diff --git a/apps/mall/locales/marketing.ts b/apps/mall/locales/marketing.ts
index e40f6b0..c128d84 100644
--- a/apps/mall/locales/marketing.ts
+++ b/apps/mall/locales/marketing.ts
@@ -19,7 +19,12 @@ export default {
       collectiveBannerAlt: "Group deal promotion",
       peopleGroup: "{n} people group",
       peopleJoined: "{n} people joined",
-      joinGroup: "View deal",
+      joinGroup: "Join",
+      openGroup: "Open a group",
+      chooseGroup: "Choose a group to join.",
+      groupFailed: "Unable to start the group order.",
+      paidMembers: "{n}/{total} joined",
+      noOpenGroups: "No open groups yet.",
       noCollectiveProducts: "No group deals right now.",
       integralTitle: "Points mall",
       integralBreadcrumb: "Points mall",
@@ -79,7 +84,12 @@ export default {
       collectiveBannerAlt: "拼团活动",
       peopleGroup: "{n}人团",
       peopleJoined: "{n}人已拼",
-      joinGroup: "查看详情",
+      joinGroup: "去拼团",
+      openGroup: "我要开团",
+      chooseGroup: "请选择一个团。",
+      groupFailed: "拼团下单失败，请稍后重试。",
+      paidMembers: "已参团 {n}/{total}",
+      noOpenGroups: "暂无进行中的团。",
       noCollectiveProducts: "当前暂无拼团商品。",
       integralTitle: "积分商城",
       integralBreadcrumb: "积分商城",
diff --git a/apps/mall/mock/api.ts b/apps/mall/mock/api.ts
index d9ba45e..9ace0c8 100644
--- a/apps/mall/mock/api.ts
+++ b/apps/mall/mock/api.ts
@@ -19,6 +19,10 @@ import type {
   FlashSaleItemInput,
   FlashSaleSession,
   FlashSaleSessionInput,
+  GroupBuyIntent,
+  GroupBuyingActivity,
+  GroupBuyingActivityInput,
+  GroupBuyingActivityView,
   HomeContent,
   IntegralOrder,
   IntegralProduct,
@@ -36,6 +40,7 @@ import type {
 import {
   BASE_CURRENCY,
   MOCK_BANNERS,
+  collectiveProducts,
   MOCK_BRANDS,
   MOCK_CATEGORIES,
   MOCK_COUPONS,
@@ -197,6 +202,46 @@ function mockFlashSales(): PublicFlashSaleSession[] {
   ];
 }
 
