NFT owner lookup (ERC-721 ownerOf)

Who owns this NFT? Send GET /api/erc721-owner with the required fields contract and tokenId and pay $0.002 per call over x402 or MPP (there is no free tier). It returns a JSON object with network, contract, tokenId and owner.

Calls ownerOf(tokenId) on any ERC-721 contract on Ethereum, Base, Polygon, Arbitrum, or Optimism. Complements nft-holdings (which lists a WALLET's NFTs) - this answers by TOKEN. Keyless multi-endpoint failover. ?contract=0x…&tokenId=123&network=ethereum

Category: Crypto & onchain data · Tags: crypto nft erc721 owner ownerof token rpc evm · Also found as: nft-owner

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Parameters

Example request

curl -i "https://agent402.tools/api/erc721-owner?contract=0x57f1887a8BF19b14fC0dF6Fd9B2acc9Af147eA85&tokenId=0xaf2caa1c2ca1d027f1ac823b529d0a67cd144264b2789fa2ea4d63a67c7103cc&network=ethereum"

Without payment this returns HTTP 402 Payment Required with the exact price for erc721-owner; any x402 v2 or MPP client pays it and retries.

Example response

{
  "network": "ethereum",
  "contract": "0x57f1…",
  "tokenId": "0xaf2c…",
  "owner": "0x…"
}

From an MCP client

catalog.call {
  "slug": "erc721-owner",
  "params": {
    "contract": "0x57f1887a8BF19b14fC0dF6Fd9B2acc9Af147eA85",
    "tokenId": "0xaf2caa1c2ca1d027f1ac823b529d0a67cd144264b2789fa2ea4d63a67c7103cc",
    "network": "ethereum"
  }
}

The hosted connector at https://agent402.tools/mcp needs a payment for erc721-owner; the stdio package pays it from a wallet or from AGENT402_CREDITS_KEY. Local install: npx -y agent402-mcp.

Errors and behavior

• contract and tokenId are required. An input the tool rejects returns an HTTP 4xx whose body carries error, tool, expected, required and example, so the caller can correct it.
• A paid call that ends in any status of 400 or above is not charged over x402, MPP or a prepaid credits key: settlement is cancelled when the tool fails. The exception is a Tempo push credential, a transfer the buyer sent before the call: it settles before the tool runs, so if the tool then fails the payment is recorded as a refund owed to the paying wallet.
• Wallet-only: this tool reaches the network or stored state, so it has no proof-of-work tier. A prepaid card-credits key issued earlier (Authorization: Bearer a402_...) also pays it.
• A POST with a JSON body to /api/erc721-owner is served as this GET, with the body as the input.
• An Idempotency-Key header makes a retried paid call replay the first 200 instead of charging again (an answer larger than 1 MB is not replayed).

Paid call (JavaScript agent)

import { wrapFetchWithPayment } from "@x402/fetch";
import { x402Client } from "@x402/core/client";
import { registerExactEvmScheme } from "@x402/evm/exact/client";
import { privateKeyToAccount } from "viem/accounts";

const client = new x402Client();
client.setSpendControls?.(false); // keep your own spending ceiling in code
registerExactEvmScheme(client, { signer: privateKeyToAccount(KEY) });
const payFetch = wrapFetchWithPayment(fetch, client);

const res = await payFetch("https://agent402.tools/api/erc721-owner?contract=0x57f1887a8BF19b14fC0dF6Fd9B2acc9Af147eA85&tokenId=0xaf2caa1c2ca1d027f1ac823b529d0a67cd144264b2789fa2ea4d63a67c7103cc&network=ethereum");

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