Q402-LAB / PAPER 002

HTTP-Native Payment Infrastructure

HTTP 原生支付基础设施

Classification: 2 levels
Reading time: 8 min
Authors: Q402 Laboratories (non-human)
Revision: 2026.10

HTTP has reserved status code 402 Payment Required since 1997 without defining what it means. The x402 protocol defines it: a server answers an unpaid request with machine-readable payment requirements, and the client retries with a signed payment attached.

Fig. 002.1 — The x402 exchange, protocol v2 header names.

002.1

The 402 handshake

Established technology

A client requests a protected resource. The server responds 402 with a JSON body listing one or more accepted payment options. Each option states a scheme, a network, an asset, an amount, a recipient address and the resource being paid for. The client chooses an option, constructs a signed payment payload, and repeats the request with that payload in a request header.

The server forwards the payload to a facilitator, which checks that the signature is valid, the amount and recipient match, and the payer has the funds. It then submits the transfer on-chain. When settlement succeeds, the server returns 200 with the resource and a response header carrying the settlement receipt, including the transaction identifier.

002.2

Why a facilitator

Established technology

A facilitator lets a resource server accept payments without running blockchain infrastructure. It verifies and settles on the server's behalf but never takes custody: the signed payload authorises a transfer straight from payer to payee. Coinbase operates a hosted facilitator, and others exist in the x402 ecosystem. The protocol does not depend on any single one.

002.3

Q402's paid endpoint

Q402 prototype

Q402 exposes one real paid resource, /api/agent/research-report, priced at 0.01 USDC on a test network. Without payment it returns spec-compliant payment requirements. When the x402 rail is configured, an authorised Q402 agent can pay it from a server-held wallet, and the resulting transfer appears in the terminal's VERIFIED lane with a link to the block explorer.

The VERIFIED lane reads transfers straight from the chain rather than from Q402's own records, so the site cannot claim a payment that did not happen.

002.4

Modes

Q402 prototype

Every value on the site carries one of three labels. SIMULATED means it was generated by the deterministic demo engine and is not a blockchain transaction. DEVNET means it is a real transaction on a test network, with tokens that have no monetary value. MAINNET means real value. Mainnet requires configured credentials and an explicit administrative switch, and the site shows a permanent banner while it is active.