+/** Synthesized activities from the fixed collective fixtures. */
+function mockGroupBuying(): GroupBuyingActivityView[] {
+  const now = Date.now();
+  return collectiveProducts().map((entry, index) => {
+    const sku = lowestSku(entry.product);
+    const price = sku?.price_minor ?? 1000;
+    return {
+      id: `gb${index + 1}`,
+      shop_id: entry.product.shop_id,
+      sku_id: sku?.id ?? "",
+      name: { en: `${entry.need}-person group`, zh: `${entry.need}人团` },
+      description: null,
+      image: entry.product.images[0] ?? null,
+      group_price_minor: Math.max(1, Math.round(price * 0.8)),
+      currency: sku?.currency ?? BASE_CURRENCY,
+      required_members: entry.need,
+      starts_at: new Date(now - 3_600_000).toISOString(),
+      ends_at: new Date(now + 7 * 24 * 3_600_000).toISOString(),
+      group_lifetime_hours: 24,
+      enabled: true,
+      created_at: "2026-01-01T00:00:00.000Z",
+      updated_at: "2026-01-01T00:00:00.000Z",
+      sku_code: sku?.sku_code ?? "",
+      product_id: entry.product.id,
+      product_slug: entry.product.slug,
+      product_name: entry.product.name,
+      original_price_minor: price,
+      original_currency: sku?.currency ?? BASE_CURRENCY,
+      open_groups: [
+        {
+          id: `g${index + 1}`,
+          paid_member_count: Math.min(1, entry.need - 1),
+          required_members: entry.need,
+          expires_at: new Date(now + 6 * 3_600_000).toISOString(),
+        },
+      ],
+    };
+  });
+}
+
 function initialState(): MockState {
   const persisted = loadPersisted();
   // Coupons and points are session-only, so a restored snapshot re-seeds them.
@@ -399,12 +444,25 @@ export function createMockApi(): ApiClient {
       shippingAddress: Address,
       currency: string,
       couponByShop: Record<string, string> = {},
+      groupBuy?: GroupBuyIntent,
     ) => {
       if (state.cart.length === 0) {
         return Promise.reject(new ApiError(400, "EMPTY_CART", "Cart is empty"));
       }
+      // A group-buy intent repriced exactly one SKU at the activity price.
+      const activity = groupBuy
+        ? mockGroupBuying().find((entry) => entry.id === groupBuy.activity_id)
+        : undefined;
+      if (groupBuy && !activity) {
+        return Promise.reject(new ApiError(409, "CONFLICT", "Group activity is not available"));
+      }
+      const cartItems = state.cart.map((item) =>
+        activity && groupBuy && item.sku_id === groupBuy.sku_id
+          ? { ...item, unit_price_minor: activity.group_price_minor }
+          : item,
+      );
       const byShop: Record<string, CartItem[]> = {};
-      for (const item of state.cart) {
+      for (const item of cartItems) {
         const product = productById(item.product_id);
         const shopId = product?.shop_id ?? "unknown";
         (byShop[shopId] ??= []).push(item);
@@ -412,8 +470,14 @@ export function createMockApi(): ApiClient {
       const created: Order[] = [];
       for (const [shopId, items] of Object.entries(byShop)) {
         const subtotal = items.reduce((sum, i) => sum + i.unit_price_minor * i.qty, 0);
+        const groupHere = activity && groupBuy && items.some((i) => i.sku_id === groupBuy.sku_id);
         const couponId = couponByShop[shopId];
         let discount = 0;
+        if (couponId && groupHere) {
+          return Promise.reject(
+            new ApiError(409, "CONFLICT", "A coupon cannot be combined with group pricing"),
+          );
+        }
         if (couponId) {
           const coupon = state.coupons.find((c) => c.id === couponId);
           if (!coupon || coupon.status !== "claimed" || coupon.shop_id !== shopId) {
@@ -436,6 +500,8 @@ export function createMockApi(): ApiClient {
           total_minor: subtotal - discount,
           discount_minor: discount,
           coupon_id: couponId ?? null,
+          group_activity_id: groupHere ? activity.id : null,
+          group_id: groupHere ? `g-${Date.now()}` : null,
           items: items.map((i, k) => ({
             id: `oi-${Date.now()}-${k}`,
             sku_id: i.sku_id,
@@ -680,6 +746,8 @@ export function createMockApi(): ApiClient {
 
     listFlashSales: () => Promise.resolve(mockFlashSales()),
 
+    listGroupBuyingActivities: () => Promise.resolve(mockGroupBuying()),
+
     listMyRedemptions: () => Promise.resolve(state.redemptions.map((o) => ({ ...o }))),
 
