diff --git a/apps/admin/e2e/locale-en.spec.ts b/apps/admin/e2e/locale-en.spec.ts
new file mode 100644
index 0000000..c865713
--- /dev/null
+++ b/apps/admin/e2e/locale-en.spec.ts
@@ -0,0 +1,36 @@
+// T15（A12）运行时抽查（P4b Task 6）：admin 主旅程 en 版。
+// 模板 i18n zh/en 键集一致性 + 错误码映射枚举覆盖由 error.spec.ts 机器守护；
+// 此处验证真实 DOM：登录页顶栏语言菜单（用户级设置，AuthTopBar）切 English →
+// 登录表单/按钮英文文案 → 正确登录 → 相册管理列表页英文（菜单/新建按钮/
+// 空态）。zh 版主旅程由既有 login/albums 系列覆盖；viewer en 旅程由
+// viewer e2e 各 spec 覆盖；embed iframe lang 两语言见 viewer e2e/embed.spec.ts。
+import { expect, test } from '@playwright/test'
+import { adminCredentials } from './helpers'
+
+test('en locale: admin 主旅程（登录 → 相册管理）全英文渲染', async ({ page }) => {
+  const { account, password } = adminCredentials()
+  await page.goto('/#/auth/login')
+
+  // 登录页右上语言菜单 → English（ElDropdown teleport 到 body）。
+  await page.locator('.language-btn').first().click()
+  await page.locator('.el-dropdown-menu__item', { hasText: 'English' }).click()
+
+  // 登录表单文案与占位符切换为英文。
+  await expect(page.getByText('Welcome back')).toBeVisible()
+  await expect(page.getByPlaceholder('Enter account')).toBeVisible()
+  await expect(page.getByPlaceholder('Please enter your password')).toBeVisible()
+  await expect(page.getByRole('button', { name: 'Login' })).toBeVisible()
+
+  // 英文凭据登录 → 落相册管理列表页，页面级文案英文。
+  await page.getByPlaceholder('Enter account').fill(account)
+  await page.getByPlaceholder('Please enter your password').fill(password)
+  await page.getByRole('button', { name: 'Login' }).click()
+  await expect(page).toHaveURL(/\/albums\/manage/, { timeout: 20_000 })
+  await expect(page.getByText('Album Management').first()).toBeVisible()
+  await expect(page.getByRole('button', { name: 'New Album' })).toBeVisible()
+
+  // 语言偏好已持久（用户级设置，刷新后仍英文）。
+  await page.reload()
+  await expect(page.getByText('Album Management').first()).toBeVisible()
+  await expect(page.getByRole('button', { name: 'New Album' })).toBeVisible()
+})
diff --git a/apps/admin/package.json b/apps/admin/package.json
index 8646613..71939c6 100644
--- a/apps/admin/package.json
+++ b/apps/admin/package.json
@@ -40,6 +40,7 @@
   },
   "devDependencies": {
     "@eslint/js": "^9.9.1",
+    "@photo/contracts": "workspace:*",
     "@playwright/test": "^1.48.0",
     "@types/node": "^24.0.5",
     "@types/qrcode": "^1.5.6",
diff --git a/apps/admin/src/locales/langs/en.json b/apps/admin/src/locales/langs/en.json
index e62ade1..03b63c6 100644
--- a/apps/admin/src/locales/langs/en.json
+++ b/apps/admin/src/locales/langs/en.json
@@ -792,6 +792,10 @@
     "PHOTO_NOT_PUBLIC": "Photo state cannot be hidden (not published)",
     "UPLOAD_VERIFY_FAILED": "Upload verification failed, please retry",
     "UPLOAD_SESSION_CONFLICT": "Upload session conflict, please start over",
-    "ALBUM_UPLOAD_DISABLED": "Upload is disabled for this album"
+    "ALBUM_UPLOAD_DISABLED": "Upload is disabled for this album",
