Arbitrum open house

A-Identity at the Arbitrum Open House Singapore Buildathon

Before an AI agent pays on Robinhood Chain or Arbitrum One, three questions should have answers: who gets paid, is the dollar real, and how much may the agent spend. A-Identity answers all three at the moment of payment, on mainnet, with a receipt or a live read behind every claim.

On 2026-09-30, 900 tokens on Robinhood Chain’s public explorer had the symbol USDG or the name “Global Dollar”; 45 of them showed more than 10,000 holders, and the biggest, 0xA913C4C2F28AA7b0B15A7C6008a6e19Ff8Bf85c0, showed 194,589 against the real USDG’s 401,824. A symbol is not an identity. That is the problem this entry is about.

Try it

• a-identity.xyz/check/robinhood: paste a token, a wallet, an agent id or an x402 link. /check/arbitrum is the same check for Arbitrum One and native Circle USDC.
• For agents: GET https://a-identity.xyz/api/robinhood/check?q=<input> (no key), the reason codes at /api/robinhood/check/codes, or the MCP tool evm_pay_check on https://a-identity.xyz/mcp.
• The evidence: /proof/robinhood and /proof/arbitrum list every transaction we claim and re-read agent #0 and agent #1259 from the chain on every load.

Built during the Buildathon (from Sep 14, 2026)

Commit or tx	Date	What
1ac21dd5	2026-10-01	Our facilitator read only x402 v1’s maxAmountRequired, so every stock v2 request failed; it reads v2’s amount now. /api/facilitator/supported quoted one chain’s minimum on every chain (Arbitrum showed 21000 while its own 402 asks 6000); each chain carries its own now. /api/facilitator/proof summed gas across chains with different gas units; it reports gas per network now. Our buyer script refuses to sign a challenge for a token the chain does not settle in.
f7f97a4d	2026-10-01	The pay check: GET /api/robinhood/check, /api/arbitrum/check and the MCP tool evm_pay_check. Token, wallet, agent id or x402 link, answered from the live chain with stable reason codes.
8870ba99	2026-10-01	/check/robinhood and /check/arbitrum, the same check for people.
96a61409	2026-10-01	Public copy corrected where it overclaimed (see “Bugs found and fixed”).
renew 0xc68bf160...d769, split 0x47d60109...7304	2026-10-01	Robinhood Chain vault: session key renewed to 2026-11-15, agent key split from the owner key.
renew 0x237457f6...960a, split 0x1e40db36...c0fc	2026-10-01	Arbitrum One vault: the same.
47a57d45	2026-09-16	Gas ceilings follow each chain’s gas unit, and the daily gas budget is kept per network. Written for the rail on every chain, including these two.
7ca6c63a	2026-09-25	An agent’s registration date comes from its mint block, not from the day it was read. Applies to agent #0 and #1259.

What the pay check does, in detail:

• A token. It must be the settlement dollar our chain registry names (Paxos USDG on Robinhood Chain, native Circle USDC on Arbitrum One), and its EIP-712 signing domain must reproduce the token’s live DOMAIN_SEPARATOR. USDG exposes no version(), so the domain is proven from candidates, never pasted. A token that copies the name and symbol is a fake; one that only shares the symbol (Arbitrum’s bridged USDC.e) is “not the dollar this chain settles in”, never “fake”. The answer always names where the real one is.
• A wallet. Does it hold an ERC-8004 agent id on this chain, and was it ever used.
• An agent id. Is it registered, and does the registration file it points to list that id back. Token #1 on Robinhood Chain was minted 7 seconds after our #0, pointing at our agent card; the check flags it.
• An x402 link. Which asset its 402 challenge asks for, which signing domain it hands the buyer, and who it pays.
• The chain. It reads arbChainID() from the ArbSys precompile at 0x64 (4663 and 42161) instead of trusting a name.
• Safety of the check itself. A pasted link is fetched with the address screen inside the connection’s own DNS lookup, so DNS rebinding cannot reach a private network, and redirects are not followed. Nothing unproven is called safe: an unprovable domain reads “Could not verify”, and a registration is labeled identity, not a review.

