diff --git a/backend/__tests__/unit/routes/billing.test.js b/backend/__tests__/unit/routes/billing.test.js
index 9e83e13d..6d89f77f 100644
--- a/backend/__tests__/unit/routes/billing.test.js
+++ b/backend/__tests__/unit/routes/billing.test.js
@@ -157,6 +157,61 @@ describe('/api/billing', () => {
       expect(arg.subscription_data.metadata.userId).toBe('u-1');
     });
 
+    // A customer created under test keys does not exist under live ones. This
+    // is not hypothetical: the first checkout after the 2026-08-06 test -> live
+    // cutover named a customer Stripe had never heard of.
+    describe('a stored customer id Stripe does not recognise', () => {
+      const missingCustomer = () => Object.assign(new Error("No such customer: 'cus_gone'"), {
+        code: 'resource_missing', param: 'customer',
+      });
+
+      test('is replaced and the checkout succeeds', async () => {
+        mockSessionsCreate
+          .mockRejectedValueOnce(missingCustomer())
+          .mockResolvedValueOnce({ url: 'https://checkout.stripe.test/s/2' });
+        mockCustomersCreate.mockResolvedValue({ id: 'cus_fresh' });
+
+        const res = await request(app).post('/api/billing/checkout').send({});
+        expect(res.status).toBe(200);
+        expect(res.body.url).toContain('/s/2');
+        expect(mockCurrentUser.value.billing.customerId).toBe('cus_fresh');
+        // The retry must use the NEW id, or it fails identically.
+        expect(mockSessionsCreate.mock.calls[1][0].customer).toBe('cus_fresh');
+      });
+
+      test('is retried exactly once, never in a loop', async () => {
+        mockSessionsCreate.mockRejectedValue(missingCustomer());
+        mockCustomersCreate.mockResolvedValue({ id: 'cus_fresh' });
+
+        const res = await request(app).post('/api/billing/checkout').send({});
+        expect(res.status).toBe(500);
+        expect(mockSessionsCreate).toHaveBeenCalledTimes(2);
+        expect(mockCustomersCreate).toHaveBeenCalledTimes(1);
+      });
+
+      // The dangerous false positive: a half-finished cutover leaves a TEST
+      // price id against LIVE keys, which also raises resource_missing. Minting
+      // a new customer for that would churn customers and still fail.
+      test('a missing PRICE is not mistaken for a stale customer', async () => {
+        mockSessionsCreate.mockRejectedValue(Object.assign(new Error('No such price'), {
+          code: 'resource_missing', param: 'line_items[0][price]',
+        }));
+        const res = await request(app).post('/api/billing/checkout').send({});
+        expect(res.status).toBe(500);
+        expect(mockCustomersCreate).not.toHaveBeenCalled();
+        expect(mockSessionsCreate).toHaveBeenCalledTimes(1);
+      });
+
+      test('the portal reports no_subscription rather than 500', async () => {
+        mockPortalCreate.mockRejectedValue(missingCustomer());
+        const res = await request(app).post('/api/billing/portal').send({});
+        expect(res.status).toBe(400);
+        expect(res.body.error).toBe('no_subscription');
+        // Never silently mint a customer here — the portal would open empty.
+        expect(mockCustomersCreate).not.toHaveBeenCalled();
+      });
+    });
+
     test('agents cannot subscribe', async () => {
       mockCurrentUser.value = { _id: 'b-1', isBot: true, save: mockSave };
       const res = await request(app).post('/api/billing/checkout').send({});
diff --git a/backend/routes/billing.ts b/backend/routes/billing.ts
index b2331f7d..36aa1615 100644
--- a/backend/routes/billing.ts
+++ b/backend/routes/billing.ts
@@ -20,6 +20,37 @@ const checkoutLimit = rateLimit({
 
 const FRONTEND = () => process.env.FRONTEND_URL || 'https://commonly.me';
 
