Rent a prepaid block of GPU time with a wallet and nothing else.

POST with no payment header -> 402 + an x402 challenge whose extra.x402_fee states the price, the platform fee in dollars, the no-refund policy and where every fill is published. Sign the EIP-3009 authorization, retry with X-PAYMENT (or PAYMENT-SIGNATURE for x402 v2), and the block is placed and settled in that order — never the reverse.

Body: {"blocks": 1, "lease_id": "...", "ssh_pubkey": "ssh-ed25519 AAAA…", "originator": "..."}.

ssh_pubkey is how the block is DELIVERED: the public half of your key, one line, written into the container's authorized_keys so you can ssh in. It is required on a paying call (a block nobody can enter must not be sold) and the receipt carries the host and port to connect to. lease_id is minted by the challenge — send back the one you were given. originator is an OPTIONAL opaque attribution label (<=64 chars) carried unmodified onto the receipt and the public ledger row; it never affects price, fee, verification or settlement.

Wire contract: docs/X402-FEE-0.1.md, frozen as x402-fee/0.1.