diff --git a/src/app/actions.ts b/src/app/actions.ts
index 2924ac3..90f6ae0 100644
--- a/src/app/actions.ts
+++ b/src/app/actions.ts
@@ -5,6 +5,7 @@ import { prisma } from '@/lib/prisma';
 import { upsertMailingListEntry } from '@/lib/mailingList';
 import { verifyTurnstileToken } from '@/lib/turnstile';
 import { getClientIp } from '@/lib/rateLimit';
+import { getCurrentCustomer } from '@/lib/auth';
 
 const EMAIL_RE = /^[^\s@]+@[^\s@]+\.[^\s@]+$/;
 
@@ -30,3 +31,8 @@ export async function subscribeToNewsletter(email: string, turnstileToken: strin
 
   return { ok: true, message: "Thanks — you're on the list." };
 }
+
+export async function checkCustomerAuth() {
+  const customer = await getCurrentCustomer();
+  return { isAuthenticated: !!customer };
+}
diff --git a/src/components/DesignCanvas.tsx b/src/components/DesignCanvas.tsx
index 760b0c8..c5855e4 100644
--- a/src/components/DesignCanvas.tsx
+++ b/src/components/DesignCanvas.tsx
@@ -6,8 +6,10 @@ import type Konva from 'konva';
 import { v4 as uuid } from 'uuid';
 import ProductMockup from './ProductMockup';
 import Price from './Price';
+import LoginPromptModal from './LoginPromptModal';
 import { useCart } from '@/lib/cartStore';
 import { getEffectivePrice } from '@/lib/pricing';
+import { checkCustomerAuth } from '@/app/actions';
 import type { DesignElement, PrintArea, ProductDTO } from '@/lib/types';
 
 const DISPLAY = 560; // css px size the mockup + stage are rendered at
@@ -310,6 +312,7 @@ export default function DesignCanvas({ product }: { product: ProductDTO }) {
   const [selectedId, setSelectedId] = useState<string | null>(null);
   const [quantity, setQuantity] = useState(1);
   const [justAdded, setJustAdded] = useState(false);
+  const [showLoginPrompt, setShowLoginPrompt] = useState(false);
   const [nodeReadyTick, setNodeReadyTick] = useState(0);
 
   const stageFrontRef = useRef<Konva.Stage | null>(null);
@@ -462,7 +465,12 @@ export default function DesignCanvas({ product }: { product: ProductDTO }) {
       ).price
     : product.effectivePrice;
 
-  const handleAddToBag = () => {
+  const handleAddToBag = async () => {
+    const { isAuthenticated } = await checkCustomerAuth();
+    if (!isAuthenticated) {
+      setShowLoginPrompt(true);
+    }
+
     if (!canPersonalize) {
       const flatPhoto = product.colorPhotos[color] ?? product.imageUrl ?? '';
       addItem({
@@ -861,6 +869,8 @@ export default function DesignCanvas({ product }: { product: ProductDTO }) {
         </div>
         {justAdded && <p className="text-sm text-pine">Added to your bag.</p>}
       </div>
+
+      <LoginPromptModal isOpen={showLoginPrompt} onClose={() => setShowLoginPrompt(false)} />
     </div>
   );
 }
diff --git a/src/components/LoginPromptModal.tsx b/src/components/LoginPromptModal.tsx
new file mode 100644
index 0000000..2ebdb61
--- /dev/null
+++ b/src/components/LoginPromptModal.tsx
@@ -0,0 +1,47 @@
+'use client';
+
+import Link from 'next/link';
+
+interface LoginPromptModalProps {
+  isOpen: boolean;
+  onClose: () => void;
+}
+
+export default function LoginPromptModal({ isOpen, onClose }: LoginPromptModalProps) {
+  if (!isOpen) return null;
+
+  return (
+    <div className="fixed inset-0 z-50 flex items-center justify-center bg-black/50">
+      <div className="mx-4 w-full max-w-sm rounded-lg border border-line bg-paper p-6 shadow-lg">
+        <h2 className="font-display text-2xl">Create an account to shop</h2>
+        <p className="mt-3 text-sm text-muted">
+          Add items to your bag and save your designs. You'll need an account to checkout.
+        </p>
+
+        <div className="mt-6 space-y-3">
+          <Link
+            href="/account/register"
+            onClick={onClose}
+            className="block w-full rounded bg-clay px-4 py-3 text-center text-sm font-medium text-paper transition-colors hover:bg-clay-dark"
+          >
+            Create account
+          </Link>
+          <Link
+            href="/account/login"
+            onClick={onClose}
+            className="block w-full rounded border border-ink px-4 py-3 text-center text-sm font-medium text-ink transition-colors hover:border-clay hover:bg-clay hover:text-paper"
+          >
+            Log in
+          </Link>
+        </div>
+
+        <button
+          onClick={onClose}
+          className="mt-4 w-full text-center text-sm text-muted hover:text-ink"
+        >
+          Continue as guest
+        </button>
+      </div>
+    </div>
+  );
+}
diff --git a/src/components/StandardProductView.tsx b/src/components/StandardProductView.tsx
index 3e37def..db28829 100644
--- a/src/components/StandardProductView.tsx
+++ b/src/components/StandardProductView.tsx
@@ -4,8 +4,10 @@ import { useState } from 'react';
 import { v4 as uuid } from 'uuid';
 import ProductMockup from './ProductMockup';
 import Price from './Price';
+import LoginPromptModal from './LoginPromptModal';
 import { useCart } from '@/lib/cartStore';
 import { getColorName } from '@/lib/colorNames';
+import { checkCustomerAuth } from '@/app/actions';
 import type { ProductDTO } from '@/lib/types';
 
 export default function StandardProductView({ product }: { product: ProductDTO }) {
@@ -14,6 +16,7 @@ export default function StandardProductView({ product }: { product: ProductDTO }
   const [view, setView] = useState<'front' | 'back'>('front');
   const [quantity, setQuantity] = useState(1);
   const [justAdded, setJustAdded] = useState(false);
+  const [showLoginPrompt, setShowLoginPrompt] = useState(false);
   const addItem = useCart((s) => s.addItem);
 
   const hasBack = Boolean(product.imageUrlBack);
@@ -44,7 +47,12 @@ export default function StandardProductView({ product }: { product: ProductDTO }
     return <img src={activePhoto} alt={product.name} className="h-full w-full object-cover" />;
   }
 
-  const handleAddToBag = () => {
+  const handleAddToBag = async () => {
+    const { isAuthenticated } = await checkCustomerAuth();
+    if (!isAuthenticated) {
+      setShowLoginPrompt(true);
+    }
+
     addItem({
       cartItemId: uuid(),
       productId: product.id,
@@ -182,6 +190,8 @@ export default function StandardProductView({ product }: { product: ProductDTO }
         </div>
         {justAdded && <p className="text-sm text-pine">Added to your bag.</p>}
       </div>
+
+      <LoginPromptModal isOpen={showLoginPrompt} onClose={() => setShowLoginPrompt(false)} />
     </div>
   );
 }