     redeemPoints: (body: RedeemPointsBody) => {
@@ -750,6 +818,10 @@ export function createMockApi(): ApiClient {
       addFlashSaleItem: (_sessionId: string, _body: FlashSaleItemInput) => unsupported(),
       updateFlashSaleItem: (_id: string, _body: FlashSaleItemInput) => unsupported(),
       deleteFlashSaleItem: (_id: string) => unsupported(),
+      listGroupBuyingActivities: () => unsupported(),
+      createGroupBuyingActivity: (_body: GroupBuyingActivityInput) => unsupported(),
+      updateGroupBuyingActivity: (_id: string, _body: GroupBuyingActivityInput) => unsupported(),
+      deleteGroupBuyingActivity: (_id: string) => unsupported(),
     },
     admin: {
       listUsers: () => unsupported(),
diff --git a/apps/mall/mock/data.ts b/apps/mall/mock/data.ts
index 912b340..2e29162 100644
--- a/apps/mall/mock/data.ts
+++ b/apps/mall/mock/data.ts
@@ -706,6 +706,8 @@ export function seedOrders(userId: string): MockOrderSeed {
       total_minor: total,
       discount_minor: 0,
       coupon_id: null,
+      group_activity_id: null,
+      group_id: null,
       items,
       shipping_address: defaultAddress(),
       created_at: createdAt,
diff --git a/apps/mall/nuxt.config.ts b/apps/mall/nuxt.config.ts
index 3742909..33f8b7f 100644
--- a/apps/mall/nuxt.config.ts
+++ b/apps/mall/nuxt.config.ts
@@ -9,7 +9,7 @@ export default defineNuxtConfig({
       // Domains served by the live backend; every other domain stays on the
       // fixed-data adapter. Override with NUXT_PUBLIC_LIVE_DOMAINS='["catalog"]'.
       // See openspec/changes/replace-mock-api-wave-1/design.md and waves 2-3.
-      liveDomains: ["catalog", "currency", "content", "shops", "brands", "auth", "account", "cart", "orders", "shipments", "invoices", "addresses", "coupons", "points", "flashSales"],
+      liveDomains: ["catalog", "currency", "content", "shops", "brands", "auth", "account", "cart", "orders", "shipments", "invoices", "addresses", "coupons", "points", "flashSales", "groupBuying"],
       appName: "mall",
     },
   },
diff --git a/apps/mall/pages/checkout/index.vue b/apps/mall/pages/checkout/index.vue
index 63e05a3..5698fb1 100644
--- a/apps/mall/pages/checkout/index.vue
+++ b/apps/mall/pages/checkout/index.vue
@@ -1,6 +1,6 @@
 <script setup lang="ts">
 import { t as pick } from "@vmall/shared";
-import type { Address, AddressBookEntry, CartItem, Coupon, LocalizedText } from "@vmall/shared";
+import type { Address, AddressBookEntry, CartItem, Coupon, GroupBuyIntent, LocalizedText } from "@vmall/shared";
 
 type CheckoutGroup = {
   shopId: string;
@@ -16,6 +16,8 @@ const { currency } = usePrefs();
 const cartStore = useCartStore();
 const router = useRouter();
 const pendingOrderIds = useState<string[]>("checkout-orders", () => []);
+/** Group-buying intent chosen on the group-buying page, if any. */
+const groupIntent = useState<GroupBuyIntent | null>("checkout-group-intent", () => null);
 
 const items = ref<CartItem[]>([]);
 const addresses = ref<AddressBookEntry[]>([]);
@@ -140,7 +142,13 @@ async function submitOrder(): Promise<void> {
     for (const [shopId, couponId] of Object.entries(selectedCoupon)) {
       if (couponId) couponByShop[shopId] = couponId;
     }
-    const orders = await $api.checkout(address, currency.value, couponByShop);
+    const orders = await $api.checkout(
+      address,
+      currency.value,
+      couponByShop,
+      groupIntent.value ?? undefined,
+    );
+    groupIntent.value = null;
     pendingOrderIds.value = orders.map((order) => order.id);
     const snapshot = await $api.getCart();
     items.value = snapshot.items;
@@ -216,7 +224,7 @@ onMounted(() => void loadCart());
                 <PriceText :amount-minor="item.unit_price_minor * item.qty" :currency="item.currency" />
               </strong>
             </div>
-            <div v-if="couponsForShop(group.shopId).length > 0" class="coupon-picker">
+            <div v-if="!groupIntent && couponsForShop(group.shopId).length > 0" class="coupon-picker">
               <span class="coupon-label">{{ t("checkout.coupon") }}</span>
               <select v-model="selectedCoupon[group.shopId]" class="minput coupon-select">
                 <option value="">{{ t("checkout.noCoupon") }}</option>
diff --git a/apps/mall/pages/collective.vue b/apps/mall/pages/collective.vue
index d6236e3..e26aeb0 100644
--- a/apps/mall/pages/collective.vue
+++ b/apps/mall/pages/collective.vue
@@ -1,22 +1,78 @@
 <script setup lang="ts">
-import { t as pick } from "@vmall/shared";
-import { collectiveProducts, lowestSku } from "~/mock/data";
+import { ApiError, t as pick } from "@vmall/shared";
+import type { GroupBuyingActivityView, GroupBuyIntent } from "@vmall/shared";
 
