> ## 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.

# Finality and block confirmations

> Block confirmation requirements and attestation timing for CCTP

Before signing an attestation, Circle waits for blockchain transactions to
achieve the appropriate level of transaction finality. The required finality
level depends on whether you use Fast Transfer or Standard Transfer.

* Fast Transfer: Attestations are issued after the transaction is confirmed and
  included in a block, typically in seconds. Because of the faster finality
  time, Fast Transfers are subject to a global allowance to mitigate
  reorganization risks.
* Standard Transfer: Attestations are issued after hard finality, when the
  transaction is unlikely to be reversed by a chain reorganization, typically in
  minutes.

## Fast Transfer attestation times

The table below shows the average time for attestations to become available when
using Fast Transfer (`minFinalityThreshold` ≤ 1000):

| Source blockchain | Block confirmations | Average time |
| - | - | - |
| **Ethereum** | 2 | \~20 seconds |
| **Arbitrum** | 1 | \~8 seconds |
| **Base** | 1 | \~8 seconds |
| **Codex** | 1 | \~8 seconds |
| **EDGE** | 1 | \~8 seconds |
| **Ink** | 1 | \~8 seconds |
| **Linea** | 1 | \~8 seconds |
| **Morph** | 1 | \~8 seconds |
| **OP Mainnet** | 1 | \~8 seconds |
| **Plume** | 1 | \~8 seconds |
| **Solana** | 2-3 | \~8 seconds |
| **Starknet** | 4 | \~20 seconds |
| **Unichain** | 1 | \~8 seconds |
| **World Chain** | 1 | \~8 seconds |
| **X Layer** | 1 | \~8 seconds |

<Note>
  **Blockchains without Fast Transfer:**

  Some blockchains don't support Fast Transfer as a source blockchain because
  their standard attestation times are already fast. For those
  [CCTP supported blockchains](/cctp/concepts/supported-chains-and-domains) where
  Fast Transfer is disabled as a source, use Standard Transfer instead.
</Note>

## Standard Transfer attestation times

The table below shows the average time for attestations to become available when
using Standard Transfer (`minFinalityThreshold` ≥ 2000):

| Source blockchain | Block confirmations | Average time |
| - | - | - |
| **Aptos** | 1 | \~0.5 seconds |
| **Ethereum** | \~65 | \~15-19 minutes |
| **Arbitrum** | \~65 ETH blocks | \~15-19 minutes |
| **Arc** | 1 | \~0.5 seconds |
| **Avalanche** | 1 | \~8 seconds |
| **Base** | \~65 ETH blocks | \~15-19 minutes |
| **BNB Smart Chain** | 3 | \~2 seconds |
| **Codex** | \~65 ETH blocks | \~15-19 minutes |
| **Cronos** | 1 | \~0.5 seconds |
| **EDGE** | \~65 ETH blocks | \~16-21 minutes |
| **HyperEVM** | 1 | \~5 seconds |
| **Injective** | 1 | \~0.65 seconds |
| **Ink** | \~65 ETH blocks | \~30 minutes |
| **Linea** | 1 | \~6-32 hours |
| **Monad** | 1 | \~5 seconds |
| **Morph** | \~65 ETH blocks | \~20-30 minutes |
| **OP Mainnet** | \~65 ETH blocks | \~15-19 minutes |
| **Pharos** | 1 | \~7 seconds |
| **Plasma** | 3 | \~1 second |
| **Plasma Testnet** | 3 | \~1 second |
| **Plume** | \~65 ETH blocks | \~15-19 minutes |
| **Polygon PoS** | 2-3 | \~8 seconds |
| **Sei** | 1 | \~5 seconds |
| **Solana** | 32 | \~25 seconds |
| **Sonic** | 1 | \~8 seconds |
| **Starknet** | \~65 ETH blocks | \~2 to 4 hours |
| **Stellar** | 1 | \~5 seconds |
| **Sui** | 1 | \~0.5 seconds |
| **Unichain** | \~65 ETH blocks | \~15-19 minutes |
| **World Chain** | \~65 ETH blocks | \~15-19 minutes |
| **X Layer** | \~65 ETH blocks | \~15-19 minutes |
| **XDC** | 3 | \~10 seconds |

## Layer 2 finality

Layer 2 (L2) blockchains built on Ethereum publish transaction data in batches
to Ethereum Layer 1. The finality characteristics of L2 chains depend on when
batches are posted and when those batches achieve finality on Ethereum L1.

<CardGroup cols={2}>
  <Card title="OP Stack chains" icon="layer-group">
    OP Stack-based chains (including Base, OP Mainnet, World Chain, and X Layer) post
    state updates using [EIP-4844](https://www.eip4844.com/) blob transactions
    approximately every 15 minutes. Circle waits for the Ethereum L1 block
    containing the batch to finalize, which typically takes \~65 blocks (15-19
    minutes) after the batch is posted.
  </Card>

  <Card title="Linea" icon="clock">
    Linea has a longer finality period compared to other L2 chains. Standard
    Transfer on Linea typically requires 6-32 hours before attestations become
    available.
  </Card>

  <Card title="Starknet" icon="clock">
    Starknet finality depends on when the zk-rollup proof is posted to Ethereum
    and when its corresponding L1 block finalizes. Attestations for Standard
    Transfer typically become available 2–4 hours after the burn, with most
    transfers closer to 3 hours.
  </Card>
</CardGroup>

## Solana finality

Solana uses a different finality model:

<CardGroup cols={2}>
  <Card title="Fast Transfer" icon="bolt">
    Circle waits for the block to be confirmed (votes from validators
    representing over two-thirds of total stake). This typically takes 2-3
    blocks (\~8 seconds).
  </Card>

  <Card title="Standard Transfer" icon="shield-check">
    Circle waits for block finality, which takes 32 blocks (\~25 seconds).
  </Card>
</CardGroup>