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Posted on Originally published at ambolt.dev

How an agent pays per call with x402

#ai #api #x402 #webdev

Most data APIs want an account, a key and a monthly plan. An agent that needs one answer once does not want any of that. x402 turns the HTTP status code 402 ("Payment Required") into a payment step that a program can complete on its own.

The flow

  1. The client sends a normal request, for example GET /v1/iban-validate?iban=….
  2. Without payment the server answers 402 with the price and the payment options in a header: network, asset (USDC) and the receiving address.
  3. The client signs a payment for exactly that amount and sends the request again with the payment attached.
  4. The server checks the payment, runs the call and returns the result. If the call fails, nothing is charged.

On Base and Polygon the client needs a little USDC and no gas token: the wallet signs an authorisation for the exact amount and a payment service submits it. Solana is also supported.

Try it without a wallet

Add free=1 and the call is free once per tool and IP address per day:

curl "https://api.ambolt.dev/v1/iban-validate?iban=DK5000400440116243&free=1"
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Without it you get the 402 answer with the price, which shows the protocol without paying anything.

From code

The JavaScript and Python clients wrap every endpoint as a method and work with any x402-capable fetch.

import { createAmbolt } from '@ambolt/sdk';
const ambolt = createAmbolt({ fetch: payingFetch });
console.log(await ambolt.ibanValidate({ iban: 'DK5000400440116243' }));
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Prices start at $0.003 a call. The full list is in the catalog, and every tool is also available as a free MCP tool at https://api.ambolt.dev/mcp.

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