diff --git a/docs/api-reference.md b/docs/api-reference.md
index ff55c46..b03ed0f 100644
--- a/docs/api-reference.md
+++ b/docs/api-reference.md
@@ -544,6 +544,41 @@
 
 购买使用 `clientRequestId` 作为幂等边界；金币不足时返回统一错误格式，`error.code` 为 `VALIDATION_ERROR`。
 
+#### POST /inventory/items/use
+
+认证：JWT
+
+请求：
+
+```json
+{
+  "itemId": "hint_feather",
+  "clientRequestId": "request-uuid",
+  "questionId": "question-uuid"
+}
+```
+
+`itemId` 取值：`heart_supply`, `double_xp_potion`, `hint_feather`, `streak_shield`。使用 `hint_feather` 时必须传 `questionId`。
+
+响应：
+
+```json
+{
+  "success": true,
+  "data": {
+    "itemId": "hint_feather",
+    "quantityRemaining": 2,
+    "effect": {
+      "type": "hint",
+      "excludedOptions": ["错误选项 A"]
+    }
+  },
+  "error": null
+}
+```
+
+效果说明：`heart_supply` 恢复当前用户爱心到上限；`double_xp_potion` 返回 15 分钟有效期 `activeUntil`；`hint_feather` 返回可排除选项；`streak_shield` 返回 `streakProtectedUntil`。`clientRequestId` 用于道具消耗幂等。
+
 #### GET /subscription
 
 认证：JWT
diff --git a/docs/gamification-server-plan.md b/docs/gamification-server-plan.md
index 89aa186..9698361 100644
--- a/docs/gamification-server-plan.md
+++ b/docs/gamification-server-plan.md
@@ -76,7 +76,7 @@
 | G3-2 | 实现宝箱奖励服务 | [x] | 支持基础概率、10 连对概率提升、高奖励次数耗尽后的概率降级 |
 | G3-3 | 实现道具库存服务 | [x] | 支持连胜护盾、双倍 XP 药水、爱心补给、提示羽毛的获得和消耗 |
 | G3-4 | 实现商店商品和购买接口 | [x] | 商品价格符合设计：提示羽毛 80、爱心补给 150、双倍 XP 250、连胜护盾 400、装扮 800-3000 |
-| G3-5 | 实现道具使用接口 | [ ] | 爱心补给恢复满心，双倍 XP 药水 15 分钟生效，提示羽毛返回可排除选项，连胜护盾可保护断签 |
+| G3-5 | 实现道具使用接口 | [x] | 爱心补给恢复满心，双倍 XP 药水 15 分钟生效，提示羽毛返回可排除选项，连胜护盾可保护断签 |
 | G3-6 | 更新 bootstrap/shop DTO | [ ] | 客户端能拿到金币、库存、可购买商品、广告商品、订阅权益 |
 | G3-7 | 添加金币/商店测试 | [ ] | 覆盖余额不足、重复购买、使用道具、药水时效、库存扣减和流水记录 |
 
@@ -84,6 +84,7 @@
 验证记录（2026-05-13）：G3-2 已通过 `./node_modules/.bin/tsc --noEmit`、`./node_modules/.bin/eslint .` 和 `git diff --check`；定向运行 `./node_modules/.bin/vitest run src/__tests__/services/gamification/chest-service.test.ts src/__tests__/services/gamification/coin-service.test.ts src/__tests__/services/gamification-rules.test.ts` 仍在启动阶段被同一个 `@rolldown/binding-darwin-x64` 原生 binding 未签名问题阻塞。
 验证记录（2026-05-13）：G3-3 已通过 `./node_modules/.bin/tsc --noEmit`、`./node_modules/.bin/eslint .` 和 `git diff --check`；定向运行 `./node_modules/.bin/vitest run src/__tests__/services/gamification/inventory-service.test.ts` 仍在启动阶段被同一个 `@rolldown/binding-darwin-x64` 原生 binding 未签名问题阻塞。
 验证记录（2026-05-13）：G3-4 已通过 `./node_modules/.bin/tsc --noEmit`、`./node_modules/.bin/eslint .` 和 `git diff --check`；定向运行 `./node_modules/.bin/vitest run src/__tests__/services/shop/shop-service.test.ts src/__tests__/services/gamification/coin-service.test.ts src/__tests__/services/gamification/inventory-service.test.ts` 仍在启动阶段被同一个 `@rolldown/binding-darwin-x64` 原生 binding 未签名问题阻塞。
+验证记录（2026-05-13）：G3-5 已通过 `./node_modules/.bin/tsc --noEmit`、`./node_modules/.bin/eslint .` 和 `git diff --check`；定向运行 `./node_modules/.bin/vitest run src/__tests__/services/gamification/item-use-service.test.ts src/__tests__/services/gamification/inventory-service.test.ts` 仍在启动阶段被同一个 `@rolldown/binding-darwin-x64` 原生 binding 未签名问题阻塞。
 
