The protocol

Payment inside the request.

x402 revives a status code the web has always had and never used. HTTP 402 — Payment Required — lets a server state its price inside the response to a request, and lets a client settle it inside the same exchange. No redirect to a checkout, no invoice, no account.

01The handshake

One round trip, five moves.

Run it below, then read the same sequence in full. The console uses the real path, the real price and the real receive address from the live catalogue.

Machine transactiondemonstration
settled · verified
  1. Agentautonomous software · no account
  2. Discover servicereads the catalogue · free
  3. Request endpointPOST /v1/research/attested-brief
  4. HTTP 402 Payment Requiredprice · payTo · network
  5. Authorise 2.00 USDCEIP-3009 transferWithAuthorization
  6. Settled on Basefacilitator broadcasts · eip155:8453
  7. Service executedresult produced
  8. Result attestedsigned · portable
  9. Verifiedthird party can check it
VERIFIED200 OK · VERIFIED
result            accepted
third_party_check passed
trust             established without an intermediary
agent             continues operating

Demonstration. Replays the protocol using values captured from the live catalogue — real endpoint path, real price, real receive address, and the real transaction hash of that endpoint’s settlement on Base. No paid request is fired from this page. The complete settlement record is on the proof page.

01

The agent requests the resource

It calls the endpoint like any other HTTP resource. No key, no token, no session. The request carries whatever the service needs to do its job — and nothing about payment.

POST /v1/research/attested-brief

02

The server answers 402

Instead of refusing, the server states its terms in the response: the scheme, the network, the asset, the amount and the address to pay. The price is machine-readable, so a program can decide whether to accept it without a human reading a pricing page.

HTTP/1.1 402 Payment Required

03

The agent authorises the payment

It signs an EIP-3009 transferWithAuthorization over the stated amount, bound to the stated address and valid for a 60-second window. It does not send funds. It produces an authorisation the facilitator can execute.

EIP-3009 · transferWithAuthorization

04

The facilitator settles on Base

The signed authorisation is broadcast on Base mainnet and the transfer executes. The agent never custodies anything and never holds a balance with us; the signature is the authorisation and the chain is the receipt.

Base mainnet · USDC

05

The result is delivered and attested

Settlement confirmed, the service executes and returns its result with a signature over the output digest. A third party can verify that attestation later at a free route, using the published public key, without paying and without asking us.

result + signature · verify for free

02The settlement rail

The exact parameters.

Non-custodial, exact-amount, single-use authorisation. These are the values a client needs in order to pay — copied from the live settlement configuration, not paraphrased.

network
eip155:8453Base mainnet
asset
USDC · 6 decimals0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913
scheme
exactEIP-3009 transferWithAuthorization
receive address
Synthex Forge0xc8fF67FAE72596478599EAAcfad8656fAe1113c8
validity window
60 secondsper authorisation
price range
per pay-per-call route$0.02 – $29.00

Settlement is enabled on the live service. 40 pay-per-call routes answer HTTP 402 — the price is in the response; 61 routes are free, including a verification route and a published public key for every family that issues attestations.

03Why it is different

Compared with the alternative.

A conventional paid API makes you prove you are a paying human company before it will speak to you. Every step below exists to satisfy that requirement — and a machine satisfies none of it.

Human-mediated integration9 steps
  1. Human
  2. Browser
  3. Sign up
  4. Verify email
  5. Dashboard
  6. API key
  7. Billing
  8. Payment
  9. Confirmation
Machine-native integration5 steps
  1. Agent
  2. Endpoint
  3. HTTP 402
  4. USDC on Base
  5. Result

Nine steps, four of which need a person. The same job on the Synthex Forge rail is five steps, and a human never appears in the sequence at all.

04Position

x402 is a rail, not the product.

Protocols are infrastructure. Anyone can implement x402. What matters is whether the services on the other end are worth paying for — deterministic, verifiable, and useful enough that an autonomous system will choose them without being told to. That catalogue is the company; the rail is how a machine reaches it.