 const { locale, t } = useI18n();
-const products = collectiveProducts();
+const { $api } = useNuxtApp();
+const session = useSessionStore();
+const cart = useCartStore();
+const router = useRouter();
+const activities = ref<GroupBuyingActivityView[]>([]);
+const loading = ref(true);
+const error = ref("");
+const workingId = ref("");
+const selectedGroup = reactive<Record<string, string>>({});
 
 const breadcrumb = computed(() => [
   { label: t("stores.breadcrumbHome"), to: "/" },
   { label: t("marketing.collectiveBreadcrumb") },
 ]);
 
-function currencyOf(product: (typeof products)[number]["product"]): string {
-  return lowestSku(product)?.currency ?? "USD";
-}
-function priceMinorOf(product: (typeof products)[number]["product"]): number {
-  return lowestSku(product)?.price_minor ?? 0;
+async function load(): Promise<void> {
+  loading.value = true;
+  try {
+    activities.value = await $api.listGroupBuyingActivities();
+  } catch {
+    activities.value = [];
+  } finally {
+    loading.value = false;
+  }
 }
 
+function chosenGroup(activityId: string): string {
+  return selectedGroup[activityId] ?? "";
+}
+
+function joined(activity: GroupBuyingActivityView, groupId: string): string {
+  const group = activity.open_groups.find((entry) => entry.id === groupId);
+  return group ? `${group.paid_member_count}/${group.required_members}` : "";
+}
+
+// Opening or joining both add the activity SKU to the cart and stash the
+// intent; the server prices and validates it at checkout.
+async function start(activity: GroupBuyingActivityView, open: boolean): Promise<void> {
+  if (!session.isLoggedIn) {
+    await navigateTo("/login");
+    return;
+  }
+  error.value = "";
+  const groupId = open ? null : chosenGroup(activity.id);
+  if (!open && !groupId) {
+    error.value = t("marketing.chooseGroup");
+    return;
+  }
+  workingId.value = activity.id;
+  try {
+    await $api.addCartItem(activity.sku_id, 1);
+    await cart.refresh();
+    useState<GroupBuyIntent | null>("checkout-group-intent", () => null).value = {
+      activity_id: activity.id,
+      sku_id: activity.sku_id,
+      group_id: groupId,
+    };
+    await router.push("/checkout");
+  } catch (err: unknown) {
+    error.value =
+      err instanceof ApiError ? err.message : t("marketing.groupFailed");
+  } finally {
+    workingId.value = "";
+  }
+}
+
+onMounted(() => {
+  session.hydrate();
+  void load();
+});
 </script>
 
