{"uid":"cap_vuLwj891IHXMiF5uEWN1t","slug":"fabler-labs-nft-owner-lookup-x402-e3c06390","name":"Fabler Labs NFT Owner Lookup (x402)","description":"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.","url":"https://x402.fablerlabs.com/nft/owner","method":"GET","headers":{},"bodySchema":{"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"}}}}}}},"responseSchema":{"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"}},"example":null,"exampleRequest":null,"tags":["x402"],"displayCostAmount":"0.003","displayCostAsset":"USDC","priceDynamic":false,"priceHint":null,"priceStatus":"priced","priceSource":"probe","requiresHandshake":false,"reviewCount":0,"rating":{"score":"0.00","successRate":"0.00","reviews":0,"stars":null,"state":"unrated"},"availabilityStatus":"unknown","priceObserved":null,"sessionDeposit":null,"pricing":{"kind":"static","summary":"$0.003/call","primary":{"kind":"static","protocol":"x402","network":"base","amountUsd":"0.003","per":"call","confidence":"exact"},"accepted":[{"kind":"static","protocol":"x402","network":"base","amountUsd":"0.003","per":"call","confidence":"exact"}]},"paymentMethods":[{"uid":"pm_dqKoefFzyrm7hZy_stqhs","protocol":"x402","methodType":"crypto","chain":"base","mode":"charge","costAmount":"0.003","costPer":"request","priority":0,"asset":"0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913","unit":"request","depositMicros":null,"planRef":null}],"brandName":null,"brandSlug":null,"brandBaseUrl":null,"brandDocsUrl":null,"whatItDoes":"Looks up the current owner address of an ERC-721 NFT token on the Base blockchain, given a contract address and token ID.","exampleAgentPrompt":"Who currently owns token ID 1234 from the NFT contract 0x03c4738ee98ae44591e1a4a4f3cab6641d95dd9a on Base mainnet?","exampleUseCases":[{"title":"Verify NFT ownership before gating access","prompt":"Can you check if wallet 0xfd2872d1304f59d400f091ba99998e00bb9180b8 actually owns token ID 42 from the contract 0x03c4738ee98ae44591e1a4a4f3cab6641d95dd9a on Base? I need to confirm before giving them access."},{"title":"Track NFT after marketplace sale","prompt":"I just saw a sale on the secondary market for token 999 from contract 0x03c4738ee98ae44591e1a4a4f3cab6641d95dd9a on Base — who owns it now?"},{"title":"Audit NFT provenance for a collection","prompt":"Look up who owns token ID 115792025850165429776940374148095744853770693818332262653016833339720940464639 from the Base NFT contract 0x03c4738ee98ae44591e1a4a4f3cab6641d95dd9a right now."}],"resultDescription":"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.","failureModes":["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"],"whenToPreferThis":"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.","instructions":null,"reviewSummary":null,"reviewSummaryHighlights":null,"reviewSummaryConcerns":null,"reviewSummaryGeneratedAt":null,"activationCount":0,"lastUsedAt":null,"lastSuccessfullyRanAt":null,"lastHealthCheckAt":"2026-09-14T12:42:47.405Z","isFirstParty":false}