Pay Per Call with x402

The Monad Indexing API can be used without an account or API key. Every endpoint is also exposed under an x402 path where each request is paid in USDC on Monad mainnet at the moment it is served. This is built for AI agents, scripts and anyone who wants on-chain data without a subscription.

Base URL

https://api.blockvision.org/x402/v2/monad

Append the same path and query parameters documented for each Monad Indexing API endpoint. For example the subscription URL https://api.blockvision.org/v2/monad/token/detail?contractAddress=0x... becomes https://api.blockvision.org/x402/v2/monad/token/detail?contractAddress=0x....

How a paid request works

x402 is an open HTTP payment standard (protocol version 2). A request goes through three steps:

1. Quote. Call the endpoint with no payment. The API answers HTTP 402 Payment Required. The PAYMENT-REQUIRED header (and the JSON body) carry the price, the USDC contract, the receiving address and a timeout.
2. Pay. Your client signs an EIP-3009 transferWithAuthorization for exactly that amount and retries the same request with the signature in the PAYMENT-SIGNATURE header. Nothing is broadcast yet.
3. Settle. The API verifies the signature through the Monad x402 facilitator, runs the query, and only if the query succeeds settles the payment on-chain. The data is returned with a PAYMENT-RESPONSE header that contains the settlement transaction hash.

x402 client libraries (for example @x402/fetch for JavaScript) do all three steps automatically. You give them a wallet and call fetch as usual.

Example 402 response

HTTP/2 402
content-type: application/json
payment-required: eyJ4NDAyVmVyc2lvbiI6MiwiZXJyb3IiOi...

{
  "x402Version": 2,
  "error": "PAYMENT-SIGNATURE header is required",
  "resource": {
    "url": "https://api.blockvision.org/x402/v2/monad/token/detail",
    "description": "Monad token detail",
    "mimeType": "application/json"
  },
  "accepts": [
    {
      "scheme": "exact",
      "network": "eip155:143",
      "amount": "5000",
      "asset": "0x754704Bc059F8C67012fEd69BC8A327a5aafb603",
      "payTo": "0x38C4Cffe8B8D9417506eB5C6a82afb5E1f5F0DB1",
      "maxTimeoutSeconds": 60,
      "extra": { "name": "USDC", "version": "2" }
    }
  ]
}

amount is in USDC base units (6 decimals), so 5000 is USD 0.005. The PAYMENT-REQUIRED header is the same JSON, base64 encoded.

Payment details

Your wallet needs USDC on Monad mainnet. Gas is paid by the facilitator, so no MON is required.

Quick start (JavaScript)

npm install @x402/fetch @x402/evm viem

import { privateKeyToAccount } from "viem/accounts";
import { wrapFetchWithPayment } from "@x402/fetch";
import { ExactEvmScheme } from "@x402/evm";

const account = privateKeyToAccount(process.env.PRIVATE_KEY as `0x${string}`);
const fetchWithPay = wrapFetchWithPayment(fetch, {
  schemes: [new ExactEvmScheme(account)],
});

const res = await fetchWithPay(
  "https://api.blockvision.org/x402/v2/monad/token/detail?contractAddress=0x754704Bc059F8C67012fEd69BC8A327a5aafb603"
);
console.log(await res.json());
console.log("settled tx:", res.headers.get("PAYMENT-RESPONSE"));

The first call returns 402, the library signs the authorization and retries, and you receive the normal JSON response. Check the x402 client documentation for the exact package names and versions for your language.

Quick start (curl, inspect the quote)

curl -i "https://api.blockvision.org/x402/v2/monad/token/detail?contractAddress=0x754704Bc059F8C67012fEd69BC8A327a5aafb603"

Decode the payment-required header with base64 -d to see the price and payment requirements.

Pricing

Prices are per successful call and may change. The live price list is always available free of charge at the discovery endpoints below.

Discovery endpoints (free)

When you are charged

• You are charged only when the query succeeds. If the API returns an error, the authorization is not settled and no USDC moves.
• An authorization is tied to one request. Reusing the same signature returns 402 with payment authorization has already been used.
• If the facilitator rejects the signature you get 402 with the reason, for example an insufficient USDC balance or an expired authorization.
• If settlement fails after the query succeeded, the API returns 402 with the message settlement status unknown, please do not retry with a new payment authorization. Keep the x-request-id from the response and contact support. We reconcile these manually so you are never charged without receiving data.

Response codes

Response bodies are identical to the subscription endpoints. See each endpoint page in this section for parameters and fields.