> ## Documentation Index
> Fetch the complete documentation index at: https://a-identity.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

# Build on Arbitrum One

> Arbitrum One network config, the canonical ERC-8004 registries already deployed there, and our x402 rail settling in native Circle USDC.

Arbitrum One is the largest Ethereum L2 by usage, and the chain whose stack Robinhood
Chain is built on. A-Identity holds an ERC-8004 agent identity here and settles paid
trust calls in native Circle USDC.

## Why Arbitrum One

<CardGroup cols={3}>
  <Card title="Native Circle USDC" icon="dollar-sign">
    Real USDC, not a bridged twin, and it implements EIP-3009, so a buyer can sign a transfer and never touch gas.
  </Card>

  <Card title="ERC-8004 already live" icon="fingerprint">
    The canonical identity and reputation registries were deployed here by their authors, at the same addresses X Layer, Celo and Robinhood Chain carry.
  </Card>

  <Card title="A well-served baseline" icon="scale-balanced">
    Other x402 facilitators already serve this chain. Ours runs beside them on purpose: it is where you can check our rail against something you can compare it to.
  </Card>
</CardGroup>

<Note>
  Arbitrum One is **live** for us: agent #1259 is ours on the canonical registry, and a
  real settlement in USDC has gone through our own facilitator. Everything on this page is
  checkable at [/proof/arbitrum](https://a-identity.xyz/proof/arbitrum).
</Note>

## Network config

| Field           | Value                                        |
| --------------- | -------------------------------------------- |
| Chain ID        | `42161`                                      |
| CAIP-2          | `eip155:42161`                               |
| Native currency | ETH (18 decimals)                            |
| RPC (HTTP)      | `https://arb1.arbitrum.io/rpc`               |
| Block explorer  | [arbiscan.io](https://arbiscan.io)           |
| USDC            | `0xaf88d065e77c8cC2239327C5EDb3A432268e5831` |
| CCTP domain     | `3`                                          |

<Warning>
  Two things worth stating plainly about this chain.

  **We are not first here, and do not claim to be.** Coinbase's x402 facilitator and
  others already serve Arbitrum One, and gasless EIP-3009 relaying on this USDC contract
  predates us by years. We run our own facilitator so that one code path and one receipt
  standard cover every chain we sell on, including chains nobody else serves.

  **"Settled" has layers on an L2.** We wait the descriptor's confirmations, which is the
  sequencer's ordering commitment, and we record the block. That is not Ethereum finality,
  and USDC cannot leave for Ethereum until the fraud-proof window passes.
</Warning>

## Connect with viem

<CodeGroup>
  ```ts chain.ts theme={null}
  import { arbitrum } from 'viem/chains'
  import { createPublicClient, http } from 'viem'

  export const client = createPublicClient({ chain: arbitrum, transport: http() })
  ```

  ```ts read.ts theme={null}
  import { client } from './chain'

  // Agent #1259 is ours on the canonical registry.
  const owner = await client.readContract({
    address: '0x8004a169fb4a3325136eb29fa0ceb6d2e539a432',
    abi: [{ type: 'function', name: 'ownerOf', stateMutability: 'view', inputs: [{ type: 'uint256' }], outputs: [{ type: 'address' }] }],
    functionName: 'ownerOf',
    args: [1259n],
  })
  ```
</CodeGroup>

## What is deployed

| Contract           | Address                                                                                                                |
| ------------------ | ---------------------------------------------------------------------------------------------------------------------- |
| IdentityRegistry   | [`0x8004a169fb4a3325136eb29fa0ceb6d2e539a432`](https://arbiscan.io/address/0x8004a169fb4a3325136eb29fa0ceb6d2e539a432) |
| ReputationRegistry | [`0x8004BAa17C55a88189AE136b182e5fdA19dE9b63`](https://arbiscan.io/address/0x8004BAa17C55a88189AE136b182e5fdA19dE9b63) |
| USDC (settlement)  | [`0xaf88d065e77c8cC2239327C5EDb3A432268e5831`](https://arbiscan.io/address/0xaf88d065e77c8cC2239327C5EDb3A432268e5831) |

We deployed none of these. The registries were already live and registering on them is
permissionless; what is ours is agent #1259 and the rail on top. There is no
ValidationRegistry in this family, so a KYA result cannot be anchored on-chain here.

## The price, and where it comes from

The base price of a tool is identical on every chain we sell on. The only difference is a
disclosed settlement fee, because we broadcast on the buyer's behalf and that costs gas.
On Arbitrum One a real settlement measured **103569 gas at 0.02 gwei**, about
**0.00000207 ETH**, so the fee here is \*\*$0.005** rather than the $0.02 charged on
Robinhood Chain, where the same settlement costs measurably more. The measurement lives in
the chain registry beside the token, so the number is traceable rather than asserted.

## Verify it yourself

```bash theme={null}
# The signing domain, straight from the token.
curl -sX POST https://arb1.arbitrum.io/rpc -H 'content-type: application/json' \
  -d '{"jsonrpc":"2.0","id":1,"method":"eth_call","params":[{"to":"0xaf88d065e77c8cC2239327C5EDb3A432268e5831","data":"0x3644e515"},"latest"]}'
# -> 0x08d11903f8419e68b1b8721bcbe2e9fc68569122a77ef18c216f10b3b5112c78

# Our provenance ledger, with a live re-read of ownerOf(1259) on every request.
curl -s https://a-identity-backend.onrender.com/api/proof/arbitrum
```

## Where Arbitrum One fits

It is the chain where our rail can be compared. [Robinhood Chain](/chains/robinhood) is an
Arbitrum L2 and is where the same engine runs on a chain with no published facilitator;
[Arc](/chains/arc) holds escrow and the spend vault; [Celo](/chains/celo) settles through
someone else's facilitator. Being live on Robinhood Chain and being live on Arbitrum One
are two different statements, and both are true.