diff --git a/workloads/ecommerce/app/api/cart/items/[lineItemId]/route.ts b/workloads/ecommerce/app/api/cart/items/[lineItemId]/route.ts
new file mode 100644
index 0000000..a144c1c
--- /dev/null
+++ b/workloads/ecommerce/app/api/cart/items/[lineItemId]/route.ts
@@ -0,0 +1,31 @@
+import { removeLineItem, updateLineItemQuantity } from "@/lib/headless/providers/medusa-cart";
+
+interface RouteParams { params: Promise<{ lineItemId: string }> }
+
+export async function POST(request: Request, { params }: RouteParams) {
+  const { lineItemId } = await params;
+  const { cartId, quantity } = await request.json();
+  if (!cartId || typeof quantity !== "number") {
+    return Response.json({ error: "cartId y quantity son obligatorios" }, { status: 400 });
+  }
+  try {
+    const cart = await updateLineItemQuantity(cartId, lineItemId, quantity);
+    return Response.json(cart);
+  } catch (error) {
+    return Response.json({ error: (error as Error).message }, { status: 502 });
+  }
+}
+
+export async function DELETE(request: Request, { params }: RouteParams) {
+  const { lineItemId } = await params;
+  const cartId = new URL(request.url).searchParams.get("cartId");
+  if (!cartId) {
+    return Response.json({ error: "cartId es obligatorio" }, { status: 400 });
+  }
+  try {
+    const cart = await removeLineItem(cartId, lineItemId);
+    return Response.json(cart);
+  } catch (error) {
+    return Response.json({ error: (error as Error).message }, { status: 502 });
+  }
+}
diff --git a/workloads/ecommerce/app/api/cart/items/route.ts b/workloads/ecommerce/app/api/cart/items/route.ts
new file mode 100644
index 0000000..86c40ac
--- /dev/null
+++ b/workloads/ecommerce/app/api/cart/items/route.ts
@@ -0,0 +1,14 @@
+import { addLineItem } from "@/lib/headless/providers/medusa-cart";
+
+export async function POST(request: Request) {
+  const { cartId, variantId, quantity } = await request.json();
+  if (!cartId || !variantId) {
+    return Response.json({ error: "cartId y variantId son obligatorios" }, { status: 400 });
+  }
+  try {
+    const cart = await addLineItem(cartId, variantId, quantity ?? 1);
+    return Response.json(cart);
+  } catch (error) {
+    return Response.json({ error: (error as Error).message }, { status: 502 });
+  }
+}
diff --git a/workloads/ecommerce/app/api/cart/route.ts b/workloads/ecommerce/app/api/cart/route.ts
new file mode 100644
index 0000000..c26e4f4
--- /dev/null
+++ b/workloads/ecommerce/app/api/cart/route.ts
@@ -0,0 +1,28 @@
+import { createCart, getCart } from "@/lib/headless/providers/medusa-cart";
+
+// cart-store.ts es "use client" y no puede pegarle directo a Medusa
+// (MEDUSA_BACKEND_URL es DNS interno del cluster, no resuelve desde el
+// navegador) -- mismo motivo que /api/search. Nunca cachear: es
+// estado mutable por sesion, no catalogo.
+
+export async function POST() {
+  try {
+    const cart = await createCart();
+    return Response.json(cart);
+  } catch (error) {
+    return Response.json({ error: (error as Error).message }, { status: 502 });
+  }
+}
+
+export async function GET(request: Request) {
+  const cartId = new URL(request.url).searchParams.get("cartId");
+  if (!cartId) {
+    return Response.json({ error: "cartId es obligatorio" }, { status: 400 });
+  }
+  try {
+    const cart = await getCart(cartId);
+    return Response.json(cart);
+  } catch (error) {
+    return Response.json({ error: (error as Error).message }, { status: 502 });
+  }
+}
diff --git a/workloads/ecommerce/lib/headless/providers/medusa-cart.ts b/workloads/ecommerce/lib/headless/providers/medusa-cart.ts
new file mode 100644
index 0000000..e1a0a1c
--- /dev/null
+++ b/workloads/ecommerce/lib/headless/providers/medusa-cart.ts
@@ -0,0 +1,164 @@
+// ADR 0009: carrito real conectado a Medusa (Store API), en vez del
+// estado 100% local que existia antes. Archivo separado de medusa.ts
+// a proposito -- ese es de solo lectura (catalogo, cacheado con
+// revalidate/tags), este es de escritura (carrito, siempre no-store,
+// nunca debe cachearse).