 <template>
@@ -27,19 +83,53 @@ function priceMinorOf(product: (typeof products)[number]["product"]): number {
       <header class="section-heading">
         <h1>{{ t("marketing.collectiveTitle") }}</h1>
       </header>
-      <div v-if="products.length" class="collective-grid">
-        <article v-for="item in products" :key="item.product.id" class="collective-card hover-lift">
-          <NuxtLink :to="`/goods/${item.product.slug}`" class="product-image">
-            <img :src="item.product.images[0]" :alt="pick(item.product.name, locale)" loading="lazy" />
+      <p v-if="error" class="collective-error" role="alert">{{ error }}</p>
+      <div v-if="loading" class="muted">{{ t("common.loading") }}</div>
+      <div v-else-if="activities.length" class="collective-grid">
+        <article v-for="activity in activities" :key="activity.id" class="collective-card hover-lift">
+          <NuxtLink :to="`/goods/${activity.product_slug}`" class="product-image">
+            <img :src="activity.image || '/mock/product-1.svg'" :alt="pick(activity.product_name, locale)" loading="lazy" />
           </NuxtLink>
           <div class="product-body">
-            <NuxtLink :to="`/goods/${item.product.slug}`" class="product-name">{{ pick(item.product.name, locale) }}</NuxtLink>
+            <NuxtLink :to="`/goods/${activity.product_slug}`" class="product-name">{{ pick(activity.name, locale) }}</NuxtLink>
             <div class="deal-meta">
-              <span class="deal-price"><PriceText :amount-minor="priceMinorOf(item.product)" :currency="currencyOf(item.product)" /></span>
-              <span class="group-tag">{{ t("marketing.peopleGroup", { n: item.need }) }}</span>
+              <span class="deal-price"><PriceText :amount-minor="activity.group_price_minor" :currency="activity.currency" /></span>
+              <span class="group-tag">{{ t("marketing.peopleGroup", { n: activity.required_members }) }}</span>
             </div>
-            <p class="joined">{{ t("marketing.peopleJoined", { n: item.joined }) }}</p>
-            <NuxtLink :to="`/goods/${item.product.slug}`" class="deal-link">{{ t("marketing.joinGroup") }} <span aria-hidden="true">›</span></NuxtLink>
+            <p class="joined">
+              <s><PriceText :amount-minor="activity.original_price_minor" :currency="activity.original_currency" /></s>
+            </p>
+
+            <div v-if="activity.open_groups.length" class="group-list">
+              <label v-for="group in activity.open_groups" :key="group.id" class="group-option">
+                <input v-model="selectedGroup[activity.id]" type="radio" :name="`group-${activity.id}`" :value="group.id" />
+                <span>{{ t("marketing.paidMembers", { n: group.paid_member_count, total: group.required_members }) }}</span>
+                <span class="group-expiry">{{ group.expires_at.slice(5, 16).replace('T', ' ') }}</span>
+              </label>
+            </div>
+            <p v-else class="joined">{{ t("marketing.noOpenGroups") }}</p>
+
+            <div class="group-actions">
+              <button
+                type="button"
+                class="mbtn red"
+                :disabled="workingId === activity.id"
+                @click="start(activity, true)"
+              >
+                {{ t("marketing.openGroup") }}
+              </button>
+              <button
+                type="button"
+                class="mbtn"
+                :disabled="workingId === activity.id || !chosenGroup(activity.id)"
+                @click="start(activity, false)"
+              >
+                {{ t("marketing.joinGroup") }}
+              </button>
+            </div>
+            <p v-if="chosenGroup(activity.id)" class="joined">
+              {{ joined(activity, chosenGroup(activity.id)) }}
+            </p>
           </div>
         </article>
       </div>
@@ -168,4 +258,40 @@ function priceMinorOf(product: (typeof products)[number]["product"]): number {
     grid-template-columns: repeat(2, minmax(0, 1fr));
   }
 }
+.collective-error {
+  margin: 12px 0 0;
+  padding: 10px 14px;
+  color: #b42318;
+  background: #fff1f0;
+  border: 1px solid #ffd6d2;
+}
+.group-list {
+  display: flex;
+  flex-direction: column;
+  gap: 4px;
+  margin: 8px 0;
+}
+.group-option {
+  display: flex;
+  align-items: center;
+  gap: 6px;
+  font-size: 12px;
+  color: var(--mall-muted);
+  cursor: pointer;
+}
+.group-expiry {
+  margin-left: auto;
+  color: var(--mall-faint);
+  font-size: 11px;
+}
+.group-actions {
+  display: flex;
+  gap: 6px;
+  margin-top: 8px;
+}
+.group-actions .mbtn {
+  flex: 1;
+  padding: 5px 8px;
+  text-align: center;
+}
 </style>
diff --git a/apps/mall/plugins/api.ts b/apps/mall/plugins/api.ts
index 60d006e..41605a7 100644
--- a/apps/mall/plugins/api.ts
+++ b/apps/mall/plugins/api.ts
@@ -22,7 +22,8 @@ type LiveDomain =
   | "addresses"
   | "coupons"
   | "points"
-  | "flashSales";
+  | "flashSales"
+  | "groupBuying";
 