 ## Phase G4：广告恢复与订阅权益对齐
 
diff --git a/src/__tests__/services/gamification/item-use-service.test.ts b/src/__tests__/services/gamification/item-use-service.test.ts
new file mode 100644
index 0000000..f4946a5
--- /dev/null
+++ b/src/__tests__/services/gamification/item-use-service.test.ts
@@ -0,0 +1,178 @@
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+import { db } from '../../../db/client.js';
+import { useInventoryItem } from '../../../services/gamification/item-use-service.js';
+
+function selectRows(rows: unknown[]) {
+  return {
+    from: vi.fn().mockReturnValue({
+      where: vi.fn().mockReturnValue({
+        limit: vi.fn().mockResolvedValue(rows),
+      }),
+    }),
+  };
+}
+
+function mockSelectQueue(queue: unknown[][]) {
+  let index = 0;
+  vi.mocked(db.select).mockImplementation((() => {
+    const rows = index < queue.length ? queue[index]! : [];
+    index += 1;
+    return selectRows(rows);
+  }) as never);
+}
+
+function mockInsert(valuesSpy: ReturnType<typeof vi.fn>) {
+  return { values: valuesSpy.mockResolvedValue(undefined) } as never;
+}
+
+function mockUpdate(setSpy: ReturnType<typeof vi.fn>) {
+  return { set: setSpy.mockReturnValue({ where: vi.fn().mockResolvedValue(undefined) }) } as never;
+}
+
+describe('item-use-service', () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+  });
+
+  it('uses heart supply to restore hearts to the user max', async () => {
+    const insertValues = vi.fn();
+    const updateSet = vi.fn();
+    mockSelectQueue([
+      [], // consume: no existing use transaction
+      [{ itemId: 'heart_supply', quantity: 2, activeUntil: null, metadata: null }],
+      [{ id: 'inventory-heart' }],
+      [{ tier: 'free' }],
+    ]);
+    vi.mocked(db.insert).mockReturnValue(mockInsert(insertValues));
+    vi.mocked(db.update).mockReturnValue(mockUpdate(updateSet));
+
+    const result = await useInventoryItem({
+      userId: 'user-1',
+      itemId: 'heart_supply',
+      clientRequestId: 'use-heart-1',
+    });
+
+    expect(result).toEqual({
+      itemId: 'heart_supply',
+      quantityRemaining: 1,
+      effect: {
+        type: 'restore_hearts',
+        hearts: 5,
+      },
+    });
+    expect(updateSet).toHaveBeenCalledWith(expect.objectContaining({
+      heartsRemaining: 5,
+      heartsLastRestore: expect.any(Object),
+    }));
+    expect(insertValues).toHaveBeenCalledWith(expect.objectContaining({
+      itemId: 'heart_supply',
+      direction: 'consume',
+      quantityDelta: -1,
+      idempotencyKey: 'use-item:use-heart-1:heart_supply',
+    }));
+  });
+
+  it('uses double XP potion and marks the effect active for 15 minutes', async () => {
+    const insertValues = vi.fn();
+    const updateSet = vi.fn();
+    mockSelectQueue([
+      [],
+      [{ itemId: 'double_xp_potion', quantity: 1, activeUntil: null, metadata: null }],
+      [{ id: 'inventory-xp' }],
+    ]);
+    vi.mocked(db.insert).mockReturnValue(mockInsert(insertValues));
+    vi.mocked(db.update).mockReturnValue(mockUpdate(updateSet));
+
+    const result = await useInventoryItem({
+      userId: 'user-1',
+      itemId: 'double_xp_potion',
+      clientRequestId: 'use-xp-1',
+    });
+
+    expect(result.itemId).toBe('double_xp_potion');
+    expect(result.quantityRemaining).toBe(0);
+    expect(result.effect.type).toBe('double_xp');
+    expect(result.effect.activeUntil).toEqual(expect.any(String));
+    expect(updateSet).toHaveBeenCalledWith(expect.objectContaining({
+      activeUntil: expect.any(Date),
+      metadata: {
+        activeEffect: 'double_xp',
+        multiplier: 2,
+      },
+    }));
+  });
+
+  it('uses hint feather and returns one distractor to exclude', async () => {
