diff --git a/.changeset/payment-option-decimals-validation.md b/.changeset/payment-option-decimals-validation.md
new file mode 100644
index 00000000..b5f3a7ad
--- /dev/null
+++ b/.changeset/payment-option-decimals-validation.md
@@ -0,0 +1,14 @@
+---
+"@agentcommercekit/ack-pay": minor
+---
+
+`paymentOptionSchema` now rejects a negative `decimals` instead of normalizing
+it. The valibot schema used `toMinValue(0)`, which is a transformation that
+clamps rather than a validation that rejects, so `decimals: -2` parsed
+successfully as `decimals: 0`.
+
+`decimals` scales `amount`, so clamping turned an invalid payment option into a
+valid one asking for a different sum, and it did so on the verification path as
+well: `verifyPaymentRequestToken` parses token payloads with this schema. The
+zod schema already rejected the same input via `nonnegative()`, so the two
+validators disagreed about which payment requests are well-formed.
diff --git a/packages/ack-pay/src/payment-request.test.ts b/packages/ack-pay/src/payment-request.test.ts
index 018c78b3..bff3a2ef 100644
--- a/packages/ack-pay/src/payment-request.test.ts
+++ b/packages/ack-pay/src/payment-request.test.ts
@@ -29,6 +29,19 @@ describe("isPaymentRequest", () => {
     ).toBe(false)
   })
 
+  // `decimals` scales `amount`, so a negative value has to be rejected rather
+  // than normalized: reading it as 0 silently changes what the request asks for.
+  it("returns false if decimals is negative", () => {
+    expect(
+      isPaymentRequest({
+        ...validPaymentRequest,
+        paymentOptions: [
+          { ...validPaymentRequest.paymentOptions[0], decimals: -2 },
+        ],
+      }),
+    ).toBe(false)
+  })
+
   it("returns false if given null", () => {
     expect(isPaymentRequest(null)).toBe(false)
   })
diff --git a/packages/ack-pay/src/schemas/valibot.ts b/packages/ack-pay/src/schemas/valibot.ts
index 06a58f1e..dfb5bcc7 100644
--- a/packages/ack-pay/src/schemas/valibot.ts
+++ b/packages/ack-pay/src/schemas/valibot.ts
@@ -7,7 +7,7 @@ const urlOrDidUri = v.union([v.pipe(v.string(), v.url()), didUriSchema])
 export const paymentOptionSchema = v.object({
   id: v.string(),
   amount: v.union([v.pipe(v.number(), v.integer(), v.gtValue(0)), v.string()]),
-  decimals: v.pipe(v.number(), v.integer(), v.toMinValue(0)),
+  decimals: v.pipe(v.number(), v.integer(), v.minValue(0)),
   currency: v.string(),
   recipient: v.string(),
   network: v.optional(v.string()),