> ## 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

Gateway Nanopayments supports gas-free USDC payments on the EVM blockchains in
the [Gateway supported blockchains](/gateway/references/supported-blockchains)
reference. Solana is the only Gateway blockchain not currently supported.

Use the `SupportedChainName` value from the 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](/gateway/references/supported-blockchains#fast-deposits) section
  offers third-party alternatives for faster deposits.
</Note>

## 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](/gateway/references/contract-addresses)
* **USDC token address** (used as the `asset` in payment requirements):
  [USDC contract addresses](/stablecoins/usdc-contract-addresses)

The SDK's `CHAIN_CONFIGS` constant provides these values programmatically. See
the [SDK reference](/sdks/gateway-nanopayments-sdk#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