+    const insertValues = vi.fn();
+    const updateSet = vi.fn();
+    mockSelectQueue([
+      [{ distractors: ['错误项 A', '错误项 B'] }],
+      [],
+      [{ itemId: 'hint_feather', quantity: 3, activeUntil: null, metadata: null }],
+      [{ id: 'inventory-hint' }],
+    ]);
+    vi.mocked(db.insert).mockReturnValue(mockInsert(insertValues));
+    vi.mocked(db.update).mockReturnValue(mockUpdate(updateSet));
+
+    const result = await useInventoryItem({
+      userId: 'user-1',
+      itemId: 'hint_feather',
+      clientRequestId: 'use-hint-1',
+      questionId: 'question-1',
+    });
+
+    expect(result).toEqual({
+      itemId: 'hint_feather',
+      quantityRemaining: 2,
+      effect: {
+        type: 'hint',
+        excludedOptions: ['错误项 A'],
+      },
+    });
+    expect(insertValues).toHaveBeenCalledWith(expect.objectContaining({
+      itemId: 'hint_feather',
+      direction: 'consume',
+      snapshot: expect.objectContaining({
+        questionId: 'question-1',
+        excludedOptions: ['错误项 A'],
+      }),
+    }));
+  });
+
+  it('requires questionId when using hint feather', async () => {
+    await expect(
+      useInventoryItem({
+        userId: 'user-1',
+        itemId: 'hint_feather',
+        clientRequestId: 'use-hint-1',
+      }),
+    ).rejects.toThrow('questionId is required');
+  });
+
+  it('uses streak shield and protects the current streak', async () => {
+    const insertValues = vi.fn();
+    const updateSet = vi.fn();
+    mockSelectQueue([
+      [],
+      [{ itemId: 'streak_shield', quantity: 1, activeUntil: null, metadata: null }],
+      [{ id: 'inventory-shield' }],
+      [{ streakDays: 7 }],
+    ]);
+    vi.mocked(db.insert).mockReturnValue(mockInsert(insertValues));
+    vi.mocked(db.update).mockReturnValue(mockUpdate(updateSet));
+
+    const result = await useInventoryItem({
+      userId: 'user-1',
+      itemId: 'streak_shield',
+      clientRequestId: 'use-shield-1',
+    });
+
+    expect(result.itemId).toBe('streak_shield');
+    expect(result.quantityRemaining).toBe(0);
+    expect(result.effect.type).toBe('streak_protection');
+    expect(result.effect.streakProtectedUntil).toEqual(expect.any(String));
+    expect(updateSet).toHaveBeenCalledWith(expect.objectContaining({
+      streakProtectedUntil: expect.any(Date),
+    }));
+  });
+});
diff --git a/src/routes/app-api.ts b/src/routes/app-api.ts
index 8476208..5840ad8 100644
--- a/src/routes/app-api.ts
+++ b/src/routes/app-api.ts
@@ -14,6 +14,7 @@ import { restoreHearts } from '../services/progress/progress-service.js';
 import { getClientLeaderboard, getClientLeaderboardMe } from '../services/learning/leaderboard-api-service.js';
 import { getShopCatalog, purchaseShopProduct } from '../services/shop/shop-service.js';
 import { getClientSubscription, verifyClientSubscription } from '../services/subscription/subscription-api-service.js';
+import { useInventoryItem } from '../services/gamification/item-use-service.js';
 
 const rewardSourceSchema = z.object({
   source: z.enum(['ad', 'check_in', 'subscription', 'admin_grant', 'debug']),
@@ -51,6 +52,12 @@ const shopPurchaseSchema = z.object({
   clientRequestId: z.string().min(1).max(80),
 });
 
+const useItemSchema = z.object({
+  itemId: z.enum(['streak_shield', 'double_xp_potion', 'heart_supply', 'hint_feather']),
+  clientRequestId: z.string().min(1).max(80),
+  questionId: z.string().min(1).optional(),
+});
+
 function getUserId(request: { user: unknown }): string {
   return (request.user as { userId: string }).userId;
 }
@@ -168,6 +175,18 @@ export async function appApiRoutes(app: FastifyInstance): Promise<void> {
     return { success: true, data, error: null };
   });
 