+/**
+ * Did Stripe reject the request because the CUSTOMER we named does not exist?
+ *
+ * `billing.customerId` is a cache of a Stripe-side object, not identity — and
+ * the two can diverge. A customer created under test keys does not exist under
+ * live ones, so the first checkout after a test -> live cutover names a ghost;
+ * the same happens if a customer is deleted in the dashboard or the account is
+ * migrated. Without this the user gets `checkout_failed` forever, with no
+ * recovery short of a database edit.
+ *
+ * `param` is what makes this safe to act on. `resource_missing` also fires for
+ * a missing PRICE — which is exactly what a half-finished cutover produces —
+ * and treating that as a stale customer would churn out a new Stripe customer
+ * on every attempt while still failing. Only `param === 'customer'` is ours.
+ */
+const isMissingCustomer = (err: any): boolean => {
+  const e = err?.raw || err;
+  return e?.code === 'resource_missing' && e?.param === 'customer';
+};
+
+/** Create a Stripe customer for this user and persist the id. */
+const attachCustomer = async (stripe: any, user: any): Promise<string> => {
+  const customer = await stripe.customers.create({
+    email: user.email,
+    metadata: { userId: String(user._id), username: user.username || '' },
+  });
+  user.billing = { ...(user.billing || {}), customerId: customer.id };
+  await user.save();
+  return customer.id;
+};
+
 /**
  * POST /api/billing/checkout — start a subscription.
  *
@@ -43,16 +74,7 @@ router.post('/checkout', checkoutLimit, auth, async (req: any, res: any) => {
     if (user.isBot) return res.status(400).json({ error: 'Agents do not hold subscriptions' });
 
     const stripe = billingService.stripe();
-    let customerId = user.billing?.customerId;
-    if (!customerId) {
-      const customer = await stripe.customers.create({
-        email: user.email,
-        metadata: { userId: String(user._id), username: user.username || '' },
-      });
-      customerId = customer.id;
-      user.billing = { ...(user.billing || {}), customerId };
-      await user.save();
-    }
+    let customerId = user.billing?.customerId || await attachCustomer(stripe, user);
 
     // No `automatic_tax` on purpose. Stripe Tax is not activated on the
     // account, and enabling it here would fail session creation outright.
@@ -61,9 +83,9 @@ router.post('/checkout', checkoutLimit, auth, async (req: any, res: any) => {
     // turning Stripe Tax on later changes what we remit, never the sticker
     // price. Enabling it also requires `customer_update: { address: 'auto' }`,
     // since an existing customer needs an address before tax can be computed.
-    const session = await stripe.checkout.sessions.create({
+    const openSession = (customer: string) => stripe.checkout.sessions.create({
       mode: 'subscription',
-      customer: customerId,
+      customer,
       line_items: [{ price: priceId, quantity: 1 }],
       // Both, deliberately: `client_reference_id` survives places metadata does
       // not, and the webhook reads either.
@@ -75,6 +97,18 @@ router.post('/checkout', checkoutLimit, auth, async (req: any, res: any) => {
       allow_promotion_codes: true,
     });
 
+    let session;
+    try {
+      session = await openSession(customerId);
+    } catch (err) {
+      if (!isMissingCustomer(err)) throw err;
+      // Stale id — re-attach and retry ONCE. A second failure is not staleness,
+      // and must surface rather than loop.
+      console.warn('[billing] stored customer id unknown to Stripe; re-creating');
+      customerId = await attachCustomer(stripe, user);
+      session = await openSession(customerId);
+    }
+
     return res.json({ url: session.url });
   } catch (err) {
     console.error('[billing] checkout failed:', (err as Error).message);
@@ -104,6 +138,14 @@ router.post('/portal', checkoutLimit, auth, async (req: any, res: any) => {
     });
     return res.json({ url: session.url });
   } catch (err) {
+    // Unlike checkout, a stale customer is NOT re-created here: a fresh
+    // customer has no subscription, so the portal would open empty and imply
+    // the subscription vanished. `no_subscription` is both true and
+    // actionable — the UI tells them to subscribe.
+    if (isMissingCustomer(err)) {
+      console.warn('[billing] portal requested for a customer unknown to Stripe');
+      return res.status(400).json({ error: 'no_subscription' });
+    }
     console.error('[billing] portal failed:', (err as Error).message);
     return res.status(500).json({ error: 'portal_failed' });
   }