diff --git a/packages/shared/src/index.ts b/packages/shared/src/index.ts
index 55b6bed..85fbe3f 100644
--- a/packages/shared/src/index.ts
+++ b/packages/shared/src/index.ts
@@ -2,3 +2,4 @@ export * from './types.js';
 export * from './moderator.js';
 export * from './protocol.js';
 export * from './geometric.js';
+export * from './phantoms.js';
diff --git a/packages/shared/src/phantoms.ts b/packages/shared/src/phantoms.ts
new file mode 100644
index 0000000..26c1bc0
--- /dev/null
+++ b/packages/shared/src/phantoms.ts
@@ -0,0 +1,41 @@
+import { FILES, isSquare, type Color, type Piece, type PieceType, type Square } from './types.js';
+
+const BACK_RANK: PieceType[] = ['r', 'n', 'b', 'q', 'k', 'b', 'n', 'r'];
+const COLORS: Color[] = ['w', 'b'];
+const TYPES: PieceType[] = ['p', 'n', 'b', 'r', 'q', 'k'];
+
+/**
+ * The standard starting position for one colour, as a square→piece map.
+ * Used to seed the phantom opponent-model layer with the opponent's army.
+ */
+export function opponentStartPosition(opponentColor: Color): Partial<Record<Square, Piece>> {
+  const backRank = opponentColor === 'w' ? '1' : '8';
+  const pawnRank = opponentColor === 'w' ? '2' : '7';
+  const out: Partial<Record<Square, Piece>> = {};
+  FILES.forEach((file, i) => {
+    out[`${file}${backRank}` as Square] = { color: opponentColor, type: BACK_RANK[i]! };
+    out[`${file}${pawnRank}` as Square] = { color: opponentColor, type: 'p' };
+  });
+  return out;
+}
+
+/**
+ * Parse a persisted phantom map (from localStorage). Tolerant: returns {} on
+ * any structural failure and silently drops individual invalid entries.
+ */
+export function deserializePhantoms(raw: string | null): Partial<Record<Square, Piece>> {
+  if (!raw) return {};
+  let parsed: unknown;
+  try { parsed = JSON.parse(raw); } catch { return {}; }
+  if (typeof parsed !== 'object' || parsed === null || Array.isArray(parsed)) return {};
+  const out: Partial<Record<Square, Piece>> = {};
+  for (const [k, v] of Object.entries(parsed as Record<string, unknown>)) {
+    if (!isSquare(k)) continue;
+    if (typeof v !== 'object' || v === null) continue;
+    const { color, type } = v as { color?: unknown; type?: unknown };
+    if (!COLORS.includes(color as Color)) continue;
+    if (!TYPES.includes(type as PieceType)) continue;
+    out[k] = { color: color as Color, type: type as PieceType };
+  }
+  return out;
+}
diff --git a/packages/shared/test/phantoms.test.ts b/packages/shared/test/phantoms.test.ts
new file mode 100644
index 0000000..881706c
--- /dev/null
+++ b/packages/shared/test/phantoms.test.ts
@@ -0,0 +1,38 @@
+import { describe, it, expect } from 'vitest';
+import { opponentStartPosition, deserializePhantoms } from '../src/phantoms.js';
+
+describe('opponentStartPosition', () => {
+  it('seeds 16 black pieces on ranks 7-8', () => {
+    const p = opponentStartPosition('b');
+    expect(Object.keys(p).length).toBe(16);
+    expect(p.e8).toEqual({ color: 'b', type: 'k' });
+    expect(p.d8).toEqual({ color: 'b', type: 'q' });
+    expect(p.a8).toEqual({ color: 'b', type: 'r' });
+    expect(p.h7).toEqual({ color: 'b', type: 'p' });
+  });
+
+  it('seeds 16 white pieces on ranks 1-2', () => {
+    const p = opponentStartPosition('w');
+    expect(Object.keys(p).length).toBe(16);
+    expect(p.e1).toEqual({ color: 'w', type: 'k' });
+    expect(p.a2).toEqual({ color: 'w', type: 'p' });
+  });
+});
+
+describe('deserializePhantoms', () => {
+  it('returns {} for null or invalid JSON', () => {
+    expect(deserializePhantoms(null)).toEqual({});
+    expect(deserializePhantoms('not json')).toEqual({});
+    expect(deserializePhantoms('[]')).toEqual({});
+  });
+
+  it('keeps valid entries and drops invalid ones', () => {
+    const raw = JSON.stringify({
+      e5: { color: 'b', type: 'n' },
+      zz: { color: 'b', type: 'p' },  // invalid square
+      a1: { color: 'x', type: 'p' },  // invalid colour
+      b2: { color: 'b', type: 'z' },  // invalid type
+    });
+    expect(deserializePhantoms(raw)).toEqual({ e5: { color: 'b', type: 'n' } });
+  });
+});