+  app.post('/inventory/items/use', async (request) => {
+    const parsed = useItemSchema.safeParse(request.body);
+    if (!parsed.success) return validationError(parsed.error.issues[0]?.message);
+    const data = await useInventoryItem({
+      userId: getUserId(request),
+      itemId: parsed.data.itemId,
+      clientRequestId: parsed.data.clientRequestId,
+      questionId: parsed.data.questionId,
+    });
+    return { success: true, data, error: null };
+  });
+
   app.get('/subscription', async (request) => {
     const data = await getClientSubscription(getUserId(request));
     return { success: true, data, error: null };
diff --git a/src/services/gamification/item-use-service.ts b/src/services/gamification/item-use-service.ts
new file mode 100644
index 0000000..93b7bec
--- /dev/null
+++ b/src/services/gamification/item-use-service.ts
@@ -0,0 +1,191 @@
+import { db } from '../../db/client.js';
+import { questions, userInventoryItems, users } from '../../db/schema.js';
+import { and, eq, sql } from 'drizzle-orm';
+import { NotFoundError, ValidationError } from '../../utils/errors.js';
+import { freezeStreak } from '../progress/streak-service.js';
+import { HEART_RULES, ITEM_RULES, type InventoryItemId } from './rules.js';
+import { consumeInventoryItem } from './inventory-service.js';
+import type { UseInventoryItemResultDto, UsableInventoryItemId } from '../../types/app-api.js';
+
+export interface UseInventoryItemInput {
+  userId: string;
+  itemId: UsableInventoryItemId;
+  clientRequestId: string;
+  questionId?: string;
+}
+
+export async function useInventoryItem(input: UseInventoryItemInput): Promise<UseInventoryItemResultDto> {
+  switch (input.itemId) {
+    case ITEM_RULES.heartSupply.id:
+      return useHeartSupply(input);
+    case ITEM_RULES.doubleXpPotion.id:
+      return useDoubleXpPotion(input);
+    case ITEM_RULES.hintFeather.id:
+      return useHintFeather(input);
+    case ITEM_RULES.streakShield.id:
+      return useStreakShield(input);
+  }
+}
+
+async function useHeartSupply(input: UseInventoryItemInput): Promise<UseInventoryItemResultDto> {
+  const consumption = await consumeUseItem(input);
+  const maxHearts = await getMaxHearts(input.userId);
+  await db
+    .update(users)
+    .set({
+      heartsRemaining: maxHearts,
+      heartsLastRestore: sql`NOW()`,
+    })
+    .where(eq(users.id, input.userId));
+
+  return {
+    itemId: input.itemId,
+    quantityRemaining: consumption.item.quantity,
+    effect: {
+      type: 'restore_hearts',
+      hearts: maxHearts,
+    },
+  };
+}
+
+async function useDoubleXpPotion(input: UseInventoryItemInput): Promise<UseInventoryItemResultDto> {
+  const consumption = await consumeUseItem(input);
+  if (!consumption.applied) {
+    return {
+      itemId: input.itemId,
+      quantityRemaining: consumption.item.quantity,
+      effect: {
+        type: 'double_xp',
+        activeUntil: consumption.item.activeUntil?.toISOString() ?? null,
+      },
+    };
+  }
+
+  const activeUntil = new Date(Date.now() + ITEM_RULES.doubleXpPotion.durationMs);
+  await db
+    .update(userInventoryItems)
+    .set({
+      activeUntil,
+      metadata: {
+        activeEffect: 'double_xp',
+        multiplier: ITEM_RULES.doubleXpPotion.multiplier,
+      },
+    })
+    .where(and(
+      eq(userInventoryItems.userId, input.userId),
+      eq(userInventoryItems.itemId, input.itemId),
+    ));
+
+  return {
+    itemId: input.itemId,
+    quantityRemaining: consumption.item.quantity,
+    effect: {
+      type: 'double_xp',
+      activeUntil: activeUntil.toISOString(),
+    },
+  };
+}
+
+async function useHintFeather(input: UseInventoryItemInput): Promise<UseInventoryItemResultDto> {
+  if (!input.questionId) {
+    throw new ValidationError('questionId is required for hint feather');
+  }