+    "RATE_LIMITED": "Too many requests, please try again later",
+    "ALBUM_NOT_PUBLIC": "Album not found or offline",
+    "DOWNLOAD_DISABLED": "Downloads are not enabled for this album",
+    "SYNC_AUTH_FAILED": "Sync service authentication failed; check service configuration"
   }
 }
diff --git a/apps/admin/src/locales/langs/zh.json b/apps/admin/src/locales/langs/zh.json
index 8a8dd3e..7d236ee 100644
--- a/apps/admin/src/locales/langs/zh.json
+++ b/apps/admin/src/locales/langs/zh.json
@@ -792,6 +792,10 @@
     "PHOTO_NOT_PUBLIC": "照片状态不可隐藏（非已发布）",
     "UPLOAD_VERIFY_FAILED": "上传核验失败，请重试",
     "UPLOAD_SESSION_CONFLICT": "上传会话状态冲突，请重新开始",
-    "ALBUM_UPLOAD_DISABLED": "该相册已关闭上传"
+    "ALBUM_UPLOAD_DISABLED": "该相册已关闭上传",
+    "RATE_LIMITED": "操作过于频繁，请稍后再试",
+    "ALBUM_NOT_PUBLIC": "相册不存在或已下线",
+    "DOWNLOAD_DISABLED": "该相册未开放下载",
+    "SYNC_AUTH_FAILED": "同步服务鉴权失败，请检查服务配置"
   }
 }
diff --git a/apps/admin/src/utils/http/error.spec.ts b/apps/admin/src/utils/http/error.spec.ts
new file mode 100644
index 0000000..3078d52
--- /dev/null
+++ b/apps/admin/src/utils/http/error.spec.ts
@@ -0,0 +1,76 @@
+// T15 全端语言验收（P4b Task 6）：admin 侧错误码→文案映射的机器守护。
+//   1. 契约 ErrorCodes 枚举全集在 API_CODE_MSG_KEYS 无缺漏（A12「主要提示
+//      无漏译」——服务端 message 仅英文诊断兜底，本表即管理端错误文案全集）；
+//   2. 表内每个文案键在 zh/en 两份 locale 文件都真实存在（点路径解析）；
+//   3. zh/en locale 键集合递归相等（模板级一致性，镜像 P3 mobile arb 完整性
+//      测试与 viewer 侧 error-coverage 守护）。
+import { describe, expect, it } from 'vitest'
+// 根入口导入（非 ./errors 子路径）：admin tsconfig 为 classic moduleResolution
+// （不走 package exports），根 main/types 字段可解析；仅测试环境（node）使用，
+// internal-sync 的 node:crypto 不入产物。production 浏览器代码如需运行时
+// 枚举请走 @photo/contracts/errors 子路径（viewer 先例）。
+import { ErrorCodes } from '@photo/contracts'
+import { API_CODE_MSG_KEYS } from './error'
+import enMessages from '@/locales/langs/en.json'
+import zhMessages from '@/locales/langs/zh.json'
+
+type Messages = Record<string, unknown>
+
+/** 递归收集 locale 文件的叶子键点路径集合（apiMsg.ALBUM_NOT_FOUND 等）。 */
+function leafPaths(messages: Messages, prefix = ''): Set<string> {
+  const paths = new Set<string>()
+  for (const [key, value] of Object.entries(messages)) {
+    const path = prefix === '' ? key : `${prefix}.${key}`
+    if (value !== null && typeof value === 'object') {
+      for (const child of leafPaths(value as Messages, path)) paths.add(child)
+    } else {
+      paths.add(path)
+    }
+  }
+  return paths
+}
+
+/** 按点路径取词条值（不存在返回 undefined）。 */
+function resolvePath(messages: Messages, path: string): unknown {
+  let current: unknown = messages
+  for (const segment of path.split('.')) {
+    if (current === null || typeof current !== 'object') return undefined
+    current = (current as Messages)[segment]
+  }
+  return current
+}
+
+describe('错误码文案覆盖（T15/A12）', () => {
+  const zhPaths = leafPaths(zhMessages as Messages)
+  const enPaths = leafPaths(enMessages as Messages)
