How an agent pays for an API with x402

What x402 is

x402 is a convention for paying for a single HTTP request. It uses 402 Payment Required, a status code reserved in the original HTTP specification and left unused for decades. The server answers an unpaid request with a challenge describing the price; the client signs an authorization for exactly that amount and retries with it attached. There is no account, no invoice, and no subscription, because the unit of commerce is one request.

For an autonomous agent that matters more than it does for a person. A human can create an account and enter a card once. An agent that discovers a useful API at three in the morning cannot, and any flow that requires it to is a flow that stops.

The exchange, step by step

1. The agent requests a paid resource with no payment attached.
2. The server answers 402 with a challenge: the accepted scheme, the network, the exact amount, the asset, and the recipient. On this service the same challenge is also published as a static document at /.well-known/x402, so a client can learn the price without spending a request to be refused.
3. The agent signs an authorization for that exact amount to that exact recipient, with an expiry and a single-use nonce. This is a signature, not a transaction — nothing has moved yet.
4. The agent retries with the signed payload in a payment header.
5. The server verifies and settles through a payment facilitator, which submits the authorization on-chain and pays the network fee.
6. The server returns the result, with a payment receipt header the agent can keep.

What you need before you start

• A wallet on the network the service accepts, holding the stablecoin it prices in. On this service that is USDC on Base.
• No native currency for gas. The facilitator submits the authorization, so the paying wallet only needs the token it is spending.
• A client that speaks the flow. Several exist; the retry-with-signature loop is short enough to implement directly if you would rather not add a dependency.

Fund the wallet for this purpose and nothing else. The bound on a single signature is one payment, but the bound on a wallet is its balance.

Trying it against a live endpoint

An unpaid request returns the challenge, so you can inspect the shape without spending anything:

curl -i https://slidingbox.ai/v1/hydrate/:pointer

That returns 402 with the challenge in the PAYMENT-REQUIRED header. The machine-readable description of every route is at /openapi.json, and a plain-text summary for agents is at /llms.txt.

If you would rather evaluate the service before wiring up payment at all, an evaluation key skips it:

Try it without an account

curl -X POST https://slidingbox.ai/v1/key

Returns an evaluation key immediately — no signup, no email, no approval step. Send it as X-API-Key on a retrieval to skip payment while the key has allowance left. Storing is free either way.