What x402 is
x402 is a payment protocol over HTTP: a resource that costs money answers 402 Payment Required with an offer, the client signs the payment and repeats the call, and the answer comes back with a settlement response. This surface implements x402 version 2 as the specification reads: the offer travels in the PAYMENT-REQUIRED header and body, the payment in PAYMENT-SIGNATURE, the settlement in PAYMENT-RESPONSE, all base64-encoded JSON.
The scheme is exact: a signed EIP-3009 transferWithAuthorization of USDC on Base mainnet, verified and settled through a facilitator. GreenCore holds no cryptocurrency; the pay-to address is the rail's, the receipt is GreenCore's.
How an agent pays here
- 1 — GET /api with no payment. The answer is 402 with the offer: amount, asset, network, pay-to address, a 60-second window.
- 2 — Sign the payment and repeat the call with the PAYMENT-SIGNATURE header.
- 3 — The facilitator verifies, the door answers the cosmetics door's statement and a signed receipt, the facilitator settles, and PAYMENT-RESPONSE carries the transaction.
- 4 — GET /x402/receipt/{nonce} resolves the receipt forever; GET /x402/settled.json is this host's register with the four-host total beside it.
Two offers
- One metered call on the cosmetics door — $0.01 · x402 — USDC on Base mainnet · the Coinbase price floor
- A block of 100 calls on the cosmetics door — $1.00 · the card rails, test mode, once GreenCore's Stripe test keys are placed · card processors do not take a one-cent charge
- Product — the cosmetics door: its statement at mcp.a2a-cosmetics.ai/health.json, read at the call, never copied.
- Machine copy — /offers.json
The receipt contract
Every receipt carries the payer, the transaction, the amount and asset, the resource, the rail, the host that issued it and the notice; the jwt verifies against /x402/jwks.json (kid a2ac-x402-receipts-2026-09) and the receipt resolves forever at /x402/receipt/{nonce}. A card-rail receipt, when those rails settle, carries the test-mode charge reference in the same shape.
This surface implements the rail it is named after. Nothing here is endorsed by any protocol's owner. Artificial intelligence makes mistakes. A2A Cosmetics is an information source, not a recommendation.
- Key — /x402/jwks.json · kid a2ac-x402-receipts-2026-09 · private half held by GreenCore Solutions Corp.
- Register — /x402/settled.json (this host, with the four-host total beside it)
The other three
- agent-mpp.ai — Machine Payments Protocol (MPP)
- agent-ucp.ai — Universal Commerce Protocol (UCP)
- agent-acp.ai — Agentic Commerce Protocol (ACP)
- Every .ai host answers all four rails; each site explains the rail it is named after.