+
+  it('契约 ErrorCodes 枚举全集在 API_CODE_MSG_KEYS 无缺漏', () => {
+    const codes = Object.values(ErrorCodes)
+    expect(codes.length).toBeGreaterThanOrEqual(18) // 契约现存 18 码，只增不减
+    for (const code of codes) {
+      expect(API_CODE_MSG_KEYS[code], `缺少错误码映射: ${code}`).toBeDefined()
+    }
+  })
+
+  it('映射表内每个文案键在 zh 与 en locale 均存在且非空', () => {
+    for (const [code, key] of Object.entries(API_CODE_MSG_KEYS)) {
+      for (const [locale, messages] of [
+        ['zh', zhMessages],
+        ['en', enMessages]
+      ] as const) {
+        const paths = locale === 'zh' ? zhPaths : enPaths
+        expect(paths.has(key), `${code} 的文案键 ${key} 未登记于 ${locale} locale`).toBe(true)
+        const value = resolvePath(messages as Messages, key)
+        expect(
+          typeof value === 'string' && value.length > 0,
+          `${key} 在 ${locale} 非非空字符串`
+        ).toBe(true)
+      }
+    }
+  })
+
+  it('zh 与 en locale 键集合递归相等（无单侧漏译）', () => {
+    expect([...zhPaths].filter((path) => !enPaths.has(path))).toEqual([])
+    expect([...enPaths].filter((path) => !zhPaths.has(path))).toEqual([])
+  })
+})
diff --git a/apps/admin/src/utils/http/error.ts b/apps/admin/src/utils/http/error.ts
index fb33790..6ad5f8a 100644
--- a/apps/admin/src/utils/http/error.ts
+++ b/apps/admin/src/utils/http/error.ts
@@ -113,12 +113,38 @@ const getErrorMessage = (status: number): string => {
   return $t(errorMap[status] || 'httpMsg.internalServerError')
 }
 
-/** 契约错误码 → 本地化文案键（服务端 message 是英文兜底，优先本地化） */
-const API_CODE_MSG_KEYS: Record<string, string> = {
+/**
+ * 契约错误码 → 本地化文案键（服务端 message 是英文兜底，优先本地化）。
+ *
+ * T15 全端语言验收（P4b Task 6）：按契约 ErrorCodes 枚举全集登记（机器守护
+ * 见同目录 error.spec.ts，遍历 @photo/contracts ErrorCodes 断言无缺漏）——
+ * 登录/会话/框架码复用 httpMsg 既有文案，业务码走 apiMsg 专属词条。
+ */
+export const API_CODE_MSG_KEYS: Record<string, string> = {
+  // 401/会话域（模板 httpMsg 既有文案）
   AUTH_INVALID_CREDENTIALS: 'apiMsg.AUTH_INVALID_CREDENTIALS',
   AUTH_TOKEN_EXPIRED: 'httpMsg.unauthorized',
   AUTH_RATE_LIMITED: 'apiMsg.AUTH_RATE_LIMITED',
+  // P2 公开互动匿名限速（管理端可达性低，防御性登记专属文案）
+  RATE_LIMITED: 'apiMsg.RATE_LIMITED',
   AUTH_FORBIDDEN: 'httpMsg.forbidden',
+  // 框架/路由 404
+  NOT_FOUND: 'httpMsg.notFound',
+  // 相册/照片域
+  ALBUM_NOT_FOUND: 'apiMsg.ALBUM_NOT_FOUND',
+  ALBUM_NOT_PUBLIC: 'apiMsg.ALBUM_NOT_PUBLIC',
+  ALBUM_UPLOAD_DISABLED: 'apiMsg.ALBUM_UPLOAD_DISABLED',
+  PHOTO_NOT_FOUND: 'apiMsg.PHOTO_NOT_FOUND',
+  PHOTO_NOT_PUBLIC: 'apiMsg.PHOTO_NOT_PUBLIC',
+  // P4a 下载两级开关（A18）
+  DOWNLOAD_DISABLED: 'apiMsg.DOWNLOAD_DISABLED',
+  // 成员/上传域
+  MEMBER_CONFLICT: 'apiMsg.MEMBER_CONFLICT',
+  UPLOAD_SESSION_CONFLICT: 'apiMsg.UPLOAD_SESSION_CONFLICT',
+  UPLOAD_VERIFY_FAILED: 'apiMsg.UPLOAD_VERIFY_FAILED',
+  // 服务间内部同步鉴权（/internal/**）：管理端不可达，防御性登记
+  SYNC_AUTH_FAILED: 'apiMsg.SYNC_AUTH_FAILED',
+  // 通用
   VALIDATION_FAILED: 'apiMsg.VALIDATION_FAILED',
