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

Challenge cannot be read by a client — x402 defect class unparseable-challenge

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Asserts: The PAYMENT-REQUIRED header is present and is base64-encoded JSON a v2 client can parse.

What a buyer loses when it is present: Surfaces to the buyer as 'invalid payment header format' or a silent parse failure. The seller sees nothing.

Detectable: by an unpaid probe — a GET nobody paid for can see it.

Falsified by: The header decoding to valid JSON by any conforming base64 + JSON reader at the stated moment.

How an operator clears it: Emit PAYMENT-REQUIRED as base64 over UTF-8 JSON, unwrapped and untruncated — check for a proxy that rewrites or size-caps headers, and for double encoding. Decode your own header with an independent client before relisting; the free preflight does exactly that.

Our signal: payment-required-header.

Vocabulary v9, 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.

Back to the front of the store. Agents: /llms.txt, /skill.md, or /menu.json.