> ## 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

> Identifiers and USDC contracts for the blockchains Facilitator Service supports

## Blockchains

* Arc
* Base
* Polygon PoS

The `upto` scheme is available on all supported mainnet networks and Arc
testnet.

## Dynamic lookup

Facilitator Service exposes the current schemes and USDC contracts dynamically
through [`/supported`](https://developers.circle.com/api-reference/facilitator-service/all/get-supported).
Advertise `upto` in your payment requirements only on networks where
`/supported` lists an `upto` kind.

## Identifiers and contracts

Use the `CAIP-2` value as the `network` field and the USDC contract as the
`asset` field in your x402 `paymentRequirements`. The schemes column lists the
x402 payment schemes Facilitator Service accepts on each blockchain.

### Mainnet

| Blockchain | CAIP-2 | USDC contract | Schemes |
| - | - | - | - |
| Arc | `eip155:5042` | `0x3600000000000000000000000000000000000000` | `exact`, `upto` |
| Base | `eip155:8453` | `0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913` | `exact`, `upto` |
| Polygon PoS | `eip155:137` | `0x3c499c542cEF5E3811e1192ce70d8cC03d5c3359` | `exact`, `upto` |

### Testnet

| Blockchain | CAIP-2 | USDC contract | Schemes |
| - | - | - | - |
| Arc | `eip155:5042002` | `0x3600000000000000000000000000000000000000` | `exact`, `upto` |
| Base Sepolia | `eip155:84532` | `0x036CbD53842c5426634e7929541eC2318f3dCF7e` | `exact` |
| Polygon PoS Amoy | `eip155:80002` | `0x41E94Eb019C0762f9Bfcf9Fb1E58725BfB0e7582` | `exact` |

## Usage-based payment contracts

[`upto`](https://developers.circle.com/facilitator-service/how-it-works#usage-based-payments) payments use
these contracts on every network that supports `upto`:

| Contract | Address |
| - | - |
| Permit2 | `0x000000000022D473030F116dDEE9F6B43aC78BA3` |
| x402 Permit2 proxy | `0x4020A4f3b7b90ccA423B9fabCc0CE57C6C240002` |

Buyers approve Permit2 to spend their USDC, onchain or with a signed EIP-2612
permit. The proxy is the `spender` the buyer signs, and the contract Facilitator
Service calls to settle.

## Confirmation thresholds

Facilitator Service reports
[`/settle`](https://developers.circle.com/api-reference/facilitator-service/all/settle-payment) and
[`/status`](https://developers.circle.com/api-reference/facilitator-service/all/get-payment-status) as
`completed` only after the threshold for the target blockchain is met.

| Blockchain | Threshold |
| - | - |
| Arc | Instant finality |
| Base | L2 block inclusion, canonical receipt, and USDC `Transfer` event verified |
| Polygon PoS | Standard Polygon PoS finality |

On Base, a verified reorg after completion moves the payment to `failed` with
`reason=reorged`. Facilitator Service never resubmits a reorged payment
automatically.