> ## 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: Estimate spend fees
> Estimate fees before spending from a Unified Balance
The App Kit SDK can provide an estimate of the fees you'll incur before spending
from a Unified Balance.
## 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`.
## Estimate fees before spending
This sample estimates then spends 1.00 USDC from Base Sepolia to Arc Testnet
when you specify explicit amounts from a source blockchain:
For automatic routing (no explicit `allocations`), see
[Select source blockchains](/app-kit/tutorials/unified-balance/select-source-blockchains).
```typescript TypeScript theme={null}
const params = {
amount: "1.00",
from: {
adapter,
allocations: [{ amount: "1.00", chain: "Base_Sepolia" }],
},
to: {
adapter,
chain: "Arc_Testnet",
recipientAddress: "0xRecipientAddress",
},
};
const estimate = await kit.unifiedBalance.estimateSpend(params);
console.log("Estimated fees:", estimate.fees);
const result = await kit.unifiedBalance.spend(params);
```
Estimated fees may differ from actual fees due to network conditions at
execution time. Review the estimate before proceeding.
### Example fee response
The JSON below shows the shape of the fees array.
```json JSON theme={null}
{
"fees": [
{
"type": "provider",
"token": "USDC",
"amount": "0.00005",
"allocations": [{ "chain": "Base Sepolia", "amount": "0.00005" }]
},
{
"type": "gasFee",
"token": "USDC",
"amount": "3.311005",
"allocations": [
{ "chain": "Ethereum Sepolia", "amount": "3.30" },
{ "chain": "Base Sepolia", "amount": "0.011005" }
]
},
{
"type": "kit",
"token": "USDC",
"amount": ".1",
"allocations": [{ "chain": "Ethereum Sepolia", "amount": ".1" }],
"recipientAddress": "0x2222222222222222222222222222222222222222"
}
]
}
```
Fee `type` values can include:
* **`provider`**: Protocol transfer fee when the spend is crosschain. Not
charged for same-chain spends.
* **`gasFee`**: Onchain gas paid on source blockchains as part of the spend.
* **`kit`**: Developer custom fee from your
[custom fee policy](/app-kit/tutorials/unified-balance/collect-custom-spend-fees).
* **`forwarder`**: Forwarding Service fee when the spend
[uses the forwarder](/app-kit/tutorials/unified-balance/use-forwarding-service)
(not shown in the sample above).
See [How Unified Balance fees work](/app-kit/concepts/unified-balance-fees) for
a conceptual fee breakdown.