifest.js` | Re-exports `static/audio/manifest.json` as `VOICES` (array) + `VOICE_IDS` (Set) + `DEFAULT_VOICE`. Manifest is generated by `scripts/generate-audio.py`. |
@@ -38,6 +39,7 @@
 | `src/lib/game-logic.test.js` | 27 unit tests: generateGrid shape (9×9, 5 per row/col, no duplicates), column ranges & ascending sort, no-3-consecutive soft constraint, row completion, waiting number detection, persistence (saveGrid/loadGrid/saveCrossedState/loadCrossedState with validators). |
 | `src/lib/settings-store.test.js` | 31 unit tests: defaults (incl. voice keys), loadSettings (restore 8 keys, apply CSS vars, toggle dark class, handle empty/corrupt), saveSettings, resetSettings, theme toggle (auto → OS pref detection), master mode, auto-call + speed, color validation, voice round-trip + invalid-id fallback. |
 | `src/lib/vietnamese-number.test.js` | 40 unit tests: ones (0–9), teens (10–19 incl. mười lăm), 20–90 incl. mốt and lăm exceptions, out-of-range fall-through. |
+| `src/lib/auto-tick.test.js` | 8 unit tests for `processAutoTick`: NEW draw flips cell, dedup on same `at`, re-cross after manual untick, mode=master/player ignored (timestamp still advances), off-board number no-op, null lastDraw, null grid + empty crossed. |
 
 ### Configuration & PWA
 | File | Purpose |
diff --git a/web/src/lib/PlayerBoard.svelte b/web/src/lib/PlayerBoard.svelte
index 6d69d9e..677320f 100644
--- a/web/src/lib/PlayerBoard.svelte
+++ b/web/src/lib/PlayerBoard.svelte
@@ -1,7 +1,7 @@
 <script>
+  import { processAutoTick } from "$lib/auto-tick.js";
   import { bus, resetBus } from "$lib/call-bus.svelte.js";
   import {
-    findUncrossedCell,
     generateGrid,
     getWaitingNumber,
     isRowComplete,
@@ -149,25 +149,19 @@
     return () => window.removeEventListener("keydown", onKey);
   });
 
-  // Auto-tick on master draw (only in "both" mode). The effect tracks
-  // `bus.lastDrawn` reactively, but `grid`/`crossed` are read inside a
-  // peek — we only want to fire when a NEW draw arrives, not when the
-  // user manually unticks, clears, or regenerates. Comparing
-  // `drawn.at` against the last-handled timestamp blocks re-marks.
+  // Auto-tick on master draw (only in "both" mode). Reads `bus.lastDrawn`
+  // reactively; the dedup-by-`at` invariant lives in `processAutoTick` —
+  // see `auto-tick.test.js` for the full case matrix.
   $effect(() => {
-    const drawn = bus.lastDrawn;
-    if (!drawn) return;
-    if (drawn.at === lastHandledDrawAt) return;
-    lastHandledDrawAt = drawn.at;
-    if (settings.mode !== "both") return;
-    if (!grid || crossed.length === 0) return;
-    const target = findUncrossedCell(grid, crossed, drawn.num);
-    if (!target) return;
-    crossed = crossed.map((row, ri) =>
-      ri === target.row
-        ? row.map((v, ci) => (ci === target.col ? true : v))
-        : row,
-    );
+    const result = processAutoTick({
+      grid,
+      crossed,
+      lastDraw: bus.lastDrawn,
+      lastHandledAt: lastHandledDrawAt,
+      mode: settings.mode,
+    });
+    lastHandledDrawAt = result.lastHandledAt;
+    if (result.changed) crossed = result.crossed;
   });
 
