diff --git a/README.md b/README.md
index 4ee6c3a..d694b6a 100644
--- a/README.md
+++ b/README.md
@@ -10,6 +10,11 @@ sr 是一個架空世界觀的 2D 六角格回合制策略遊戲：**手牌 + 
    再由使用者選擇 iframe 嵌入或另開新分頁。
 3. 服務目錄、探活與分流屬於獨立的 `hoshi-svc`，不在這個靜態網站的部署物內。
 
+公開頁面使用 `hoshi_cookie_consent` 區分「只用必要 Cookie」與「記住偏好」。訪客明確
+選擇語言或接受偏好後，語言會寫入 `.hoshivel.com` 範圍的 `hoshi_lang`，供 Hoshivel、
+Hoshi ID 與遊戲入口共用；本站維持固定深色美術方向，不會覆寫其他頁面的外觀偏好。
+偏好 Cookie 不承載分析或追蹤資料，登入與安全 Cookie 也不受這個選項停用。
+
 具體流程：官網 Play → `GET https://svc.hoshivel.com/v1/services/sr-game/route?endpoint=web`
 → 取得建議與候選節點 → 使用者進入後才載入該節點原始 `web` endpoint。
 
diff --git a/package.json b/package.json
index 2a50136..fcd41cc 100644
--- a/package.json
+++ b/package.json
@@ -11,7 +11,7 @@
   "scripts": {
     "dev": "astro dev",
     "start": "astro dev",
-    "test": "node --experimental-strip-types --test test/play.test.mjs",
+    "test": "node --experimental-strip-types --test test/*.test.mjs",
     "build": "npm test && astro check && astro build",
     "preview": "astro preview",
     "check": "astro check",
diff --git a/src/components/CookiePreferences.astro b/src/components/CookiePreferences.astro
new file mode 100644
index 0000000..66c6a71
--- /dev/null
+++ b/src/components/CookiePreferences.astro
@@ -0,0 +1,133 @@
+---
+import { useTranslations, type Locale } from "@/i18n/utils";
+import { globalLanguageForLocale } from "@/lib/preferences";
+
+interface Props {
+  locale: Locale;
+}
+
+const { locale } = Astro.props;
+const t = useTranslations(locale);
+const language = globalLanguageForLocale(locale);
+---
+
+<details class="sr-cookie" data-cookie-panel data-language={language} hidden>
+  <summary>{t("cookie.settings")}</summary>
+  <div class="sr-cookie__card">
+    <h2>{t("cookie.title")}</h2>
+    <p>{t("cookie.body")}</p>
+    <div class="sr-cookie__actions">
+      <button class="sr-btn sr-btn--ghost" type="button" data-cookie-essential>
+        {t("cookie.essential")}
+      </button>
+      <button class="sr-btn sr-btn--primary" type="button" data-cookie-accept>
+        {t("cookie.remember")}
+      </button>
+    </div>
+  </div>
+</details>
+
+<style>
+  .sr-cookie {
+    position: fixed;
+    right: 1rem;
+    bottom: 1rem;
+    z-index: var(--sr-z-overlay);
+    width: min(30rem, calc(100% - 2rem));
+  }
+  .sr-cookie > summary {
+    width: fit-content;
+    max-width: 100%;
+    margin-left: auto;
+    padding: 0.45rem 0.8rem;
+    border: 1px solid var(--sr-border-strong);
+    border-radius: var(--sr-radius);
+    list-style: none;
+    color: var(--sr-text-muted);
+    background: var(--sr-surface-2);
+    box-shadow: var(--sr-shadow-2);
+    font-size: var(--sr-text-xs);
