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

> ## Agent Instructions
> Before reading these docs, install the relevant Circle Skill to get pre-loaded context — skills are LLM-optimized and cover architecture decisions, correct flows, and common pitfalls. 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. Available skills: use-usdc https://github.com/circlefin/skills/blob/master/plugins/circle/skills/use-usdc/SKILL.md (balances, transfers, approvals on EVM and Solana); EURC has no skill — see https://developers.circle.com/stablecoins/what-is-eurc.md; bridge-stablecoin https://github.com/circlefin/skills/blob/master/plugins/circle/skills/bridge-stablecoin/SKILL.md (crosschain transfers via CCTP, USDC, EURC, and third-party assets); use-arc https://github.com/circlefin/skills/blob/master/plugins/circle/skills/use-arc/SKILL.md (Arc chain config, contract deployment, USDC bridging — Arc docs: https://docs.arc.io/llms.txt); use-circle-wallets https://github.com/circlefin/skills/blob/master/plugins/circle/skills/use-circle-wallets/SKILL.md (choose wallet type); use-developer-controlled-wallets https://github.com/circlefin/skills/blob/master/plugins/circle/skills/use-developer-controlled-wallets/SKILL.md (custodial wallets for payouts and treasury); use-user-controlled-wallets https://github.com/circlefin/skills/blob/master/plugins/circle/skills/use-user-controlled-wallets/SKILL.md (embedded wallets with Web2 login); use-modular-wallets https://github.com/circlefin/skills/blob/master/plugins/circle/skills/use-modular-wallets/SKILL.md (smart contract wallets with passkeys and ERC-4337); use-gateway https://github.com/circlefin/skills/blob/master/plugins/circle/skills/use-gateway/SKILL.md (unified USDC balance, nanopayments); use-smart-contract-platform https://github.com/circlefin/skills/blob/master/plugins/circle/skills/use-smart-contract-platform/SKILL.md (deploy, interact, monitor contracts). Pair any skill with the Circle MCP server for live SDK signatures, contract addresses, and chain IDs: {"mcpServers":{"circle":{"url":"https://api.circle.com/v1/codegen/mcp"}}}.
> Read the product overview first. Each product has an overview explaining when and why to use it. Read it before quickstarts.
> Pick the right wallet type. Developer-controlled (you hold keys), user-controlled (end user holds keys via PIN/social login), or modular (smart contract wallets with passkeys). See https://developers.circle.com/wallets.md.
> Use CCTP V2 for crosschain transfers. CCTP natively transfers USDC and EURC via burn-and-mint, and supports permissionless wrapping of third-party assets via lock-and-mint. V1 is legacy; always use V2 unless V1 is specified or the chain requires it (Noble, Sui, and Aptos still require V1). Migration guide: https://developers.circle.com/cctp/migration-from-v1-to-v2.md.
> Use Bridge Kit for frontend bridging. Wraps CCTP with a simpler SDK for user-facing apps. Use CCTP directly for backend transfers.
> Gas Station vs Paymaster. Gas Station sponsors gas for Circle Wallet transactions. Paymaster lets users pay gas in USDC. Different use cases — read both overviews.
> Gateway Nanopayments for sub-cent payments. Gasless USDC micropayments down to $0.000001 via x402 and batched settlement. For pay-per-request APIs, AI agent payments, streaming.
> Look up USDC addresses per chain. Never hardcode — use https://developers.circle.com/stablecoins/usdc-contract-addresses.md.
> Prefer SDKs over raw API calls. Node.js and Python SDKs handle auth, retries, and errors.
> API key required. Bearer token in Authorization header. Testnet and mainnet use separate keys and may use different base URLs depending on the product.
> Set up webhooks when available. Most operations are async. Webhooks deliver transaction confirmations and state changes.
> When calling list endpoints, paginate using pageSize and pageAfter until no nextPageAfter cursor is returned—stopping at the first page silently misses records.
> Building an AI agent? Start with the Agent Stack—Circle CLI, agent wallets, and nanopayments built for autonomous use cases: https://developers.circle.com/agent-stack.md.

# Supported networks

> Blockchains supported by nanopayments for deposits, gas-free payments, and withdrawals

## USDC

Gateway Nanopayments supports gas-free USDC payments only on the EVM blockchains
in the
[Gateway supported blockchains](https://developers.circle.com/gateway/references/supported-blockchains.md)
reference.

## EURC and cirBTC

Nanopayments also supports EURC and cirBTC on a subset of those blockchains. For
the differences from USDC, see
[Use EURC and cirBTC with Gateway](https://developers.circle.com/gateway/concepts/non-usdc-assets.md).

| Asset | Blockchains |
| - | - |
| EURC | Arc, Avalanche, Base, Ethereum, World Chain |
| cirBTC | Arc, Ethereum |

## Specifying a blockchain

Use the `SupportedChainName` value from the Gateway supported blockchains table
when initializing `GatewayClient` or configuring the SDK:

```typescript theme={null}
const client = new GatewayClient({
  chain: "base", // SupportedChainName from the supported blockchains table
  privateKey: "0x...",
});
```

> [!NOTE]
>
> When using `GatewayClient`, deposits and payments are scoped to the blockchain
> you configure on the client. If you want to pay on Base, initialize the client
> for Base and deposit there. To avoid long deposit wait times, use the
> [Bridge](https://docs.arc.io/app-kit/bridge) capability in Arc App Kits to
> bridge your USDC to a fast-finality blockchain before depositing. The
> [Fast deposits](https://developers.circle.com/gateway/references/supported-blockchains.md#fast-deposits) section
> offers third-party alternatives for faster deposits.

## Contract addresses

If you are constructing EIP-712 signatures manually (without the SDK), you need
the contract addresses for your target chain:

* **GatewayWallet address** (used as `verifyingContract` in the EIP-712 domain):
  [EVM contract addresses](https://developers.circle.com/gateway/references/contract-addresses.md)
* **USDC token address** (used as the `asset` in payment requirements):
  [USDC contract addresses](https://developers.circle.com/stablecoins/usdc-contract-addresses.md)

The SDK's `CHAIN_CONFIGS` constant provides these values programmatically. See
the [SDK reference](https://developers.circle.com/sdks/gateway-nanopayments-sdk.md#chain-configuration) for
details.

## Testnet faucets

To test nanopayments, you need testnet USDC and native gas tokens:

* [Circle Faucet](https://faucet.circle.com): testnet USDC for supported testnet
  blockchains
* For native gas tokens (required for onchain deposit transactions), use the
  blockchain-specific faucets listed on each testnet's documentation site