diff --git a/.changeset/clean-amount-strings.md b/.changeset/clean-amount-strings.md
new file mode 100644
index 00000000..86fe130d
--- /dev/null
+++ b/.changeset/clean-amount-strings.md
@@ -0,0 +1,5 @@
+---
+"@agentcommercekit/ack-pay": patch
+---
+
+Reject malformed, fractional, zero, and negative string payment amounts while preserving positive integer strings for values larger than JavaScript's 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
new file mode 100644
index 00000000..166b4496
--- /dev/null
+++ b/packages/ack-pay/src/schemas/payment-option.test.ts
@@ -0,0 +1,37 @@
+import * as v from "valibot"
+import { describe, expect, it } from "vitest"
+
+import { paymentOptionSchema as valibotPaymentOptionSchema } from "./valibot"
+import { paymentOptionSchema as zodPaymentOptionSchema } from "./zod"
+
+const paymentOption = {
+  id: "test-payment-option-id",
+  decimals: 2,
+  currency: "USD",
+  recipient: "did:example:recipient",
+}
+
+function acceptsAmount(amount: number | string) {
+  const value = { ...paymentOption, amount }
+
+  return {
+    valibot: v.safeParse(valibotPaymentOptionSchema, value).success,
+    zod: zodPaymentOptionSchema.safeParse(value).success,
+  }
+}
+
+describe("paymentOptionSchema amount", () => {
+  it.each([1, 100, "1", "100", "9007199254740993"])(
+    "accepts positive integer amount %s",
+    (amount) => {
+      expect(acceptsAmount(amount)).toEqual({ valibot: true, zod: true })
+    },
+  )
+
+  it.each([0, -1, 1.5, "", "0", "-1", "1.5", "abc"])(
+    "rejects invalid 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 06a58f1e..fc04172e 100644
--- a/packages/ack-pay/src/schemas/valibot.ts
+++ b/packages/ack-pay/src/schemas/valibot.ts
@@ -3,10 +3,14 @@ import { jwtStringSchema } from "@agentcommercekit/jwt/schemas/valibot"
 import * as v from "valibot"
 
 const urlOrDidUri = v.union([v.pipe(v.string(), v.url()), didUriSchema])
+const positiveIntegerString = v.pipe(v.string(), v.regex(/^[1-9]\d*$/))
 
 export const paymentOptionSchema = v.object({
   id: v.string(),
-  amount: v.union([v.pipe(v.number(), v.integer(), v.gtValue(0)), v.string()]),
+  amount: v.union([
+    v.pipe(v.number(), v.integer(), v.gtValue(0)),
+    positiveIntegerString,
+  ]),
   decimals: v.pipe(v.number(), v.integer(), v.toMinValue(0)),
   currency: v.string(),
   recipient: v.string(),
diff --git a/packages/ack-pay/src/schemas/zod.ts b/packages/ack-pay/src/schemas/zod.ts
index 66ceffd9..40cec4e2 100644
--- a/packages/ack-pay/src/schemas/zod.ts
+++ b/packages/ack-pay/src/schemas/zod.ts
@@ -3,10 +3,11 @@ import { jwtStringSchema } from "@agentcommercekit/jwt/schemas/zod"
 import * as z from "zod"
 
 const urlOrDidUri = z.union([z.url(), didUriSchema])
+const positiveIntegerString = z.string().regex(/^[1-9]\d*$/)
 
 export const paymentOptionSchema = z.object({
   id: z.string(),
-  amount: z.union([z.number().int().positive(), z.string()]),
+  amount: z.union([z.number().int().positive(), positiveIntegerString]),
   decimals: z.number().int().nonnegative(),
   currency: z.string(),
   recipient: z.string(),