diff --git a/src/contracts/checkout.ts b/src/contracts/checkout.ts
index 73cb570..465f651 100644
--- a/src/contracts/checkout.ts
+++ b/src/contracts/checkout.ts
@@ -57,6 +57,7 @@ export const PaymentReceivedInputSchema = z.object({
 		z.object({
 			paymentHash: z.string(),
 			amountSats: z.number(),
+			sandbox: z.boolean().default(false),
 		}),
 	),
 });
diff --git a/tests/contracts/checkout.test.ts b/tests/contracts/checkout.test.ts
index 8d43da9..ac49cc0 100644
--- a/tests/contracts/checkout.test.ts
+++ b/tests/contracts/checkout.test.ts
@@ -270,6 +270,20 @@ describe('Checkout Contracts', () => {
       expect(result.success).toBe(true);
     });
 
+    test('should allow sandbox flag on payment', () => {
+      const input = {
+        payments: [{
+          paymentHash: 'hash-sandbox',
+          amountSats: 1500,
+          sandbox: true,
+        }],
+      };
+
+      const result = PaymentReceivedInputSchema.safeParse(input);
+      expect(result.success).toBe(true);
+      expect(result.success && result.data.payments[0]?.sandbox).toBe(true);
+    });
+
     test('should reject without paymentHash', () => {
       const input = {
         amountSats: 1500,
@@ -364,4 +378,4 @@ describe('Checkout Contracts', () => {
       expect(PaymentReceivedInputSchema.safeParse(paymentInput).success).toBe(true);
     });
   });
-}); 
\ No newline at end of file
+});