# ERC-721 NFT Balance Checker

> ERC-721 NFT Balance Checker is a paid API for AI agents from twin.unykorn.org, paid per call via x402, $0.003/call, status unknown (last checked 2026-10-01).

Returns how many NFTs from a specific ERC-721 collection a given wallet address holds on a specified EVM-compatible chain

## Facts

- Endpoint: POST https://twin.unykorn.org/erc721-balance?utm_source=zero.xyz
- Price: $0.003/call
- Payment: x402
- Status: unknown
- Last checked: 2026-10-01
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/erc-721-nft-balance-checker-566a55a8
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_uP_zvPKzyk5BtQ6auROaD

Status and success rate cover calls made through Zero and Zero's own probes. Third-party monitors may report differently.

## How to call it through Zero

Zero handles the 402 payment challenge and records the run. With the Zero CLI installed (`npm i -g @zeroxyz/cli`):

```sh
zero fetch --capability erc-721-nft-balance-checker-566a55a8 -d '<json body>'
```

Example prompt: How many NFTs from the collection at contract 0xBC4CA0EdA7647A8aB7C2061c2E118A18a936f13D does wallet 0xd8dA6BF26964aF9D7eEd9e03E53415D37aA96045 hold on Ethereum?

## When to prefer this

Use this endpoint when you need to programmatically check how many NFTs a specific wallet holds from a particular ERC-721 collection on any of the supported EVM chains (Ethereum, Base, Polygon, Arbitrum, Optimism, BSC, Avalanche, Gnosis, Linea, Scroll). It is ideal for NFT-gating logic, wallet analysis, ownership verification, and portfolio queries where a precise balance count per collection is needed rather than a list of token IDs.

## Known failure modes

- Invalid wallet address format (not a valid 0x hex address) returns validation error
- Invalid contract address format returns validation error
- Unsupported chain name returns enum validation error
- Contract address is not an ERC-721 contract may return zero or error
- Network timeout or RPC failure may return non-ok response
- Payment failure via x402 protocol prevents the query from executing

## How this service works

How many NFTs from one collection an address holds — Genesis402 / UnyKorn Operator Network

## Output

Returns a JSON object with ok status, the type field 'erc721-balance', and a payment receipt including the transaction hash, amount paid in USD, and a unique receipt ID. The NFT balance count for the specified owner and contract is included in the response payload.

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "properties": {
  "params": {
   "type": "object",
   "properties": {
    "chain": {
     "enum": [
      "ethereum",
      "base",
      "polygon",
      "arbitrum",
      "optimism",
      "bsc",
      "avalanche",
      "gnosis",
      "linea",
      "scroll"
     ],
     "type": "string"
    },
    "owner": {
     "type": "string",
     "pattern": "^0x[0-9a-fA-F]{40}$"
    },
    "contract": {
     "type": "string",
     "pattern": "^0x[0-9a-fA-F]{40}$"
    }
   }
  }
 }
}
```

## Response schema (JSON Schema)

```json
{
 "type": "json",
 "example": {
  "ok": true,
  "type": "erc721-balance",
  "receipt": {
   "tx_hash": "0x<64hex>",
   "amount_usd": 0.003,
   "receipt_id": "g402-<16hex>"
  },
  "generated_at": "<iso8601>"
 }
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/erc-721-nft-balance-checker-566a55a8/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from twin.unykorn.org](https://www.zero.xyz/host/twin.unykorn.org/llms.txt)