+//
+// Formas de respuesta verificadas en vivo contra la API real
+// (2026-08-29), no asumidas contra documentacion:
+// - crear/agregar/actualizar linea: { cart: {...} }
+// - eliminar linea: { id, object, deleted, parent: {...} } -- el
+//   carrito actualizado viene en "parent", no en "cart".
+// - agregar el mismo variant_id dos veces hace merge de cantidad en la
+//   misma linea (confirmado con un carrito de prueba real), no crea
+//   una linea duplicada -- por eso addLineItem siempre manda
+//   quantity=1 sin revisar antes si ya existe.
+
+export interface CartLineProduct {
+  id: string;
+  name: string;
+  thumbnail: string;
+  price: number | null;
+  currency: string;
+}
+
+export interface CartLine {
+  lineItemId: string;
+  product: CartLineProduct;
+  quantity: number;
+  variantId: string;
+}
+
+export interface CartSummary {
+  cartId: string;
+  currency: string;
+  subtotal: number;
+  items: CartLine[];
+}
+
+function backendUrl(): string {
+  const value = process.env.MEDUSA_BACKEND_URL;
+  if (!value) {
+    throw new Error("MEDUSA_BACKEND_URL no está configurado.");
+  }
+  return value.replace(/\/$/, "");
+}
+
+function regionId(): string {
+  const value = process.env.MEDUSA_REGION_ID;
+  if (!value) {
+    throw new Error("MEDUSA_REGION_ID no está configurado.");
+  }
+  return value;
+}
+
+type MedusaLineItem = {
+  id: string;
+  product_id: string;
+  variant_id: string;
+  title?: string | null;
+  product_title?: string | null;
+  thumbnail?: string | null;
+  quantity: number;
+  unit_price?: number | null;
+};
+
+type MedusaCart = {
+  id: string;
+  currency_code: string;
+  subtotal?: number | null;
+  items?: MedusaLineItem[] | null;
+};
+
+function mapCart(cart: MedusaCart): CartSummary {
+  return {
+    cartId: cart.id,
+    currency: cart.currency_code,
+    subtotal: cart.subtotal ?? 0,
+    items: (cart.items ?? []).map((item) => ({
+      lineItemId: item.id,
+      variantId: item.variant_id,
+      quantity: item.quantity,
+      product: {
+        id: item.product_id,
+        name: item.product_title || item.title || "Producto",
+        // La linea del carrito no trae el arreglo de imagenes del producto,
+        // solo este campo (a veces null) -- mismo fallback final que ya usa
+        // mapProduct() en medusa.ts.