   INTERNAL_ERROR: 'httpMsg.internalServerError'
 }
diff --git a/apps/mobile/lib/core/error_codes.dart b/apps/mobile/lib/core/error_codes.dart
index d921372..0760222 100644
--- a/apps/mobile/lib/core/error_codes.dart
+++ b/apps/mobile/lib/core/error_codes.dart
@@ -2,7 +2,8 @@
 ///
 /// 1:1 对照 `packages/contracts/src/errors.ts` 的 `ErrorCodes` 与
 /// `packages/contracts/openapi.yaml` `components.schemas.Error` 的 enum
-/// （17 项）。契约约定：客户端按 code 映射多语言文案（l10n 解析留在 UI
+/// （18 项；P4a 增补 DOWNLOAD_DISABLED，P4b Task 6 语言验收时补齐镜像）。
+/// 契约约定：客户端按 code 映射多语言文案（l10n 解析留在 UI
 /// 层），`message` 仅供诊断。
 ///
 /// 文件末尾的 [networkError] / [networkTimeout] / [cancelled] /
@@ -51,6 +52,10 @@ abstract final class ErrorCodes {
   /// 照片不存在、未发布或不属于该相册（404，观众端）。
   static const String photoNotPublic = 'PHOTO_NOT_PUBLIC';
 
+  /// 相册可见但未开放下载（403，P4a 下载两级开关；App 无下载面，
+  /// 仅为与 errors.ts 全量 1:1 对齐而镜像——文案走 errorGeneric 兜底）。
+  static const String downloadDisabled = 'DOWNLOAD_DISABLED';
+
   /// 参数校验失败（400，经全局校验管道）。
   static const String validationFailed = 'VALIDATION_FAILED';
 
diff --git a/apps/viewer/e2e/embed.spec.ts b/apps/viewer/e2e/embed.spec.ts
index c7455e2..700030b 100644
--- a/apps/viewer/e2e/embed.spec.ts
+++ b/apps/viewer/e2e/embed.spec.ts
@@ -90,6 +90,26 @@ test.describe.serial('embed carousel widget (P4b Task 4)', () => {
     await popup.close();
   });
 
+  // T15（A12）embed lang 参数两语言渲染断言之 en 侧（zh 侧由 lang=auto →
+  // defaultLocale 各用例覆盖）：loader data-lang=en → iframe 内英文标题，
+  // 深链 lang=en 跟随。
+  test('lang=en renders English copy in the iframe and passes en to the photo deep link', async ({ page }) => {
+    const feed = await seedFeed('langen');
+
+    await page.goto(`/__test__/embed-host.html?slot=${feed.slotToken}&lang=en`);
+    const frame = page.frameLocator('iframe');
+    await expect(frame.locator('.embed-title')).toHaveText('langen EN Album');
+    await expect(frame.locator('.embed-photo').first()).toBeVisible();
+
+    // 点击 → 新标签深链携带 lang=en（resolvedLocale）。
+    const popupPromise = page.context().waitForEvent('page');
+    await frame.locator('.embed-photo.active').click();
+    const popup = await popupPromise;
+    await expect(popup).toHaveURL(new RegExp(`/a/${feed.albumToken}/p/`));
+    await expect(popup).toHaveURL(/lang=en/);
+    await popup.close();
+  });
+
   test('collapse button hides the widget, persists after reload, and the handle expands it', async ({ page }) => {
     const feed = await seedFeed('collapse');
 
diff --git a/apps/viewer/src/__tests__/download.spec.ts b/apps/viewer/src/__tests__/download.spec.ts
index e523bb0..b5e1a4b 100644
--- a/apps/viewer/src/__tests__/download.spec.ts
+++ b/apps/viewer/src/__tests__/download.spec.ts
@@ -2,6 +2,7 @@ import { flushPromises, mount, type VueWrapper } from '@vue/test-utils';
 import { defineComponent, ref } from 'vue';
 import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