+    cursor: pointer;
+  }
+  .sr-cookie > summary::-webkit-details-marker {
+    display: none;
+  }
+  .sr-cookie[open] > summary {
+    margin-bottom: 0.5rem;
+    color: var(--sr-accent-ice);
+    border-color: var(--sr-accent);
+  }
+  .sr-cookie__card {
+    padding: 1.25rem;
+    border: 1px solid color-mix(in srgb, var(--sr-accent) 62%, var(--sr-border));
+    border-radius: var(--sr-radius-lg);
+    color: var(--sr-text);
+    background: var(--sr-surface-1);
+    box-shadow: var(--sr-shadow-3);
+  }
+  .sr-cookie__card h2 {
+    color: var(--sr-text-bright);
+    font-size: var(--sr-text-lg);
+  }
+  .sr-cookie__card p {
+    margin-top: 0.5rem;
+    color: var(--sr-text-muted);
+    font-size: var(--sr-text-sm);
+    line-height: var(--sr-leading);
+  }
+  .sr-cookie__actions {
+    display: flex;
+    justify-content: flex-end;
+    flex-wrap: wrap;
+    gap: 0.6rem;
+    margin-top: 1rem;
+  }
+  .sr-cookie__actions .sr-btn {
+    min-height: 2.5rem;
+    padding: 0.55rem 0.95rem;
+  }
+  @media (max-width: 520px) {
+    .sr-cookie {
+      right: 0.6rem;
+      bottom: 0.6rem;
+      width: calc(100% - 1.2rem);
+    }
+    .sr-cookie__actions,
+    .sr-cookie__actions .sr-btn {
+      width: 100%;
+    }
+  }
+</style>
+
+<script>
+  import {
+    acceptPreferences,
+    globalLanguageForLocale,
+    readConsent,
+    rememberLanguage,
+    useEssentialCookies,
+  } from "@/lib/preferences";
+
+  const panel = document.querySelector<HTMLDetailsElement>("[data-cookie-panel]");
+  if (panel) {
+    panel.hidden = false;
+    panel.open = readConsent() === null;
+    const current = globalLanguageForLocale(panel.dataset.language ?? "zh-TW");
+    panel.querySelector("[data-cookie-accept]")?.addEventListener("click", () => {
+      acceptPreferences(current);
+      panel.open = false;
+    });
+    panel.querySelector("[data-cookie-essential]")?.addEventListener("click", () => {
+      useEssentialCookies();
+      panel.open = false;
+    });
+  }
+
+  document.addEventListener("click", (event) => {
+    const link = (event.target as HTMLElement | null)?.closest<HTMLElement>(
+      "[data-hoshi-language]",
+    );
+    if (!link) return;
+    rememberLanguage(globalLanguageForLocale(link.dataset.hoshiLanguage ?? ""));
+  });
+</script>
diff --git a/src/components/LanguageSwitcher.astro b/src/components/LanguageSwitcher.astro
index 425680c..20610c3 100644
--- a/src/components/LanguageSwitcher.astro
+++ b/src/components/LanguageSwitcher.astro
@@ -14,6 +14,7 @@ import {
   HTML_LANG,
   type Locale,
 } from "@/i18n/utils";
+import { globalLanguageForLocale } from "@/lib/preferences";
 
