---
title: "MPP request that does not decode — x402 defect class mpp-request-undecodable"
description: "MPP request that does not decode (mpp-request-undecodable), an x402 defect class: The request parameter is base64url of a JSON object. Detectable by an unpaid probe. The price, currency and recipient are inside the request; a client that cannot decode it has nothing to sign and nothing to show its human. The seller sees nothing."
canonical: "https://scvd.store/defects/mpp-request-undecodable"
url: "https://scvd.store/defects/mpp-request-undecodable"
term_code: "mpp-request-undecodable"
defined_in: "https://scvd.store/defects"
data: "https://scvd.store/defects.json"
vocabulary_version: "22"
license: "https://creativecommons.org/licenses/by/4.0/"
---

# MPP request that does not decode

`mpp-request-undecodable` — an x402 defect class.

**Asserts:** The request parameter is base64url of a JSON object.

**Costs:** The price, currency and recipient are inside the request; a client that cannot decode it has nothing to sign and nothing to show its human. The seller sees nothing.

**Detectable:** unpaid — a free probe can see it

**Falsified by:** The request decoding, by any conforming base64url and JSON reader, to a JSON object at the stated moment.

**How an operator clears it:** Encode the request as base64url (no padding, URL-safe alphabet) of UTF-8 JSON, and decode your own header with an independent client before shipping; a proxy that rewrites or size-caps headers is the usual cause.

**What a buyer does:** Do not guess the terms: a request you could not decode is a door you cannot sign for. Report the decode failure by name and move on.

**Our signal:** `mpp-request-decodes`

---

Vocabulary v22, CC BY 4.0. Every class: https://scvd.store/defects. Machine-readable, with falsifiers and cross-instrument mappings: https://scvd.store/defects.json.

A class describes a property of one endpoint at one moment and never accumulates into a judgment on an operator.