 import {
+  ApiError,
   getPhotoDownload,
   getPublicPhotos,
   type PublicAlbum,
@@ -250,6 +251,20 @@ describe('photo download (P4a Task 4)', () => {
     expect(openSpy).not.toHaveBeenCalled();
   });
 
+  it('DOWNLOAD_DISABLED surfaces the dedicated localized notice (T15 漏译修复)', async () => {
+    // 相册中途关闭下载开关（载荷仍开但端点 403）→ 专属文案而非通用失败。
+    mockedGetPhotoDownload.mockReset().mockRejectedValue(
+      new ApiError('DOWNLOAD_DISABLED', 'downloads disabled', 403, null),
+    );
+    const wrapper = mountLightbox(makeAlbum());
+    await flushPromises();
+
+    await wrapper.find('.control.download').trigger('click');
+    await flushPromises();
+    expect(wrapper.find('.download-failed[role="alert"]').text()).toContain('该相册未开放下载');
+    expect(wrapper.find('.download-failed').text()).not.toContain('下载失败');
+  });
+
   it('WeChat webview: click opens the guidance overlay instead of hitting the endpoint', async () => {
     stubUserAgent(WECHAT_IOS_UA);
     const wrapper = mountLightbox(makeAlbum());
diff --git a/apps/viewer/src/__tests__/error-coverage.spec.ts b/apps/viewer/src/__tests__/error-coverage.spec.ts
new file mode 100644
index 0000000..0573592
--- /dev/null
+++ b/apps/viewer/src/__tests__/error-coverage.spec.ts
@@ -0,0 +1,71 @@
+import { describe, expect, it } from 'vitest';
+import { ErrorCodes } from '@photo/contracts/errors';
+import { errorCodeI18nKeys } from '../api/errors';
+import en from '../i18n/locales/en';
+import zhCN from '../i18n/locales/zh-CN';
+
+// T15 全端语言验收（P4b Task 6）：错误码→文案映射的机器守护。
+//   1. 契约 ErrorCodes 枚举全集在本表无缺漏（A12「主要提示无漏译」）；
+//   2. 表内每个文案键在 zh-CN / en 两份词条都真实存在（防键注册了但一侧
+//      没写文案的真漏译；点路径解析）；
+//   3. zh-CN / en 两份词条键集合递归相等（模板级 zh/en 一致性，镜像 admin
+//      侧 locale 守护与 P3 mobile arb 完整性测试）。
+// 服务端不做文案本地化（错误体 message 仅英文诊断兜底，客户端按稳定 code
+// 映射——见 openapi.yaml info.description），故本表即观众侧错误文案全集。
+
+type Messages = Record<string, unknown>;
+
+/** 递归收集词条文件的叶子键点路径集合（errors.albumNotFound 等）。 */
+function leafPaths(messages: Messages, prefix = ''): Set<string> {
+  const paths = new Set<string>();
+  for (const [key, value] of Object.entries(messages)) {
+    const path = prefix === '' ? key : `${prefix}.${key}`;
+    if (value !== null && typeof value === 'object') {
+      for (const child of leafPaths(value as Messages, path)) paths.add(child);
+    } else {
+      paths.add(path);
+    }
+  }
+  return paths;
+}
+
+/** 按点路径取词条值（不存在返回 undefined）。 */
+function resolvePath(messages: Messages, path: string): unknown {
+  let current: unknown = messages;
+  for (const segment of path.split('.')) {
+    if (current === null || typeof current !== 'object') return undefined;
+    current = (current as Messages)[segment];
+  }
+  return current;
+}
+
+describe('错误码文案覆盖（T15/A12）', () => {
+  const zhPaths = leafPaths(zhCN as Messages);
+  const enPaths = leafPaths(en as Messages);
+
+  it('契约 ErrorCodes 枚举全集在 errorCodeI18nKeys 无缺漏', () => {