 interface Props {
   locale: Locale;
@@ -36,6 +37,7 @@ const t = useTranslations(locale);
           hreflang={HTML_LANG[l]}
           title={LOCALE_LABEL[l]}
           aria-current={active ? "true" : undefined}
+          data-hoshi-language={globalLanguageForLocale(l)}
         >
           {LOCALE_SHORT[l]}
         </a>
diff --git a/src/components/PreferenceBootstrap.astro b/src/components/PreferenceBootstrap.astro
new file mode 100644
index 0000000..247ef88
--- /dev/null
+++ b/src/components/PreferenceBootstrap.astro
@@ -0,0 +1,52 @@
+---
+import {
+  LOCALES,
+  DEFAULT_LOCALE,
+  localizedPath,
+  type Locale,
+} from "@/i18n/utils";
+import { globalLanguageForLocale } from "@/lib/preferences";
+
+interface Props {
+  locale: Locale;
+  path: string;
+}
+
+const { locale, path } = Astro.props;
+const routes = Object.fromEntries(
+  LOCALES.map((candidate) => [
+    globalLanguageForLocale(candidate),
+    localizedPath(candidate, path),
+  ]),
+);
+const redirectFromDefault = locale === DEFAULT_LOCALE && path === "/";
+---
+
+<script is:inline define:vars={{ routes, redirectFromDefault }}>
+  // A static site can only honor a saved locale in the browser. Explicit
+  // locale URLs still win, so only the unprefixed home route redirects.
+  if (redirectFromDefault) {
+    var readCookie = function (name) {
+      var prefix = name + "=";
+      var parts = document.cookie.split(";");
+      for (var i = 0; i < parts.length; i += 1) {
+        var part = parts[i].trim();
+        if (!part.startsWith(prefix)) continue;
+        try {
+          return decodeURIComponent(part.slice(prefix.length));
+        } catch (_error) {
+          return null;
+        }
+      }
+      return null;
+    };
+    if (readCookie("hoshi_cookie_consent") === "preferences-v1") {
+      var saved = readCookie("hoshi_lang");
+      if (saved === "zh-Hant") saved = "zh-TW";
+      var target = routes[saved];
+      if (target && target !== location.pathname) {
+        location.replace(target + location.search + location.hash);
+      }
+    }
+  }
+</script>
diff --git a/src/i18n/ui.ts b/src/i18n/ui.ts
index 1955216..a65a3c6 100644
--- a/src/i18n/ui.ts
+++ b/src/i18n/ui.ts
@@ -82,6 +82,13 @@ const zhHant = {
   "footer.summary": "破碎星空之下，啟程未竟之旅",
   "footer.rights": "碎界 Shattered Realms",
 
+  "cookie.settings": "Cookie 設定",
+  "cookie.title": "記住你的顯示偏好",
+  "cookie.body":
+    "我們不使用分析或追蹤 Cookie。你可以讓 Hoshivel 旗下網站共用語言偏好；外觀只在支援的頁面套用。登入與安全所需的 Cookie 永遠啟用。",
+  "cookie.essential": "只使用必要 Cookie",
+  "cookie.remember": "記住偏好",
+
   "hero.badge": "正式上線",
 
   // 預覽 Teaser（#teaser）—— 只有眉標與播放器，不加說明文字
@@ -236,6 +243,13 @@ const zhCN: Record<UIKey, string> = {
   "footer.summary": "破碎星空之下，启程未竟之旅",
   "footer.rights": "碎界 Shattered Realms",
 
+  "cookie.settings": "Cookie 设置",
+  "cookie.title": "记住你的显示偏好",
+  "cookie.body":
+    "我们不使用分析或跟踪 Cookie。你可以让 Hoshivel 旗下网站共享语言偏好；外观只在支持的页面应用。登录与安全所需的 Cookie 始终启用。",
+  "cookie.essential": "仅使用必要 Cookie",
+  "cookie.remember": "记住偏好",
+
   "hero.badge": "正式上线",
 
   "teaser.eyebrow": "预览",
@@ -380,6 +394,13 @@ const en: Record<UIKey, string> = {
   "footer.summary": "Beneath a shattered sky, the journey begins",
   "footer.rights": "Shattered Realms",
 
+  "cookie.settings": "Cookie settings",
+  "cookie.title": "Remember your display preferences",
+  "cookie.body":
+    "We do not use analytics or tracking cookies. You can share your language across Hoshivel sites; appearance applies only on pages that support it. Cookies required for sign-in and security are always enabled.",
+  "cookie.essential": "Essential only",
+  "cookie.remember": "Remember preferences",
+
   "hero.badge": "Now live",
 
   "teaser.eyebrow": "Preview",
@@ -533,6 +554,13 @@ const ja: Record<UIKey, string> = {
   "footer.summary": "砕けた星空の下、未完の旅へ",
   "footer.rights": "砕界 Shattered Realms",
 
