> ## 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.
# Facilitator Service
> Circle's hosted x402 facilitator for USDC settlement on Arc, Base, and Polygon PoS
Facilitator Service is Circle's hosted [x402](/x402-facilitators/x402)
facilitator for sellers. Point your HTTP API at Facilitator Service and start
accepting x402 payments in USDC without operating a relayer, funding a gas
wallet, or building your own sanctions screening. Circle verifies each payment
authorization signed by the buyer and submits the USDC transfer onchain.
Facilitator Service settles USDC on Arc, Base, and Polygon PoS through the
[EIP-3009](https://eips.ethereum.org/EIPS/eip-3009) `exact` scheme. It requires
a Circle [API key](/api-reference/keys#api-keys) to settle, but you can start
with the [keyless trial](/facilitator-service/keyless-trial) without a Circle
account.
**Disclaimer**: Facilitator Service is offered by Circle Technology Services,
LLC, a software provider. Facilitator Service verifies buyer-signed EIP-3009
USDC payment authorizations under the x402 protocol only — it does not hold,
control, or transmit user funds, verify transaction purpose, or confirm agent
authority scope.
On Base and Polygon PoS, settlements are subject to blockchain reorganization
risk; Circle does not guarantee fund receipt. Arc settlements are final.
Use of the Facilitator Service is subject to
[Circle Console Service Terms](https://console.circle.com/legal/service-terms)
and [Developer Terms](https://console.circle.com/legal/developer-terms). Not
reviewed or approved by any regulatory authority. Features may change at any
time. Nothing herein constitutes a commitment, warranty, or guarantee. Use
involves risk.
## Key features
Price your API, data, or agent service per request and get paid in USDC on
Arc, with Circle handling settlement onchain.
Point your seller at Facilitator Service once and accept x402 payments on
Arc, Base, and Polygon PoS from the same integration.
Circle broadcasts every transfer, pays settlement gas, and screens both
parties before any money state is created.
## What you can build
Charge per API call, per dataset query, or per agent task. Your seller asks for
payment over x402 and calls Facilitator Service to settle the USDC transfer
onchain, with no billing relationship to set up per buyer.
Add Arc coverage to your existing x402 seller without losing Base or Polygon
PoS. One facilitator settles USDC across all three blockchains from the same
integration.
Settle USDC payments between participants on your platform, with address
screening running before any payment record is created.
Add an agent payment acceptance path in USDC without building relayer or gas
infrastructure. Buyers can pay from a standard wallet address or a deployed
smart contract account.
## Get started
Understand the x402 protocol, the roles, and how Facilitator Service settles
USDC through EIP-3009
Accept your first x402 payment on Arc testnet with the keyless trial
Understand the trial allowance, seller accounts, and how Facilitator Service
tracks ownership per `payTo`
Chain IDs, USDC contracts, and confirmation thresholds for Arc, Base, and
Polygon PoS