Already there before Sep 14 (the baseline, not claimed as Buildathon work)

• 2026-08-12: agent #0 minted on Robinhood Chain mainnet’s canonical ERC-8004 IdentityRegistry, tx 0x602ce85ad044836b39918311a3031dcd689e4be0d23aed9ed0ac9227d46ec79e.
• 2026-08-13: our x402 facilitator settling in USDG on Robinhood Chain; agent #1259 on Arbitrum One, tx 0x23275840eb9a8b85a752769c113109a753f39b592236c85093cf94f6a517b2f3.
• 2026-08-28: the two AgentSpendPolicy vaults deployed, Robinhood Chain 0x05a6aad7124c2f1c7b82b03e0bbe3867bc500073 and Arbitrum One 0xac6c5c9af62bc482ffeef882a6ac4678513be6db.

Numbers (2026-10-01)

• 1,480 unit tests declared across 98 files, all passing, plus 48 end-to-end and 149 guardrail assertions against a running server.
• Vault policy on both chains: 1 USDG or USDC a day and 0.25 per payment for the agent key; above that the contract refuses with AboveAutoApprove and the payment is the owner’s to make.
• Each vault operation on 2026-10-01 cost about 31,000 gas, about 0.0000006 ETH.

Bugs found and fixed

We audited our own Arbitrum and Robinhood Chain surfaces during the Buildathon. What we found was wrong, and is now fixed:

• Both mainnet vaults’ session keys had lapsed on 2026-09-04, so every agent payment reverted, while the proof pages still described a working 7-day key. Renewed, and the agent key now differs from the owner key, which it did not before.
• The facilitator rejected every stock x402 v2 request (the amount field).
• /supported advertised one chain’s minimum on every chain.
• The proof’s gas total added Arc’s USDC-denominated gas to ETH gas; 99% of the figure was the former.
• Nine public surfaces said no published facilitator served Robinhood Chain. Others do. The surfaces now say why we run our own instead: one code path and one receipt standard on every chain we sell on.
• A doc called agent #0 “the first agent ever registered on this chain”. It is the first token the canonical registry minted, which is narrower and true.

Known limits

• Every settlement so far is our own buyer wallet paying our own address. It shows the rail works, not that there is demand, and the proof pages label it internal.
• We did not deploy the ERC-8004 registries; their authors did.
• The vault owner is a server-held key, and the contract has no owner transfer. The split limits what the agent key can do; it does not make the owner a hardware wallet.
• There is no ValidationRegistry in this mainnet registry family, so a KYA result cannot be anchored on chain here.
• These are L2s: we wait for confirmations and record the block, and we do not claim Ethereum finality.
• A-Identity also runs on Arc, Base, Stellar and Algorand. This page is only about Robinhood Chain and Arbitrum One.

Prior art

Other x402 facilitators serve Robinhood Chain, hosted and open-source, some predating our August launch; Coinbase and thirdweb serve Arbitrum One. Gasless EIP-3009 relaying on USDG predates us. Other projects verify stock tokens by bytecode and deployer. We do not claim to be first at any of these. What this entry combines is the payment moment: who, is the dollar real, and how much, in one flow, on mainnet.

Verify it yourself

# The impostor with the most holders, and the real USDG.
curl -s 'https://a-identity.xyz/api/robinhood/check?q=0xA913C4C2F28AA7b0B15A7C6008a6e19Ff8Bf85c0'
curl -s 'https://a-identity.xyz/api/robinhood/check?q=0x5fc5360D0400a0Fd4f2af552ADD042D716F1d168'

# Agent #0, and the clone minted 7 seconds later.
curl -s 'https://a-identity.xyz/api/robinhood/check?q=%230'
curl -s 'https://a-identity.xyz/api/robinhood/check?q=%231'

# Each chain's own minimum, and gas per network.
curl -s https://a-identity-backend.onrender.com/api/facilitator/supported
curl -s https://a-identity-backend.onrender.com/api/facilitator/proof

# The vault's agent key and its expiry, read from the chain (operator() and sessionKeyExpiry()).
cast call 0x05a6aad7124c2f1c7b82b03e0bbe3867bc500073 'operator()(address)' --rpc-url https://rpc.mainnet.chain.robinhood.com
cast call 0xac6c5c9af62bc482ffeef882a6ac4678513be6db 'sessionKeyExpiry()(uint256)' --rpc-url https://arb1.arbitrum.io/rpc

Source: github.com/getA-Identity/A-Identity. The pay check is in mcp/src/evm-pay-check/, the rail in mcp/src/x402-3009/, the vault in mcp/contracts/AgentSpendPolicy.sol.