 /**
  * Explicit per-domain method picks rather than a string allowlist: indexing
@@ -80,6 +81,9 @@ const LIVE_PICKS = {
     redeemPoints: a.redeemPoints,
   }),
   flashSales: (a: ApiClient) => ({ listFlashSales: a.listFlashSales }),
+  groupBuying: (a: ApiClient) => ({
+    listGroupBuyingActivities: a.listGroupBuyingActivities,
+  }),
 } satisfies Record<LiveDomain, (a: ApiClient) => Partial<ApiClient>>;
 
 const KNOWN_DOMAINS = Object.keys(LIVE_PICKS) as LiveDomain[];
@@ -101,6 +105,7 @@ const DEFAULT_LIVE_DOMAINS: LiveDomain[] = [
   "coupons",
   "points",
   "flashSales",
+  "groupBuying",
 ];
 
 export default defineNuxtPlugin(() => {
diff --git a/packages/shared/src/api.ts b/packages/shared/src/api.ts
index a95fdf4..08a690b 100644
--- a/packages/shared/src/api.ts
+++ b/packages/shared/src/api.ts
@@ -17,6 +17,10 @@ import type {
   FlashSaleItemInput,
   FlashSaleSession,
   FlashSaleSessionInput,
+  GroupBuyIntent,
+  GroupBuyingActivity,
+  GroupBuyingActivityInput,
+  GroupBuyingActivityView,
   HomeContent,
   IntegralOrder,
   IntegralProduct,
@@ -182,6 +186,8 @@ export interface ApiClient {
     currency: string,
     /** At most one owned coupon per generated shop order. */
     couponByShop?: Record<string, string>,
+    /** Optional group-buying intent for one activity SKU at quantity 1. */
+    groupBuy?: GroupBuyIntent,
   ): Promise<Order[]>;
   listMyOrders(page?: number): Promise<Paged<Order>>;
   getOrder(id: string): Promise<Order>;
@@ -216,6 +222,8 @@ export interface ApiClient {
   redeemPoints(body: RedeemPointsBody): Promise<IntegralOrder>;
   /** Public: active flash-sale sessions with their purchasable items. */
   listFlashSales(): Promise<PublicFlashSaleSession[]>;
+  /** Public: active group-buying activities with their open groups. */
+  listGroupBuyingActivities(): Promise<GroupBuyingActivityView[]>;
   shop: {
     getMyShop(): Promise<Shop>;
     listMyProducts(q?: ShopProductQuery): Promise<Paged<Product>>;
@@ -248,6 +256,13 @@ export interface ApiClient {
     addFlashSaleItem(sessionId: string, body: FlashSaleItemInput): Promise<FlashSaleItem>;
     updateFlashSaleItem(id: string, body: FlashSaleItemInput): Promise<FlashSaleItem>;
     deleteFlashSaleItem(id: string): Promise<void>;
+    listGroupBuyingActivities(): Promise<GroupBuyingActivityView[]>;
+    createGroupBuyingActivity(body: GroupBuyingActivityInput): Promise<GroupBuyingActivity>;
+    updateGroupBuyingActivity(
+      id: string,
+      body: GroupBuyingActivityInput,
+    ): Promise<GroupBuyingActivity>;
+    deleteGroupBuyingActivity(id: string): Promise<void>;
   };
   admin: {
     listUsers(page?: number): Promise<Paged<User>>;
@@ -289,11 +304,12 @@ export function createApi(opts: ApiClientOptions): ApiClient {
     addCartItem: (skuId, qty) => r("POST", "/cart/items", { sku_id: skuId, qty }),
     updateCartItem: (skuId, qty) => r("PUT", `/cart/items/${skuId}`, { qty }),
     removeCartItem: (skuId) => r("DELETE", `/cart/items/${skuId}`),
-    checkout: (shippingAddress, currency, couponByShop = {}) =>
+    checkout: (shippingAddress, currency, couponByShop = {}, groupBuy) =>
       r("POST", "/orders/checkout", {
         shipping_address: shippingAddress,
         currency,
         coupon_by_shop: couponByShop,
+        group_buy: groupBuy ?? null,
       }),
     listMyOrders: (page = 1) => r("GET", "/orders", undefined, { page }),
     getOrder: (id) => r("GET", `/orders/${id}`),
@@ -319,6 +335,7 @@ export function createApi(opts: ApiClientOptions): ApiClient {
     listMyRedemptions: () => r("GET", "/points/redemptions"),
     redeemPoints: (body) => r("POST", "/points/redemptions", body),
     listFlashSales: () => r("GET", "/flash-sales"),
+    listGroupBuyingActivities: () => r("GET", "/group-buying/activities"),
     shop: {
       getMyShop: () => r("GET", "/shop/profile"),
       listMyProducts: (q = {}) => r("GET", "/shop/products", undefined, { ...q }),
@@ -352,6 +369,11 @@ export function createApi(opts: ApiClientOptions): ApiClient {
         r("POST", `/shop/flash-sales/${sessionId}/items`, body),
       updateFlashSaleItem: (id, body) => r("PUT", `/shop/flash-sale-items/${id}`, body),
       deleteFlashSaleItem: (id) => r("DELETE", `/shop/flash-sale-items/${id}`),
+      listGroupBuyingActivities: () => r("GET", "/shop/group-buying-activities"),
+      createGroupBuyingActivity: (body) => r("POST", "/shop/group-buying-activities", body),
+      updateGroupBuyingActivity: (id, body) =>
+        r("PUT", `/shop/group-buying-activities/${id}`, body),
+      deleteGroupBuyingActivity: (id) => r("DELETE", `/shop/group-buying-activities/${id}`),
     },
     admin: {
       listUsers: (page = 1) => r("GET", "/admin/users", undefined, { page }),
diff --git a/packages/shared/src/types.ts b/packages/shared/src/types.ts
index e1b4358..e5e7207 100644
--- a/packages/shared/src/types.ts
+++ b/packages/shared/src/types.ts
@@ -161,6 +161,10 @@ export interface Order {
   discount_minor: number;
   /** The coupon this order redeemed, or null. */
   coupon_id: string | null;
+  /** The group-buying activity this order joined, or null. */
+  group_activity_id: string | null;
+  /** The concrete group this order belongs to, or null. */
+  group_id: string | null;
   items: OrderItem[];
   shipping_address: Address;
   created_at: string;
@@ -398,6 +402,69 @@ export interface PublicFlashSaleSession extends FlashSaleSession {
   items: PublicFlashSaleItem[];
 }
 
