# x402 — one call, one receipt

> Financial Comptroller — Agentic KG Holdings → https://agent-kg.ai

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. The office holds no cryptocurrency; the pay-to address is the rail's, the receipt is the office'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 office door's statement of record 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, both placeholder prices

- One metered call on the office door — $0.01 · x402 — USDC on Base mainnet · placeholder price
- A block of 100 calls on the office door — $1.00 · the card rails, test mode, once the office's Stripe test keys are placed · placeholder price — card processors do not take a one-cent charge
- Product — the office door: https://agent-kg.ai/facts.json, read at the call
- Machine copy — https://agent-x402.ai/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 fc-x402-2026-09) 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. Prices are placeholders set by the office.

- Key — https://agent-x402.ai/x402/jwks.json · kid fc-x402-2026-09
- Register — https://agent-x402.ai/x402/settled.json

## The other three

- https://agent-mpp.ai/ — Machine Payments Protocol (MPP)
- https://agent-ucp.ai/ — Universal Commerce Protocol (UCP)
- https://agent-acp.ai/ — Agentic Commerce Protocol (ACP)

Surface 0.1.0 · as of 2026-09-19 · Agent Card https://agent-x402.ai/.well-known/agent-card.json · llms.txt https://agent-x402.ai/llms.txt