e2747f39-c387-4c14-aca4-8d70b1048d4aIMD JOB

`artifacts/` is git-ignored by the environment and I was told not to force-add outputs, so nothing is committed. Both files are in `artifacts/` for the daemon to upload. I've written the field guide to `artifacts/report.md`, with a short `README.md` beside it. One thing first: part of the brief doesn't match the x402 standard. The standard x402 v2 `exact`/Permit2 flow has no second client-signed EIP-712 signature that binds the payment to the quote, and no "payment hash." **What the specs actually have:** - **One client signature:** the Permit2 witness signature. It is the only thing the client signs. - **A second EIP-712 signature exists, but the server signs it.** It's the optional signed Offer in the `offer-receipt` extension. Nothing on chain checks it, and it isn't tied to the client's signature. - **The nearest thing to a "payment hash"** is either in a different scheme (`auth-capture`) or in the repository's test code. The report says this plainly in §0.1. It then shows how to get most of the quote-binding effect from standard pieces: the signed Offer, the `accepted` echo, and `payment-identifier`. If a particular facilitator requires a client-signed quote binding, it's vendor-specific and you'll need its format from that vendor (open question Q1). **What the report covers:** - **Message flow:** the 402 challenge, the exact Permit2 type string and domain, how the reference client fills each field, what the facilitator verifies (including a 6-second deadline buffer), and settlement with the facilitator paying gas. - **What the signature authorizes and why it can't be replayed:** a table covering chain, spender, recipient, amount, token, nonce and deadline. It also names what is *not* bound: the resource URL, which request it's for, and which facilitator submits it. - **Nonces, deadlines and quote expiry:** Permit2 nonces are unordered and shared across everything the wallet signs, a failed settlement doesn't use up the nonce, and outstanding signatures can be cancelled. It includes a rule for choosing the deadline from the quote timeout and the offer expiry. - **Failure modes:** missing allowance, insufficient balance, expired window, mismatch and reused nonce, each with the reference facilitator's error code and a fix. - **Checklist and spending controls:** a one-page pre-flight checklist, plus operator controls. One thing to watch: the reference SDK's approval helper grants an unlimited (`MAX_UINT256`) allowance by default, which the report says to override. **Evidence:** every claim is tagged as fact, inference, uncertain or open question. Facts cite pinned commits of the x402 specs and reference code (as of today) and Uniswap Permit2. I also read Ethereum mainnet directly: both Permit2 and the x402 exact proxy are deployed, and the proxy points at the standard Permit2 address. All cited URLs resolve. **Limits:** I made no live payment and didn't query any production facilitator, so I don't know which ones support mainnet today or what they charge (open questions Q2 and Q3). The spec also disagrees with itself on whether a missing allowance returns HTTP 412 or 402 (noted as U4). No one has independently reviewed the report.