+// ---- group buying ----
+
+export type CollectiveGroupStatus = "open" | "successful" | "expired" | "cancelled";
+
+/** Shop-owned timed activity on one SKU; checkout takes it at quantity 1. */
+export interface GroupBuyingActivity {
+  id: string;
+  shop_id: string;
+  sku_id: string;
+  name: LocalizedText;
+  description: LocalizedText | null;
+  image: string | null;
+  group_price_minor: number;
+  currency: string;
+  required_members: number;
+  starts_at: string;
+  ends_at: string;
+  group_lifetime_hours: number;
+  enabled: boolean;
+  created_at: string;
+  updated_at: string;
+}
+
+export interface GroupBuyingActivityInput {
+  sku_id: string;
+  name: LocalizedText;
+  description?: LocalizedText | null;
+  image?: string | null;
+  group_price_minor: number;
+  currency: string;
+  required_members: number;
+  starts_at: string;
+  ends_at: string;
+  group_lifetime_hours: number;
+  enabled?: boolean;
+}
+
+/** A group a shopper can join. */
+export interface OpenGroup {
+  id: string;
+  paid_member_count: number;
+  required_members: number;
+  expires_at: string;
+}
+
+/** Activity plus the SKU/catalog details both surfaces need. */
+export interface GroupBuyingActivityView extends GroupBuyingActivity {
+  sku_code: string;
+  product_id: string;
+  product_slug: string;
+  product_name: LocalizedText;
+  original_price_minor: number;
+  original_currency: string;
+  open_groups: OpenGroup[];
+}
+
+/** Checkout intent: one activity SKU at quantity 1, joining or opening. */
+export interface GroupBuyIntent {
+  activity_id: string;
+  sku_id: string;
+  group_id?: string | null;
+}
+
 export type ShipmentStatus = "pending" | "shipped" | "delivered";export interface Shipment {
   id: string;
   shipment_no: string;