diff --git a/app/lib/cart.ts b/app/lib/cart.ts
index 28f5f3c..cef2330 100644
--- a/app/lib/cart.ts
+++ b/app/lib/cart.ts
@@ -132,18 +132,39 @@ export function useCart(): CartItem[] {
 // logout — the local cart is always either empty (no guest additions since
 // the last logout) or holds only genuinely new guest-session items, never a
 // stale copy of what's already in the server cart, so this add never
-// double-counts. CartSync then picks up the resulting change and pushes the
-// merged cart back to the server on its own, closing the loop without a
-// separate save call here.
+// double-counts.
+//
+// Always pushes the resulting cart to the server itself at the end,
+// explicitly — does NOT rely on CartSync's own change-triggered push.
+// That was the bug (confirmed live 2026-08-01): logging in with an empty
+// server cart but a non-empty guest cart means the merge loop below never
+// actually runs (nothing in `data.cart` to add), so `writeCart` is never
+// called, no change event fires, and CartSync's effect — which only
+// re-fires when the cart *reference* actually changes — never pushes the
+// guest cart to the server at all. The cart looked fine on the device
+// that was already logged in, but never reached any other device. An
+// explicit push here doesn't depend on anything having changed.
 export async function mergeServerCartIntoLocal(): Promise<void> {
   try {
     const res = await fetch("/api/account/cart");
-    if (!res.ok) return;
-    const data: { cart?: CartItem[] } = await res.json();
-    for (const item of data.cart ?? []) addToCart(item.id, item.qty, item.variant);
+    if (res.ok) {
+      const data: { cart?: CartItem[] } = await res.json();
+      for (const item of data.cart ?? []) addToCart(item.id, item.qty, item.variant);
+    }
   } catch {
     // Best-effort — a failed merge just means the server-side cart stays
-    // as it was; nothing local is lost either way.
+    // as it was; nothing local is lost either way. Still fall through to
+    // push below — the local (guest) cart is genuine either way.
+  }
+  try {
+    await fetch("/api/account/cart", {
+      method: "POST",
+      headers: { "Content-Type": "application/json" },
+      body: JSON.stringify({ cart: readCart() }),
+    });
+  } catch {
+    // Best-effort — CartSync's own push will retry on the next cart
+    // change, or the next login-time merge tries again.
   }
 }