+
+  const excludedOptions = await getHintExcludedOptions(input.questionId);
+  const consumption = await consumeUseItem(input, {
+    questionId: input.questionId,
+    excludedOptions,
+  });
+
+  return {
+    itemId: input.itemId,
+    quantityRemaining: consumption.item.quantity,
+    effect: {
+      type: 'hint',
+      excludedOptions,
+    },
+  };
+}
+
+async function useStreakShield(input: UseInventoryItemInput): Promise<UseInventoryItemResultDto> {
+  const consumption = await consumeUseItem(input);
+  if (!consumption.applied) {
+    const protectedUntil = await getStreakProtectedUntil(input.userId);
+    return {
+      itemId: input.itemId,
+      quantityRemaining: consumption.item.quantity,
+      effect: {
+        type: 'streak_protection',
+        streakProtectedUntil: protectedUntil,
+      },
+    };
+  }
+
+  const protectedUntil = new Date();
+  protectedUntil.setUTCDate(protectedUntil.getUTCDate() + ITEM_RULES.streakShield.protectDays);
+  await db
+    .update(users)
+    .set({ streakProtectedUntil: protectedUntil })
+    .where(eq(users.id, input.userId));
+  await freezeStreak(input.userId);
+
+  return {
+    itemId: input.itemId,
+    quantityRemaining: consumption.item.quantity,
+    effect: {
+      type: 'streak_protection',
+      streakProtectedUntil: protectedUntil.toISOString(),
+    },
+  };
+}
+
+async function getStreakProtectedUntil(userId: string): Promise<string | null> {
+  const [user] = await db
+    .select({ streakProtectedUntil: users.streakProtectedUntil })
+    .from(users)
+    .where(eq(users.id, userId))
+    .limit(1);
+
+  const value = user?.streakProtectedUntil ?? null;
+  if (!value) return null;
+  return value instanceof Date ? value.toISOString() : new Date(value).toISOString();
+}
+
+async function consumeUseItem(input: UseInventoryItemInput, snapshot: Record<string, unknown> = {}) {
+  return consumeInventoryItem({
+    userId: input.userId,
+    itemId: input.itemId as InventoryItemId,
+    quantity: 1,
+    sourceType: 'system_adjust',
+    sourceId: `use-item:${input.clientRequestId}`,
+    idempotencyKey: `use-item:${input.clientRequestId}:${input.itemId}`,
+    snapshot,
+  });
+}
+
+async function getMaxHearts(userId: string): Promise<number> {
+  const [user] = await db
+    .select({ tier: users.tier })
+    .from(users)
+    .where(eq(users.id, userId))
+    .limit(1);
+
+  return user?.tier === 'pro' || user?.tier === 'proplus'
+    ? HEART_RULES.subscribedMax
+    : HEART_RULES.freeMax;
+}
+
+async function getHintExcludedOptions(questionId: string): Promise<readonly string[]> {
+  const [question] = await db
+    .select({ distractors: questions.distractors })
+    .from(questions)
+    .where(eq(questions.id, questionId))
+    .limit(1);
+
+  if (!question) throw new NotFoundError('Question');
+  const distractors = Array.isArray(question.distractors)
+    ? question.distractors.filter((item): item is string => typeof item === 'string')
+    : [];
+
+  return distractors.slice(0, ITEM_RULES.hintFeather.excludedDistractors);
+}
diff --git a/src/types/app-api.ts b/src/types/app-api.ts
index 781f0f0..93e9746 100644
--- a/src/types/app-api.ts
+++ b/src/types/app-api.ts
@@ -108,6 +108,20 @@ export interface ShopPurchaseResultDto {
   }>;
 }
 
+export type UsableInventoryItemId = 'streak_shield' | 'double_xp_potion' | 'heart_supply' | 'hint_feather';
+
+export interface UseInventoryItemResultDto {
+  itemId: UsableInventoryItemId;
+  quantityRemaining: number;
+  effect: {
+    type: 'restore_hearts' | 'double_xp' | 'hint' | 'streak_protection';
+    activeUntil?: string | null;
+    hearts?: number;
+    excludedOptions?: readonly string[];
+    streakProtectedUntil?: string | null;
+  };
+}
+
 export interface BootstrapDto {
   user: UserBriefDto;
   progress: ProgressSummaryDto;