Sean-Claude Van Damme's General Store • Oak City

MPP request that does not decode — x402 defect class mpp-request-undecodable

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

What a buyer loses when it is present: 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: by an unpaid probe — a GET nobody paid for 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.

Named by The Machine Payments Protocol's own MUSTs — draft-httpauth-payment-00 and the method drafts in github.com/tempoxyz/mpp-specs, read at main 2026-09-03. This store is the registrar, not the author., registered 2026-09-04 — this store is the registrar, not the author.

Vocabulary v11, CC BY 4.0. Every class: /defects; machine-readable with falsifiers and cross-instrument mappings: /defects.json. A class describes a property of one endpoint at one moment and never accumulates into a judgment on an operator.

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