+    const codes = Object.values(ErrorCodes);
+    expect(codes.length).toBeGreaterThanOrEqual(18); // 契约现存 18 码，只增不减
+    for (const code of codes) {
+      expect(errorCodeI18nKeys[code], `缺少错误码映射: ${code}`).toBeDefined();
+    }
+  });
+
+  it('映射表内每个文案键在 zh-CN 与 en 词条均存在且非空', () => {
+    for (const [code, key] of Object.entries(errorCodeI18nKeys)) {
+      for (const [locale, paths] of [
+        ['zh-CN', zhPaths],
+        ['en', enPaths],
+      ] as const) {
+        expect(paths.has(key), `${code} 的文案键 ${key} 未登记于 ${locale} 词条`).toBe(true);
+        const value = resolvePath((locale === 'zh-CN' ? zhCN : en) as Messages, key);
+        expect(typeof value === 'string' && value.length > 0, `${key} 在 ${locale} 非非空字符串`).toBe(true);
+      }
+    }
+  });
+
+  it('zh-CN 与 en 词条键集合递归相等（无单侧漏译）', () => {
+    expect([...zhPaths].filter((path) => !enPaths.has(path))).toEqual([]);
+    expect([...enPaths].filter((path) => !zhPaths.has(path))).toEqual([]);
+  });
+});
diff --git a/apps/viewer/src/api/errors.ts b/apps/viewer/src/api/errors.ts
index 79f821e..e4b5fe9 100644
--- a/apps/viewer/src/api/errors.ts
+++ b/apps/viewer/src/api/errors.ts
@@ -2,19 +2,38 @@ import { NETWORK_ERROR_CODE } from './client';
 
 // 错误码 → i18n 文案键映射表：401/404 等观众可认知的码给出确定性文案，
 // 未登记码统一走 errors.unknown（原始 message 仅用于日志/排查）。
+//
+// T15 全端语言验收（P4b Task 6）：本表按契约枚举全集登记（机器守护见
+// __tests__/error-coverage.spec.ts，遍历 @photo/contracts ErrorCodes 断言
+// 无缺漏）。viewer 可达码给专属文案；管理/上传/内部域码（登录凭据、成员
+// 冲突、上传校验、服务间鉴权——viewer 无对应界面）按移动端先例防御性
+// 注册到通用文案，杜绝未来路由扩展时静默漏译。
 export const errorCodeI18nKeys: Readonly<Record<string, string>> = {
   // 客户端合成码
   [NETWORK_ERROR_CODE]: 'errors.network',
   // 401 域
   AUTH_TOKEN_EXPIRED: 'errors.authExpired',
-  AUTH_FORBIDDEN: 'errors.forbidden',
+  // 登录凭据错误：viewer 无登录面（App/admin 域），防御性注册。
+  AUTH_INVALID_CREDENTIALS: 'errors.unknown',
   AUTH_RATE_LIMITED: 'errors.rateLimited',
+  // P2 公开互动（likes/visits）匿名限速 429——viewer 直面。
+  RATE_LIMITED: 'errors.rateLimited',
+  AUTH_FORBIDDEN: 'errors.forbidden',
   // 404 域
   NOT_FOUND: 'errors.notFound',
   ALBUM_NOT_FOUND: 'errors.albumNotFound',
   ALBUM_NOT_PUBLIC: 'errors.albumNotAvailable',
   PHOTO_NOT_FOUND: 'errors.photoNotFound',
   PHOTO_NOT_PUBLIC: 'errors.photoNotFound',
+  // P4a 下载两级开关（A18）：相册可见但禁止下载——下载入口与文案直达。
+  DOWNLOAD_DISABLED: 'errors.downloadDisabled',
+  // 上传/成员域（admin 与 App 上传链路）：viewer 不可达，防御性注册。
+  ALBUM_UPLOAD_DISABLED: 'errors.unknown',
+  MEMBER_CONFLICT: 'errors.unknown',
+  UPLOAD_SESSION_CONFLICT: 'errors.unknown',
+  UPLOAD_VERIFY_FAILED: 'errors.unknown',
+  // 服务间内部同步鉴权（/internal/**）：外部客户端不可达，防御性注册。
+  SYNC_AUTH_FAILED: 'errors.unknown',
   // 其余
   VALIDATION_FAILED: 'errors.validationFailed',
   INTERNAL_ERROR: 'errors.internal',
diff --git a/apps/viewer/src/components/PhotoLightbox.vue b/apps/viewer/src/components/PhotoLightbox.vue