+        thumbnail: item.thumbnail || "/images/brand/logo-ari.png",
+        price: typeof item.unit_price === "number" ? item.unit_price : null,
+        currency: cart.currency_code,
+      },
+    })),
+  };
+}
+
+async function medusaCartFetch(
+  path: string,
+  init?: { method?: string; body?: unknown },
+): Promise<Record<string, unknown>> {
+  const publishableKey = process.env.MEDUSA_PUBLISHABLE_KEY;
+
+  const response = await fetch(`${backendUrl()}${path}`, {
+    method: init?.method ?? "GET",
+    headers: {
+      "Content-Type": "application/json",
+      ...(publishableKey ? { "x-publishable-api-key": publishableKey } : {}),
+    },
+    body: init?.body ? JSON.stringify(init.body) : undefined,
+    cache: "no-store",
+  });
+
+  if (!response.ok) {
+    const text = await response.text().catch(() => "");
+    throw new Error(`Medusa cart API ${response.status}: ${text.slice(0, 300)}`);
+  }
+
+  return response.json();
+}
+
+export async function createCart(): Promise<CartSummary> {
+  const data = await medusaCartFetch("/store/carts", {
+    method: "POST",
+    body: { region_id: regionId() },
+  });
+  return mapCart(data.cart as MedusaCart);
+}
+
+export async function getCart(cartId: string): Promise<CartSummary> {
+  const data = await medusaCartFetch(`/store/carts/${cartId}`);
+  return mapCart(data.cart as MedusaCart);
+}
+
+export async function addLineItem(
+  cartId: string,
+  variantId: string,
+  quantity: number,
+): Promise<CartSummary> {
+  const data = await medusaCartFetch(`/store/carts/${cartId}/line-items`, {
+    method: "POST",
+    body: { variant_id: variantId, quantity },
+  });
+  return mapCart(data.cart as MedusaCart);
+}
+
+export async function updateLineItemQuantity(
+  cartId: string,
+  lineItemId: string,
+  quantity: number,
+): Promise<CartSummary> {
+  const data = await medusaCartFetch(`/store/carts/${cartId}/line-items/${lineItemId}`, {
+    method: "POST",
+    body: { quantity },
+  });
+  return mapCart(data.cart as MedusaCart);
+}
+
+export async function removeLineItem(cartId: string, lineItemId: string): Promise<CartSummary> {
+  const data = await medusaCartFetch(`/store/carts/${cartId}/line-items/${lineItemId}`, {
+    method: "DELETE",
+  });
+  // Forma distinta a las demas: el carrito actualizado viene en
+  // "parent", no en "cart" -- verificado en vivo, no documentado.
+  return mapCart(data.parent as MedusaCart);
+}
diff --git a/workloads/ecommerce/store/cart-store.ts b/workloads/ecommerce/store/cart-store.ts
index f24b9b8..f5a9e4d 100644
--- a/workloads/ecommerce/store/cart-store.ts
+++ b/workloads/ecommerce/store/cart-store.ts
@@ -2,60 +2,117 @@
 
 import { create } from "zustand";
 import { createJSONStorage, persist } from "zustand/middleware";
-import type { Product } from "@/lib/headless/types";
+import type { CartLine } from "@/lib/headless/providers/medusa-cart";
 
-export interface CartItem {
-  product: Product;
-  quantity: number;
-  variantId?: string;
-}
+// ADR 0009: carrito real conectado a un cart_id de Medusa (Store API,
+// via /api/cart/*) en vez de estado 100% local -- precios y existencia
+// de linea son los que confirma el servidor, no un calculo optimista
+// en el cliente. Primer corte a proposito: sin UI de direccion/envio
+// todavia, el checkout sigue cerrando por WhatsApp (CartDrawer).
