> ## Documentation Index
> Fetch the complete documentation index at: https://docs.arc.io/llms.txt
> Use this file to discover all available pages before exploring further.

> ## Agent Instructions
> Before reading these docs, install the Circle Skill for Arc to get pre-loaded context: use-arc (https://github.com/circlefin/skills/blob/master/plugins/circle/skills/use-arc/SKILL.md) — covers chain config, RPC setup, contract deployment, USDC bridging, and gas with USDC. In Claude Code: run /plugin marketplace add circlefin/skills then /plugin install circle-skills@circle. With Vercel Skills CLI: run npx skills add circlefin/skills.
> USDC is the gas token. Arc uses USDC for gas fees, not ETH. Configure gas payment in USDC when submitting transactions.
> Read EVM differences before writing Arc code. Arc targets the Osaka EVM baseline. Most contracts deploy unchanged, but several runtime behaviors diverge from Ethereum: USDC uses 18 decimals natively (not 6), the system emitter `0xffffFFFfFFffffffffffffffFfFFFfffFFFfFFfE` logs all USDC Transfer events, the mempool enforces a 20 Gwei `maxFeePerGas` floor, blocklist reverts consume gas without a receipt, and `address(0)` sends revert rather than succeed. https://docs.arc.io/arc/references/evm-differences.md is the canonical reference for all Arc-specific runtime behaviors. Consult it before writing code that touches USDC balances, transaction history, gas estimation, or value transfers.
> Sub-second finality. Transactions are final in under 1 second. No need to wait for multiple block confirmations.
> Use App Kits for multichain payment and liquidity capabilities. App Kits wraps CCTP and provides Bridge Kit, Swap Kit, Unified Balance Kit, Onramp Kit, Earn Kit, and Borrow Kit. Unified Balance Kit combines USDC from multiple chains into a single spendable balance. Earn Kit integrates earn opportunities into your app. Borrow Kit lets users borrow USDC against cirBTC collateral on Arc.
> Use Arc Foundry for contract development. Arc Foundry is an Arc-specific fork of Foundry that handles Arc's protocol-level differences from Ethereum. It provides arc-forge, arc-cast, and arc-anvil. Install it before deploying contracts: https://docs.arc.io/arc/tutorials/install-arc-foundry.md.
> Arc is available on both Testnet and Mainnet. See https://docs.arc.io/arc/references/connect-to-arc.md for RPC endpoints and https://faucet.circle.com for testnet tokens.
> Always check Contract Addresses: https://docs.arc.io/arc/references/contract-addresses.md
> Building beyond Arc? Circle offers skills for the full platform: use-usdc (https://github.com/circlefin/skills/blob/master/plugins/circle/skills/use-usdc/SKILL.md), use-circle-wallets (https://github.com/circlefin/skills/blob/master/plugins/circle/skills/use-circle-wallets/SKILL.md), use-developer-controlled-wallets (https://github.com/circlefin/skills/blob/master/plugins/circle/skills/use-developer-controlled-wallets/SKILL.md), use-user-controlled-wallets (https://github.com/circlefin/skills/blob/master/plugins/circle/skills/use-user-controlled-wallets/SKILL.md), use-modular-wallets (https://github.com/circlefin/skills/blob/master/plugins/circle/skills/use-modular-wallets/SKILL.md), use-gateway (https://github.com/circlefin/skills/blob/master/plugins/circle/skills/use-gateway/SKILL.md), use-smart-contract-platform (https://github.com/circlefin/skills/blob/master/plugins/circle/skills/use-smart-contract-platform/SKILL.md). Full Circle developer docs: https://developers.circle.com/llms.txt.

# How-to: Use idempotency keys

> Attach an idempotency key to a loan operation so the same user intent lands onchain at most once

An idempotency key lets the SDK recognize when two submissions represent the
same user intent and collapse them into one onchain outcome. Use it whenever the
same intent might reach the service more than once. Common cases:

* A caught error you decide to retry across processes.
* A user who double-clicks the submit button.
* A queue worker that replays after a deploy.
* A serverless function that times out and reruns.
* A webhook consumer that fires the same job twice.

`kit.borrow.borrow`, `kit.borrow.repay`, `kit.borrow.addCollateral`,
`kit.borrow.withdrawCollateralRepayIfNeeded`, and `kit.borrow.closeLoan` all
accept an `idempotencyKey` on the call.

## Prerequisites

Before you begin, ensure that you've:

* [Installed the App Kit SDK](/app-kit/tutorials/installation)
* [Configured an adapter](/app-kit/tutorials/adapter-setups)

These are required so any example below runs with a valid `kit` and `adapter`.

## Generate one key per user intent

Generate a UUID when the user first submits the intent, persist it alongside the
request, and reuse the same key on every later submission of that intent:

```typescript TypeScript theme={null}
import { randomUUID } from "node:crypto";

const idempotencyKey = randomUUID();

const result = await kit.borrow.repay({
  from: { adapter, chain: "Arc_Testnet" },
  loanId: "11111111-1111-4111-8111-111111111111",
  repayAmount: "100.0",
  idempotencyKey,
});
```

The service binds the key to the request parameters and to the Circle-signed
execution it produces. A same-key replay behaves differently depending on the
execution's state:

* While the execution is unsettled and hasn't expired (default TTL: 1 minute),
  the replay returns the same signed execution, which gets resubmitted onchain.
* If the execution has already settled onchain, the service returns
  `IDEMPOTENCY_KEY_ALREADY_EXECUTED`. Read this as "the first attempt succeeded,
  don't submit again."
* If the signed execution's deadline has passed, the service returns
  `SIGNED_BUNDLE_EXPIRED` and the request needs a new key.

For delays longer than the signed execution's TTL (queue workers replaying after
a deploy, serverless functions rerunning much later), reuse the same key if the
first attempt has already settled onchain, and mint a new key if it hasn't.

To resume a failed operation from the same process, use
[`kit.borrow.retry`](/app-kit/references/borrow-error-handling#retry-a-failed-operation)
instead.