index e8abcce..2c49015 100644
--- a/apps/viewer/src/components/PhotoLightbox.vue
+++ b/apps/viewer/src/components/PhotoLightbox.vue
@@ -439,9 +439,9 @@ onBeforeUnmount(() => {
 
     <p v-if="extending" class="loading">{{ t('album.loading') }}</p>
 
-    <!-- 下载失败轻提示（端点失败/网络；可重点击重试）。 -->
+    <!-- 下载失败轻提示（端点失败/网络；DOWNLOAD_DISABLED 专属文案；可重点击重试）。 -->
     <p v-if="download !== undefined && download.failed" class="download-failed" role="alert">
-      {{ t('download.failed') }}
+      {{ t(download.failed) }}
     </p>
 
     <!-- 微信内置浏览器引导（P4a Task 4）：替代直接下载——长按保存 + 复制深链。 -->
diff --git a/apps/viewer/src/download/useDownload.ts b/apps/viewer/src/download/useDownload.ts
index 7f2d530..9fbc9fb 100644
--- a/apps/viewer/src/download/useDownload.ts
+++ b/apps/viewer/src/download/useDownload.ts
@@ -17,10 +17,12 @@
 // 打开后可正常下载）。clipboard 不可用时降级展示可选中文本供手动长按复制。
 import { computed, onBeforeUnmount, reactive, ref, type Ref } from 'vue';
 import {
+  ApiError,
   getPhotoDownload,
   type DownloadVariant,
   type PublicAlbum,
 } from '../api/client';
+import { ErrorCodes } from '@photo/contracts/errors';
 
 export interface DownloadInputs {
   token: Ref<string>;
@@ -63,7 +65,9 @@ export const COPY_NOTICE_MS = 2500;
 
 export function useDownload(inputs: DownloadInputs) {
   const busy = ref(false);
-  const failed = ref(false);
+  /** 失败文案键（''=无失败）：默认通用失败；DOWNLOAD_DISABLED 给专属文案
+   * （T15 漏译修复——相册中途关闭下载开关时观众可理解的确定性提示）。 */
+  const failed = ref<'' | 'download.failed' | 'errors.downloadDisabled'>('');
   /** 微信引导浮层可见（点击下载时处于微信内置浏览器）。 */
   const wechatGuide = ref(false);
   const copyNotice = ref<'' | 'copied' | 'failed'>('');
@@ -90,7 +94,7 @@ export function useDownload(inputs: DownloadInputs) {
     const photoId = inputs.photoId();
     if (photoId === null || busy.value) return;
     busy.value = true;
-    failed.value = false;
+    failed.value = '';
     try {
       let credential = await getPhotoDownload(inputs.token.value, photoId, variant);
       if (isExpired(credential.expiresAt)) {
@@ -99,8 +103,11 @@ export function useDownload(inputs: DownloadInputs) {
         credential = await getPhotoDownload(inputs.token.value, photoId, variant);
       }
       openDownloadUrl(credential.url);
-    } catch {
-      failed.value = true;
+    } catch (error) {
+      failed.value =
+        error instanceof ApiError && error.code === ErrorCodes.DOWNLOAD_DISABLED
+          ? 'errors.downloadDisabled'
+          : 'download.failed';
     } finally {
       busy.value = false;
     }
diff --git a/apps/viewer/src/i18n/locales/en.ts b/apps/viewer/src/i18n/locales/en.ts
index 00d8cad..298632a 100644
--- a/apps/viewer/src/i18n/locales/en.ts
+++ b/apps/viewer/src/i18n/locales/en.ts
@@ -51,6 +51,8 @@ export default {
     loadFailed: 'Failed to load image',
     // PHOTO_NOT_PUBLIC 专用：not found / unpublished / hidden share one code (anti-enumeration).
