> ## 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.

# Gateway supported blockchains

> List of blockchains that Gateway supports

Gateway supports a subset of blockchains where USDC is natively issued. The
following table shows the blockchains that Gateway supports.

Circle identifies each blockchain by a numeric domain identifier. These domain
identifiers are not tied to any existing public chain ID. Gateway uses the same
domain identifiers as [CCTP](/cctp/cctp-supported-blockchains). The Mainnet and
Testnet columns show the `SupportedChainName` value to pass in API requests.

| Blockchain | Domain | Mainnet | Testnet |
| - | - | - | - |
| Arbitrum | 3 | `arbitrum` | `arbitrumSepolia` |
| Arc | 26 | `arc` | `arcTestnet` |
| Avalanche | 1 | `avalanche` | `avalancheFuji` |
| Base | 6 | `base` | `baseSepolia` |
| Ethereum | 0 | `ethereum` | `sepolia` |
| HyperEVM | 19 | `hyperEvm` | `hyperEvmTestnet` |
| OP | 2 | `optimism` | `optimismSepolia` |
| Polygon PoS | 7 | `polygon` | `polygonAmoy` |
| Sei | 16 | `sei` | `seiAtlantic` |
| Solana | 5 | — | — |
| Sonic | 13 | `sonic` | `sonicTestnet` |
| Unichain | 10 | `unichain` | `unichainSepolia` |
| World Chain | 14 | `worldChain` | `worldChainSepolia` |

Gateway supports nanopayments on all blockchains except Solana.

## Required block confirmations

Before updating the unified USDC balance or issuing
[attestations](/gateway/references/technical-guide#attestation), the Gateway API
waits for a specific number of block confirmations so that the deposit
transaction is unlikely to be reverted.

The following table shows the number of block confirmations and the average time
to attestation for each supported blockchain. The average time to attestation is
the time it takes for a transaction to be included in a block and confirmed by
the network.

| Blockchain | Number of blocks | Average time |
| - | - | - |
| Arbitrum | \~65 ETH blocks | \~13 to 19 minutes |
| Arc | \~1 | \~0.5 seconds |
| Avalanche | 1 | \~8 seconds |
| Base | \~65 ETH blocks | \~13 to 19 minutes |
| Ethereum | \~65 | \~13 to 19 minutes |
| HyperEVM | \~1 | \~5 seconds |
| OP | \~65 ETH blocks | \~13 to 19 minutes |
| Polygon PoS | \~2-3 | \~8 seconds |
| Sei | \~1 | \~5 seconds |
| Solana | \~2-3 | \~8 seconds |
| Sonic | \~1 | \~8 seconds |
| Unichain | \~65 ETH blocks | \~13 to 19 minutes |
| World Chain | \~65 ETH blocks | \~13 to 19 minutes |

## Fast deposits

Standard deposits wait for the deposit transaction to finalize on the source
blockchain before crediting a unified USDC balance, which can take several
minutes. Gateway Fast Deposit uses
[CCTP Fast Transfer](/cctp/concepts/finality-and-block-confirmations) to
accelerate eligible deposits and credit the balance in seconds. Fast deposits
settle to Avalanche or Polygon PoS, and developers pay the CCTP Fast Transfer
fee for each transfer.

Gateway Fast Deposit is available through Unified Balance in Arc App Kits. For
the integration walkthrough, see
[Use fast deposits](https://docs.arc.io/app-kit/tutorials/unified-balance/use-fast-deposits)
in the Arc documentation.