   function handleGenerate() {
diff --git a/web/src/lib/auto-tick.js b/web/src/lib/auto-tick.js
new file mode 100644
index 0000000..8c1bdaa
--- /dev/null
+++ b/web/src/lib/auto-tick.js
@@ -0,0 +1,52 @@
+/**
+ * Pure helper for the master→player auto-tick path. Extracted from
+ * PlayerBoard.svelte so the dedup-by-`at` invariant is unit-testable
+ * without mounting the component.
+ * @module lib/auto-tick
+ */
+
+import { findUncrossedCell } from "$lib/game-logic.js";
+
+/**
+ * Decide what `crossed` should become given a new bus draw.
+ *
+ * Always advance `lastHandledAt` to the draw's timestamp on a NEW draw,
+ * even when no cell ends up flipped (mode mismatch, number off-board,
+ * already crossed). This blocks reactive re-runs caused by `crossed` /
+ * `grid` changes (manual untick, clear, regen) from re-firing the same
+ * draw — only a fresh `at` should ever advance state.
+ *
+ * @param {object} args
+ * @param {number[][] | null} args.grid
+ * @param {boolean[][]} args.crossed
+ * @param {{ num: number, at: number } | null} args.lastDraw
+ * @param {number} args.lastHandledAt
+ * @param {"player" | "master" | "both"} args.mode
+ * @returns {{ crossed: boolean[][], lastHandledAt: number, changed: boolean }}
+ */
+export function processAutoTick({
+  grid,
+  crossed,
+  lastDraw,
+  lastHandledAt,
+  mode,
+}) {
+  if (!lastDraw) return { crossed, lastHandledAt, changed: false };
+  if (lastDraw.at === lastHandledAt) {
+    return { crossed, lastHandledAt, changed: false };
+  }
+  // From here on, the draw is consumed: lastHandledAt advances.
+  const advanced = lastDraw.at;
+  if (mode !== "both") return { crossed, lastHandledAt: advanced, changed: false };
+  if (!grid || crossed.length === 0) {
+    return { crossed, lastHandledAt: advanced, changed: false };
+  }
+  const target = findUncrossedCell(grid, crossed, lastDraw.num);
+  if (!target) return { crossed, lastHandledAt: advanced, changed: false };
+  const updated = crossed.map((row, ri) =>
+    ri === target.row
+      ? row.map((v, ci) => (ci === target.col ? true : v))
+      : row,
+  );
+  return { crossed: updated, lastHandledAt: advanced, changed: true };
+}
diff --git a/web/src/lib/auto-tick.test.js b/web/src/lib/auto-tick.test.js
new file mode 100644
index 0000000..db48382
--- /dev/null
+++ b/web/src/lib/auto-tick.test.js
@@ -0,0 +1,150 @@
+// @vitest-environment happy-dom
+import { describe, expect, it } from "vitest";
+import { processAutoTick } from "./auto-tick.js";
+
+/**
+ * Build a minimal grid where row 0 col 2 = 42 and row 2 col 5 = 17.
+ * Other cells are 0 (empty).
+ */
+function makeGrid() {
+  const grid = Array.from({ length: 9 }, () => new Array(9).fill(0));
+  grid[0][2] = 42;
+  grid[2][5] = 17;
+  return grid;
+}
+
+function makeCrossed() {
+  return Array.from({ length: 9 }, () => new Array(9).fill(false));
+}
+
+describe("processAutoTick", () => {
+  it("crosses the cell on a NEW draw when mode=both", () => {
+    const grid = makeGrid();
+    const crossed = makeCrossed();
+    const result = processAutoTick({
+      grid,
+      crossed,
+      lastDraw: { num: 42, at: 1000 },
+      lastHandledAt: 0,
+      mode: "both",
+    });
+    expect(result.changed).toBe(true);
+    expect(result.lastHandledAt).toBe(1000);
+    expect(result.crossed[0][2]).toBe(true);
+    // Original input not mutated (immutable update).
+    expect(crossed[0][2]).toBe(false);
+  });
+
+  it("ignores a re-fire with the same `at` (dedup guard)", () => {
+    const crossed = makeCrossed();
+    const result = processAutoTick({
+      grid: makeGrid(),
+      crossed,
+      lastDraw: { num: 42, at: 5000 },
+      lastHandledAt: 5000,
+      mode: "both",
+    });
+    expect(result.changed).toBe(false);
+    expect(result.lastHandledAt).toBe(5000);
+    expect(result.crossed).toBe(crossed);
+  });
+
+  it("re-crosses on a NEW `at` after a manual untick", () => {
+    // Step 1: auto-tick fires.
+    const grid = makeGrid();
+    const first = processAutoTick({
+      grid,
+      crossed: makeCrossed(),
+      lastDraw: { num: 42, at: 1000 },
+      lastHandledAt: 0,
+      mode: "both",
+    });
+    expect(first.crossed[0][2]).toBe(true);
+
+    // Step 2: user manually unticks (simulated by editing).
+    const manualUntick = first.crossed.map((row) => row.slice());
+    manualUntick[0][2] = false;
+
+    // Step 3: same number arrives with NEW timestamp → re-crosses.
+    const second = processAutoTick({
+      grid,
+      crossed: manualUntick,
+      lastDraw: { num: 42, at: 2000 },
+      lastHandledAt: first.lastHandledAt,
+      mode: "both",
+    });
+    expect(second.changed).toBe(true);
+    expect(second.lastHandledAt).toBe(2000);
+    expect(second.crossed[0][2]).toBe(true);
+  });
+
+  it("ignores draws when mode=master but still consumes the timestamp", () => {
+    const crossed = makeCrossed();
+    const result = processAutoTick({
+      grid: makeGrid(),
+      crossed,
+      lastDraw: { num: 42, at: 4000 },
+      lastHandledAt: 0,
+      mode: "master",
+    });
+    expect(result.changed).toBe(false);
+    expect(result.crossed).toBe(crossed);
+    expect(result.lastHandledAt).toBe(4000);
+  });
+
+  it("ignores draws when mode=player but still consumes the timestamp", () => {
+    const crossed = makeCrossed();
+    const result = processAutoTick({
+      grid: makeGrid(),
+      crossed,
+      lastDraw: { num: 42, at: 9000 },
+      lastHandledAt: 0,
+      mode: "player",
+    });
+    expect(result.changed).toBe(false);
+    expect(result.crossed).toBe(crossed);
+    // lastHandledAt still advances — solo player switching to "both"
+    // mid-game shouldn't replay a stale draw.
+    expect(result.lastHandledAt).toBe(9000);
+  });
+
+  it("no-ops when the number is not on the grid", () => {
+    const crossed = makeCrossed();
+    const result = processAutoTick({
+      grid: makeGrid(),
+      crossed,
+      lastDraw: { num: 88, at: 3000 },
+      lastHandledAt: 0,
+      mode: "both",
+    });
+    expect(result.changed).toBe(false);
+    expect(result.crossed).toBe(crossed);
+    expect(result.lastHandledAt).toBe(3000);
+  });
+
+  it("returns unchanged state when lastDraw is null", () => {
+    const crossed = makeCrossed();
+    const result = processAutoTick({
+      grid: makeGrid(),
+      crossed,
+      lastDraw: null,
+      lastHandledAt: 1234,
+      mode: "both",
+    });
+    expect(result.changed).toBe(false);
+    expect(result.crossed).toBe(crossed);
+    expect(result.lastHandledAt).toBe(1234);
+  });
+
+  it("no-ops when grid is null or crossed is empty", () => {
+    const result = processAutoTick({
+      grid: null,
+      crossed: [],
+      lastDraw: { num: 42, at: 7000 },
+      lastHandledAt: 0,
+      mode: "both",
+    });
+    expect(result.changed).toBe(false);
+    expect(result.lastHandledAt).toBe(7000);
+  });
+});