     photoUnavailable: 'Photo not found or unavailable',
+    // 轮播中被移除（ws photo.hidden）的轻提示。
+    photoRemoved: 'This photo has been removed',
     backToAlbum: 'Back to album',
   },
   route: { tokenLabel: 'Album token', photoLabel: 'Photo' },
@@ -67,5 +69,7 @@ export default {
     internal: 'Service temporarily unavailable, please try again later',
     unknown: 'Something went wrong, please try again later',
     network: 'Network error, please check your connection',
+    // DOWNLOAD_DISABLED (P4a two-tier download switches): visible but not downloadable.
+    downloadDisabled: 'Downloads are not enabled for this album',
   },
 };
diff --git a/apps/viewer/src/i18n/locales/zh-CN.ts b/apps/viewer/src/i18n/locales/zh-CN.ts
index 7a0e402..15804be 100644
--- a/apps/viewer/src/i18n/locales/zh-CN.ts
+++ b/apps/viewer/src/i18n/locales/zh-CN.ts
@@ -69,5 +69,7 @@ export default {
     internal: '服务暂时不可用，请稍后再试',
     unknown: '出错了，请稍后再试',
     network: '网络连接失败，请检查网络',
+    // DOWNLOAD_DISABLED（P4a 下载两级开关）：相册可见但未开放下载。
+    downloadDisabled: '该相册未开放下载',
   },
 };
diff --git a/packages/contracts/package.json b/packages/contracts/package.json
index 359feb2..1ad710e 100644
--- a/packages/contracts/package.json
+++ b/packages/contracts/package.json
@@ -3,7 +3,7 @@
   "version": "0.1.0",
   "private": true,
   "type": "module",
-  "description": "API contract single source of truth (OpenAPI 3.1 + error codes); types generated via openapi-typescript and committed under src/generated/. Subpath ./constants is the browser-safe entry (root index re-exports internal-sync with node:crypto — browser apps like viewer's embed page must import constants from there).",
+  "description": "API contract single source of truth (OpenAPI 3.1 + error codes); types generated via openapi-typescript and committed under src/generated/. Subpaths ./constants and ./errors are the browser-safe entries (root index re-exports internal-sync with node:crypto — browser apps like viewer must import runtime values from those; errors.ts is pure TS).",
   "//": "lint runs `eslint .` against the root flat config (Task 7). test is ts-jest with full type-checking (Task 6 type tests).",
   "main": "./dist/index.js",
   "types": "./dist/index.d.ts",
@@ -16,6 +16,10 @@
       "types": "./dist/constants.d.ts",
       "default": "./dist/constants.js"
     },
+    "./errors": {
+      "types": "./dist/errors.d.ts",
+      "default": "./dist/errors.js"
+    },
     "./realtime": {
       "types": "./dist/realtime.d.ts",
       "default": "./dist/realtime.js"
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index 1666984..3e4c499 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -99,6 +99,9 @@ importers:
       '@eslint/js':
         specifier: ^9.9.1
         version: 9.39.5
+      '@photo/contracts':
+        specifier: workspace:*
+        version: link:../../packages/contracts
       '@playwright/test':
         specifier: ^1.48.0
         version: 1.63.0