+  "cookie.settings": "Cookie 設定",
+  "cookie.title": "表示設定を保存",
+  "cookie.body":
+    "解析や追跡の Cookie は使用しません。Hoshivel の各サイトで言語を共有できます。外観は対応ページにのみ適用されます。ログインとセキュリティに必要な Cookie は常に有効です。",
+  "cookie.essential": "必須 Cookie のみ",
+  "cookie.remember": "設定を保存",
+
   "hero.badge": "正式サービス中",
 
   "teaser.eyebrow": "プレビュー",
diff --git a/src/layouts/Layout.astro b/src/layouts/Layout.astro
index e894a36..a2d8b46 100644
--- a/src/layouts/Layout.astro
+++ b/src/layouts/Layout.astro
@@ -2,6 +2,8 @@
 import "@/styles/global.css";
 import Header from "@/components/Header.astro";
 import Footer from "@/components/Footer.astro";
+import PreferenceBootstrap from "@/components/PreferenceBootstrap.astro";
+import CookiePreferences from "@/components/CookiePreferences.astro";
 import {
   useTranslations,
   localizedPath,
@@ -56,6 +58,7 @@ const xDefault = Astro.site
     <script is:inline>
       document.documentElement.classList.add("sr-js");
     </script>
+    <PreferenceBootstrap locale={locale} path={logicalPath} />
     <link rel="icon" type="image/svg+xml" href="/favicon.svg" />
     <link rel="canonical" href={canonical} />
     <link rel="sitemap" type="application/xml" href="/sitemap.xml" />
@@ -88,6 +91,7 @@ const xDefault = Astro.site
     <Header locale={locale} path={chromePath} isHome={isHomePage} />
     <slot />
     <Footer locale={locale} path={chromePath} isHome={isHomePage} />
+    <CookiePreferences locale={locale} />
 
