diff --git a/src/lib/stores/game.svelte.ts b/src/lib/stores/game.svelte.ts
new file mode 100644
index 0000000..f9d6f7c
--- /dev/null
+++ b/src/lib/stores/game.svelte.ts
@@ -0,0 +1,164 @@
+import { DuplicateGame } from '../../engine/game';
+import { legalSyncedMoves, selectionHighlight, type SelectionHighlight } from '../../engine/legality';
+import { ghosts } from '../../engine/ghosts';
+import { evaluateStatus } from '../../engine/endgame';
+import { serialize, deserialize } from '../../engine/notation';
+import { PLAYER_BOARDS } from '../../engine/boards';
+import type {
+  BoardId, Player, Square, SyncMove, HistoryEntry, GhostMarker, GameStatus,
+} from '../../engine/types';
+
+/** A plain, reactivity-friendly snapshot of everything the UI renders. */
+export interface GameView {
+  /** Piece-placement FEN field per board. */
+  fen: Record<BoardId, string>;
+  currentPlayer: Player;
+  ply: number;
+  ghosts: GhostMarker[];
+  status: GameStatus;
+  history: HistoryEntry[];
+}
+
+function buildView(game: DuplicateGame): GameView {
+  return {
+    fen: {
+      NW: game.boards.NW.fen(),
+      NE: game.boards.NE.fen(),
+      SW: game.boards.SW.fen(),
+      SE: game.boards.SE.fen(),
+    },
+    currentPlayer: game.currentPlayer,
+    ply: game.ply,
+    ghosts: ghosts(game),
+    status: evaluateStatus(game),
+    history: [...game.history],
+  };
+}
+
+class GameStore {
+  /** The authoritative live game — deliberately NOT a $state proxy. */
+  #game = new DuplicateGame();
+
+  /** Snapshot the UI renders. While scrubbing it reflects a past ply. */
+  view = $state<GameView>(buildView(this.#game));
+  /** The grabbed square, or null. */
+  selected = $state<Square | null>(null);
+  /** Triple-highlight for the grabbed piece, or null. */
+  highlight = $state<SelectionHighlight | null>(null);
+  /** A pawn move awaiting a promotion choice, or null. */
+  pendingPromotion = $state<{ from: Square; to: Square } | null>(null);
+  /** Ply currently being viewed; null means the live position. */
+  scrubPly = $state<number | null>(null);
+
+  get isScrubbing(): boolean {
+    return this.scrubPly !== null;
+  }
+
+  /** Which boards belong to the player to move (for the turn glow). */
+  get activeBoards(): [BoardId, BoardId] {
+    return PLAYER_BOARDS[this.#game.currentPlayer];
+  }
+
+  /** Grab a piece: must be the current player's turn and a live (non-scrub) view. */
+  select(square: Square): void {
+    if (this.isScrubbing) return;
+    if (this.selected === square) { this.clearSelection(); return; }
+    this.selected = square;
+    this.highlight = selectionHighlight(this.#game, square);
+  }
+
+  clearSelection(): void {
+    this.selected = null;
+    this.highlight = null;
+  }
+
+  /** Attempt to play the grabbed piece to `to`. Opens the promotion dialog if needed. */
+  commitTo(to: Square): void {
+    const from = this.selected;
+    if (from === null || this.highlight === null) return;
+    if (!this.highlight.playable.includes(to)) return; // not a synchronized-legal square
+
+    const moves = legalSyncedMoves(this.#game).filter((m) => m.from === from && m.to === to);
+    if (moves.length === 0) return;
+    if (moves.some((m) => m.promotion)) {
+      this.pendingPromotion = { from, to };
+      return;
+    }
+    this.#apply(moves[0]);
+  }
+
+  /** Finish a promotion started by commitTo. */
+  choosePromotion(piece: SyncMove['promotion']): void {
+    if (this.pendingPromotion === null) return;
+    this.#apply({ ...this.pendingPromotion, promotion: piece });
+    this.pendingPromotion = null;
+  }
+
+  cancelPromotion(): void {
+    this.pendingPromotion = null;
+  }
+
+  #apply(move: SyncMove): void {
+    this.#game.applyMove(move);
+    this.clearSelection();
+    this.scrubPly = null;
+    this.view = buildView(this.#game);
+  }
+
+  undo(): void {
+    this.#game.undo();
+    this.clearSelection();
+    this.scrubPly = null;
+    this.view = buildView(this.#game);
+  }
+
+  newGame(): void {
+    this.#game = new DuplicateGame();
+    this.clearSelection();
+    this.pendingPromotion = null;
+    this.scrubPly = null;
+    this.view = buildView(this.#game);
+  }
+
+  /** Scrub the move history; null returns to the live position. */
+  scrubTo(ply: number | null): void {
+    this.clearSelection();
+    if (ply === null || ply >= this.#game.ply) {
+      this.scrubPly = null;
+      this.view = buildView(this.#game);
+      return;
+    }
+    this.scrubPly = ply;
+    this.view = buildView(new DuplicateGame(this.#game.history.slice(0, ply)));
+  }
+
+  /** Manually declare a draw (provisional: insufficient material is not auto-detected). */
+  declareDraw(): void {
+    this.view = {
+      ...this.view,
+      status: { state: 'draw', reason: 'manual', checks: [],
+        result: { N: 'draw', S: 'draw', E: 'draw', W: 'draw' } },
+    };
+  }
+
+  save(): void {
+    const blob = new Blob([serialize(this.#game.history)], { type: 'application/json' });
+    const url = URL.createObjectURL(blob);
+    const a = document.createElement('a');
+    a.href = url;
+    a.download = `duplicate-chess-${Date.now()}.json`;
+    a.click();
+    URL.revokeObjectURL(url);
+  }
+
+  async load(file: File): Promise<void> {
+    const history = deserialize(await file.text());
+    this.#game = new DuplicateGame(history);
+    this.clearSelection();
+    this.pendingPromotion = null;
+    this.scrubPly = null;
+    this.view = buildView(this.#game);
+  }
+}
+
+export const gameStore = new GameStore();