> ## 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.

# CCTP block confirmations V1

> Block confirmation requirements for attestations by chain

<Warning>
  **This is CCTP V1 (Legacy) version. For the latest version, see [CCTP](/cctp)**.
</Warning>

Before signing an attestation for a source chain event, Circle waits for a
specified number of onchain block confirmations to achieve
<Tooltip tip="The point at which a transaction is fully finalized on-chain and can no longer be reversed or reorganized. This ensures the highest level of security and certainty before attestation. Hard Finality requires the transaction to reach full consensus on the source blockchain, typically resulting in longer processing times.">hard finality</Tooltip>.

The table below shows the average time required for an attestation to become
available after a message is emitted onchain.

<Note>**Note:** These values are subject to change.</Note>

## CCTP V1 Attestation Times

| Source Chain | Number of Blocks | Average Time |
| - | - | - |
| **Ethereum** | \~65\* | \~13 to 19 minutes\* |
| **Avalanche** | 1 | \~8 seconds |
| **OP Mainnet** | \~65 ETH blocks\* | \~13 to 19 minutes\* |
| **Arbitrum** | \~65 ETH blocks\* | \~13 to 19 minutes\* |
| **Noble** | 1 | \~20 seconds |
| **Base** | \~65 ETH blocks\* | \~13 to 19 minutes\* |
| **Polygon PoS** | \~33 | \~75 to 120 seconds |
| **Solana** | 32 | \~25 seconds |
| **Sui** | 1 | \~8 seconds |
| **Aptos** | 1 | \~8 seconds |
| **Unichain** | \~65 ETH blocks\* | \~13 to 19 minutes\* |

<Note>
  **Block confirmations for L2s to Ethereum**

  Layer 2 (L2) blockchains publish transaction data in batches to Ethereum L1, and
  the frequency of these posts varies by chain. Some submit batches every few
  minutes, while others are less frequent. After a batch is posted, Circle waits
  for the Ethereum L1 block containing the batch to finalize, which typically
  happens after \~65 blocks, or 13 to 19 minutes, before issuing an attestation.
</Note>