     <script>
       // 輕量互動啟動：reveal-on-scroll（IntersectionObserver）＋磁吸按鈕。
diff --git a/src/lib/preferences.ts b/src/lib/preferences.ts
new file mode 100644
index 0000000..51f15af
--- /dev/null
+++ b/src/lib/preferences.ts
@@ -0,0 +1,98 @@
+export const LANGUAGE_COOKIE = "hoshi_lang";
+export const THEME_COOKIE = "hoshi_theme";
+export const CONSENT_COOKIE = "hoshi_cookie_consent";
+
+export const CONSENT_ESSENTIAL = "essential-v1";
+export const CONSENT_PREFERENCES = "preferences-v1";
+
+export type CookieConsent =
+  | typeof CONSENT_ESSENTIAL
+  | typeof CONSENT_PREFERENCES;
+export type GlobalLanguage = "zh-TW" | "zh-CN" | "ja" | "en";
+
+const COOKIE_TTL = 365 * 24 * 60 * 60;
+
+const GLOBAL_LANGUAGE: Record<string, GlobalLanguage> = {
+  "zh-Hant": "zh-TW",
+  "zh-TW": "zh-TW",
+  "zh-CN": "zh-CN",
+  "zh-Hans": "zh-CN",
+  ja: "ja",
+  en: "en",
+};
+
+export function globalLanguageForLocale(locale: string): GlobalLanguage {
+  return GLOBAL_LANGUAGE[locale] ?? "zh-TW";
+}
+
+export function normalizeGlobalLanguage(value: string | null): GlobalLanguage | null {
+  if (!value) return null;
+  return GLOBAL_LANGUAGE[value] ?? null;
+}
+
+export function parseCookie(cookieHeader: string, name: string): string | null {
+  for (const part of cookieHeader.split(";")) {
+    const [rawName, ...rawValue] = part.trim().split("=");
+    if (rawName !== name) continue;
+    try {
+      return decodeURIComponent(rawValue.join("="));
+    } catch {
+      return null;
+    }
+  }
+  return null;
+}
+
+export function preferenceCookieDomain(hostname: string): string | null {
+  const host = hostname.toLowerCase().replace(/\.$/, "");
+  return host === "hoshivel.com" || host.endsWith(".hoshivel.com")
+    ? "hoshivel.com"
+    : null;
+}
+
+function cookieSuffix(maxAge: number, domain: string | null): string {
+  const secure = typeof location !== "undefined" && location.protocol === "https:"
+    ? "; Secure"
+    : "";
+  return `; Path=/; Max-Age=${maxAge}; SameSite=Lax${domain ? `; Domain=${domain}` : ""}${secure}`;
+}
+
+function writeCookie(name: string, value: string): void {
+  if (typeof document === "undefined" || typeof location === "undefined") return;
+  const domain = preferenceCookieDomain(location.hostname);
+  if (domain) {
+    // Clear the previous host-only shape before writing the shared cookie.
+    document.cookie = `${name}=${cookieSuffix(0, null)}`;
+  }
+  document.cookie = `${name}=${encodeURIComponent(value)}${cookieSuffix(COOKIE_TTL, domain)}`;
+}
+
+function removeCookie(name: string): void {
+  if (typeof document === "undefined" || typeof location === "undefined") return;
+  document.cookie = `${name}=${cookieSuffix(0, null)}`;
+  const domain = preferenceCookieDomain(location.hostname);
+  if (domain) document.cookie = `${name}=${cookieSuffix(0, domain)}`;
+}
+
+export function readConsent(): CookieConsent | null {
+  if (typeof document === "undefined") return null;
+  const value = parseCookie(document.cookie, CONSENT_COOKIE);
+  return value === CONSENT_ESSENTIAL || value === CONSENT_PREFERENCES
+    ? value
+    : null;
+}
+
+export function acceptPreferences(language: GlobalLanguage): void {
+  writeCookie(CONSENT_COOKIE, CONSENT_PREFERENCES);
+  writeCookie(LANGUAGE_COOKIE, language);
+}
+
+export function rememberLanguage(language: GlobalLanguage): void {
+  acceptPreferences(language);
+}
+
+export function useEssentialCookies(): void {
+  removeCookie(LANGUAGE_COOKIE);
+  removeCookie(THEME_COOKIE);
+  writeCookie(CONSENT_COOKIE, CONSENT_ESSENTIAL);
+}
diff --git a/test/preferences.test.mjs b/test/preferences.test.mjs
new file mode 100644
index 0000000..5aef1f1
--- /dev/null
+++ b/test/preferences.test.mjs
@@ -0,0 +1,29 @@
+import assert from "node:assert/strict";
+import test from "node:test";
+
+import {
+  globalLanguageForLocale,
+  normalizeGlobalLanguage,
+  parseCookie,
+  preferenceCookieDomain,
+} from "../src/lib/preferences.ts";
+
+test("normalizes the locale vocabularies used by the public sites", () => {
+  assert.equal(globalLanguageForLocale("zh-Hant"), "zh-TW");
+  assert.equal(normalizeGlobalLanguage("zh-Hans"), "zh-CN");
+  assert.equal(normalizeGlobalLanguage("en"), "en");
+  assert.equal(normalizeGlobalLanguage("de"), null);
+});
+
+test("parses preference cookies and rejects malformed encoding", () => {
+  assert.equal(parseCookie("a=1; hoshi_lang=ja", "hoshi_lang"), "ja");
+  assert.equal(parseCookie("token=a%3Db", "token"), "a=b");
+  assert.equal(parseCookie("hoshi_lang=%E0%A4%A", "hoshi_lang"), null);
+});
+
+test("shares preferences only within the Hoshivel parent domain", () => {
+  assert.equal(preferenceCookieDomain("sr.hoshivel.com"), "hoshivel.com");
+  assert.equal(preferenceCookieDomain("play.sr.hoshivel.com."), "hoshivel.com");
+  assert.equal(preferenceCookieDomain("not-hoshivel.com"), null);
+  assert.equal(preferenceCookieDomain("localhost"), null);
+});