# Fabler Labs NFT Owner Lookup (x402)

> Fabler Labs NFT Owner Lookup (x402) is a paid API for AI agents from x402.fablerlabs.com, paid per call via x402, $0.003/call, status unknown (last checked 2026-09-14).

Looks up the current owner address of an ERC-721 NFT token on the Base blockchain, given a contract address and token ID.

## Facts

- Endpoint: GET https://x402.fablerlabs.com/nft/owner
- Price: $0.003/call
- Payment: x402
- Status: unknown
- Last checked: 2026-09-14
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/fabler-labs-nft-owner-lookup-x402-e3c06390
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_vuLwj891IHXMiF5uEWN1t

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 fabler-labs-nft-owner-lookup-x402-e3c06390
```

Example prompt: Who currently owns token ID 1234 from the NFT contract 0x03c4738ee98ae44591e1a4a4f3cab6641d95dd9a on Base mainnet?

## When to prefer this

Choose this endpoint when you need a lightweight, pay-per-call lookup of the current owner of any ERC-721 NFT on the Base mainnet without standing up your own RPC node or managing indexer infrastructure. It is especially useful for AI agents that need occasional, real-time on-chain ownership checks and can handle x402 micropayments in USDC. Prefer alternatives (e.g. a free public RPC) if you need high-volume bulk lookups or if your agent cannot handle the x402 payment flow.

## Known failure modes

- Invalid contract address format (not a valid 0x-prefixed 40-char hex) returns a validation error
- Invalid or out-of-range token ID returns a validation error
- Unpaid request returns HTTP 402 with an empty body and a base64-encoded PAYMENT-REQUIRED header containing payment challenge details
- Token ID does not exist on the contract may return an error or zero address
- RPC upstream (Base node) temporarily unavailable may cause a timeout or 5xx error
- Malformed PAYMENT-SIGNATURE header causes payment verification failure

## How this service works

Machine-payable endpoints for AI agents, over x402 protocol v2. Unpaid requests to a paid endpoint return `402 Payment Required` with an empty `{}` body and a typed challenge (price, network, asset, pay-to address) in a base64-encoded `PAYMENT-REQUIRED` response header; decode it, pay the USDC, and replay with a base64 `PAYMENT-SIGNATURE` header to get the result. Built and operated by an autonomous AI agent, filmed for transparency. Live status and prices are authoritative in https://fablerlabs.com/products.json.

## Output

Returns a JSON object containing the current owner's Ethereum address, the contract address, token ID, chain info (name, chain ID, CAIP-2 identifier), the RPC upstream used, block tag (latest), and the timestamp of the lookup.

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "$schema": "https://json-schema.org/draft/2020-12/schema",
 "required": [
  "input"
 ],
 "properties": {
  "input": {
   "type": "object",
   "required": [
    "type",
    "method"
   ],
   "properties": {
    "type": {
     "type": "string",
     "const": "http"
    },
    "method": {
     "enum": [
      "GET"
     ],
     "type": "string"
    },
    "queryParams": {
     "type": "object",
     "required": [
      "contract",
      "token_id"
     ],
     "properties": {
      "contract": {
       "type": "string",
       "pattern": "^0x[0-9a-fA-F]{40}$",
       "description": "ERC-721 contract address on Base."
      },
      "token_id": {
       "type": "string",
       "pattern": "^(0|[1-9][0-9]{0,77})$",
       "description": "Canonical unsigned decimal uint256 token ID."
      }
     },
     "additionalProperties": false
    }
   },
   "additionalProperties": false
  },
  "output": {
   "type": "object",
   "required": [
    "type"
   ],
   "properties": {
    "type": {
     "type": "string"
    },
    "example": {
     "type": "object",
     "required": [
      "chain",
      "contract",
      "token_id",
      "owner",
      "block_tag",
      "upstream",
      "as_of"
     ],
     "properties": {
      "as_of": {
       "type": "string",
       "format": "date-time"
      },
      "chain": {
       "type": "object",
       "required": [
        "name",
        "chain_id",
        "caip2"
       ],
       "properties": {
        "name": {
         "type": "string",
         "const": "Base"
        },
        "caip2": {
         "type": "string",
         "const": "eip155:8453"
        },
        "chain_id": {
         "type": "integer",
         "const": 8453
        }
       }
      },
      "owner": {
       "type": "string",
       "pattern": "^0x[0-9a-f]{40}$"
      },
      "contract": {
       "type": "string",
       "pattern": "^0x[0-9a-f]{40}$"
      },
      "token_id": {
       "type": "string",
       "pattern": "^(0|[1-9][0-9]*)$"
      },
      "upstream": {
       "enum": [
        "https://mainnet.base.org",
        "https://base-rpc.publicnode.com"
       ],
       "type": "string"
      },
      "block_tag": {
       "type": "string",
       "const": "latest"
      }
     }
    }
   }
  }
 }
}
```

## Response schema (JSON Schema)

```json
{
 "type": "json",
 "example": {
  "as_of": "2026-07-10T13:45:00.000Z",
  "chain": {
   "name": "Base",
   "caip2": "eip155:8453",
   "chain_id": 8453
  },
  "owner": "0xfd2872d1304f59d400f091ba99998e00bb9180b8",
  "contract": "0x03c4738ee98ae44591e1a4a4f3cab6641d95dd9a",
  "token_id": "115792025850165429776940374148095744853770693818332262653016833339720940464639",
  "upstream": "https://mainnet.base.org",
  "block_tag": "latest"
 }
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/fabler-labs-nft-owner-lookup-x402-e3c06390/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from x402.fablerlabs.com](https://www.zero.xyz/host/x402.fablerlabs.com/llms.txt)