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

Serves the goods twice for one settled payment — x402 defect class replay-accepted

Front of the store What is this?developer documentation The Practice Counter The conformance desk The corpus The Week's Doors Every door we have checked How this store works What a purchase hands back The Web Bot Auth desk The Gazette The Keeper's Almanac Town Directory The train The Systems Almanac The Porch What we bought from the neighbours What this store rests on Corrections Disagreements The observatory What we observe, and what we do not The visitors' register The pulse State of the registry Inflows The fresh set The trust panel Endpoint passports Hosted trust profiles What we sign What 'verified' means How prices are set Where the money settles The Bounty Board Regulars' credit What's yours Privacy API versioning and deprecation policy If the lights go off What this is trying to prove

Asserts: A byte-identical, already-settled payment presented a second time is refused.

What a buyer loses when it is present: The seller gives its product away. The authorization's nonce is spent, so nothing reaches them on the second pass — and from their side both requests look like successful sales.

Detectable: only by a paid probe — a settled payment reveals it.

Falsified by: The endpoint refusing the identical presented payment on the second attempt at the stated moment.

How an operator clears it: Record each settled authorization nonce at settle time and refuse a byte-identical presentation BEFORE fulfillment runs — one keyed read at the till. Refusing with a reference to the original settlement also keeps you clear of the nonce-unbound class below.

Our signal: launch_check stage: replay.

As other instruments name it: {"instrument":"Cairn (cairnwake.com)","as":"replay-accepted / check replay_rejected","verify":"cairnwake.com/scoreboard.json, read 2026-08-23: a public machine-readable rollup of its published reports","falsified_by":"Their check asserting something other than the refusal of an identical already-settled payment."}.

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.