+export type CartItem = CartLine;
 
 interface CartState {
+  cartId: string | null;
   items: CartItem[];
   isOpen: boolean;
   hasHydrated: boolean;
   openCart: () => void;
   closeCart: () => void;
   setHasHydrated: (value: boolean) => void;
-  addItem: (product: Product, variantId?: string) => void;
-  removeItem: (productId: string) => void;
-  updateQuantity: (productId: string, quantity: number) => void;
-  clearCart: () => void;
+  addItem: (product: { id: string }, variantId?: string) => Promise<void>;
+  removeItem: (productId: string) => Promise<void>;
+  updateQuantity: (productId: string, quantity: number) => Promise<void>;
+  clearCart: () => Promise<void>;
+  syncCart: () => Promise<void>;
+}
+
+async function postJson(url: string, body: unknown) {
+  const response = await fetch(url, {
+    method: "POST",
+    headers: { "Content-Type": "application/json" },
+    body: JSON.stringify(body),
+  });
+  return response.json();
 }
 
 export const useCartStore = create<CartState>()(
   persist(
-    (set) => ({
+    (set, get) => ({
+      cartId: null,
       items: [],
       isOpen: false,
       hasHydrated: false,
       openCart: () => set({ isOpen: true }),
       closeCart: () => set({ isOpen: false }),
       setHasHydrated: (value) => set({ hasHydrated: value }),
-      addItem: (product, variantId) => set((state) => {
-        const existing = state.items.find(
-          (item) => item.product.id === product.id && item.variantId === variantId,
+
+      addItem: async (product, variantId) => {
+        set({ isOpen: true });
+        if (!variantId) return;
+
+        let cartId = get().cartId;
+        if (!cartId) {
+          const cart = await (await fetch("/api/cart", { method: "POST" })).json();
+          if (!cart.cartId) return;
+          cartId = cart.cartId;
+          set({ cartId, items: cart.items ?? [] });
+        }
+
+        const cart = await postJson("/api/cart/items", { cartId, variantId, quantity: 1 });
+        if (cart.items) set({ items: cart.items });
+      },
+
+      removeItem: async (productId) => {
+        const { cartId, items } = get();
+        const line = items.find((item) => item.product.id === productId);
+        if (!cartId || !line) return;
+        const response = await fetch(`/api/cart/items/${line.lineItemId}?cartId=${cartId}`, { method: "DELETE" });
+        const cart = await response.json();
+        if (cart.items) set({ items: cart.items });
+      },
+
+      updateQuantity: async (productId, quantity) => {
+        if (quantity <= 0) return get().removeItem(productId);
+        const { cartId, items } = get();
+        const line = items.find((item) => item.product.id === productId);
+        if (!cartId || !line) return;
+        const cart = await postJson(`/api/cart/items/${line.lineItemId}`, { cartId, quantity });
+        if (cart.items) set({ items: cart.items });
+      },
+
+      clearCart: async () => {
+        const { cartId, items } = get();
+        if (!cartId) return;
+        await Promise.all(
+          items.map((item) => fetch(`/api/cart/items/${item.lineItemId}?cartId=${cartId}`, { method: "DELETE" })),
         );
-        const items = existing
-          ? state.items.map((item) => item === existing ? { ...item, quantity: item.quantity + 1 } : item)
-          : [...state.items, { product, quantity: 1, variantId }];
-        return { items, isOpen: true };
-      }),
-      removeItem: (productId) => set((state) => ({
-        items: state.items.filter((item) => item.product.id !== productId),
-      })),
-      updateQuantity: (productId, quantity) => set((state) => ({
-        items: quantity <= 0
-          ? state.items.filter((item) => item.product.id !== productId)
-          : state.items.map((item) => item.product.id === productId ? { ...item, quantity } : item),
-      })),
-      clearCart: () => set({ items: [] }),
+        set({ items: [] });
+      },
+
+      // Al recargar la pagina, el cart_id guardado en localStorage puede
+      // llevar dias -- se refresca contra el servidor en vez de confiar en
+      // el ultimo snapshot local (precios/stock pudieron cambiar).
+      syncCart: async () => {
+        const cartId = get().cartId;
+        if (!cartId) return;
+        try {
+          const response = await fetch(`/api/cart?cartId=${cartId}`);
+          if (!response.ok) return;
+          const cart = await response.json();
+          if (cart.items) set({ items: cart.items });
+        } catch {
+          // Sin red: se queda con el ultimo estado local conocido.
+        }
+      },
     }),
     {
-      name: "ari-shopping-cart-v4",
+      name: "ari-shopping-cart-v5",
       storage: createJSONStorage(() => localStorage),
-      partialize: (state) => ({ items: state.items }),
-      onRehydrateStorage: () => (state) => state?.setHasHydrated(true),
+      partialize: (state) => ({ cartId: state.cartId, items: state.items }),
+      onRehydrateStorage: () => (state) => {
+        state?.setHasHydrated(true);
+        void state?.syncCart();
+      },
     },
   ),
 );