diff --git a/.changeset/payment-option-safe-integer-amount.md b/.changeset/payment-option-safe-integer-amount.md
new file mode 100644
index 0000000..0069538
--- /dev/null
+++ b/.changeset/payment-option-safe-integer-amount.md
@@ -0,0 +1,5 @@
+---
+"@agentcommercekit/ack-pay": patch
+---
+
+Reject unsafe integer numbers on the payment option amount number arm so valibot matches zod (and Number.isSafeInteger), keeping the string arm for amounts larger than the safe integer range.
diff --git a/packages/ack-pay/src/schemas/payment-option.test.ts b/packages/ack-pay/src/schemas/payment-option.test.ts
index 166b449..f6f016d 100644
--- a/packages/ack-pay/src/schemas/payment-option.test.ts
+++ b/packages/ack-pay/src/schemas/payment-option.test.ts
@@ -34,4 +34,11 @@ describe("paymentOptionSchema amount", () => {
       expect(acceptsAmount(amount)).toEqual({ valibot: false, zod: false })
     },
   )
+
+  it.each([Number.MAX_SAFE_INTEGER + 1, 1e21])(
+    "rejects an unsafe integer number amount %s",
+    (amount) => {
+      expect(acceptsAmount(amount)).toEqual({ valibot: false, zod: false })
+    },
+  )
 })
diff --git a/packages/ack-pay/src/schemas/valibot.ts b/packages/ack-pay/src/schemas/valibot.ts
index 75fc980..8de7239 100644
--- a/packages/ack-pay/src/schemas/valibot.ts
+++ b/packages/ack-pay/src/schemas/valibot.ts
@@ -14,7 +14,8 @@ const timestampSchema = v.pipe(
 export const paymentOptionSchema = v.object({
   id: v.string(),
   amount: v.union([
-    v.pipe(v.number(), v.integer(), v.gtValue(0)),
+    // Unsafe integers are not exact; use the string arm for large amounts.
+    v.pipe(v.number(), v.safeInteger(), v.gtValue(0)),
     positiveIntegerString,
   ]),
   decimals: v.pipe(v.number(), v.integer(), v.toMinValue(0)),