> ## 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. # Get supported x402 payment kinds > Returns the x402 payment kinds and extensions Facilitator Service accepts. ## OpenAPI ````yaml openapi/facilitator-service.yaml get /v1/facilitator/x402/supported openapi: 3.0.3 info: title: Facilitator Service version: 1.0.0 description: > Facilitator Service is Circle's hosted x402 facilitator. Sellers call Facilitator Service to verify and settle x402 payments in USDC on Arc, Base, and Polygon PoS without operating their own relayer, gas wallet, or sanctions screening. Facilitator Service implements the x402 v2 `exact` scheme and settles USDC through EIP-3009 authorizations. Facilitator Service also implements the x402 v2 `upto` scheme. With `upto`, the buyer signs a Permit2 authorization for a maximum amount, and the seller settles the actual amount, from zero up to that maximum. The buyer either approves the Permit2 contract onchain or attaches a signed EIP-2612 permit through the `eip2612GasSponsoring` extension, which needs no buyer transaction. `/supported` lists the networks where `upto` is available. servers: - url: https://api.circle.com description: Production security: - CircleApiKey: [] - FacilitatorSellerProof: [] tags: - name: Payments description: | x402 payment verification, settlement, and status. - name: Supported payment kinds description: | x402 payment kinds and extensions Facilitator Service accepts. paths: /v1/facilitator/x402/supported: get: tags: - Supported payment kinds summary: Get supported x402 payment kinds description: > Returns the x402 payment kinds and extensions Facilitator Service accepts. operationId: GetSupported responses: '200': description: Supported payment kinds and extensions content: application/json: schema: $ref: '#/components/schemas/SupportedResponse' examples: exactAndUpto: summary: Exact and upto kinds description: > Arc testnet advertises both `exact` and `upto`. Base Sepolia advertises `exact` only. The `upto` kind's `facilitatorAddress` is one of the wallets listed under `signers` for that network. value: x402Version: 2 kinds: - x402Version: 2 scheme: exact network: eip155:5042002 asset: '0x3600000000000000000000000000000000000000' decimals: 6 extra: name: USDC version: '2' - x402Version: 2 scheme: upto network: eip155:5042002 asset: '0x3600000000000000000000000000000000000000' decimals: 6 extra: name: USDC version: '2' assetTransferMethod: permit2 facilitatorAddress: '0x2240278661b83f55d31b2506f77266f6ffd21d55' - x402Version: 2 scheme: exact network: eip155:84532 asset: '0x036CbD53842c5426634e7929541eC2318f3dCF7e' decimals: 6 extra: name: USDC version: '2' extensions: - payment-identifier - settlement-status - eip2612GasSponsoring signers: eip155:5042002: - '0x2240278661b83f55d31b2506f77266f6ffd21d55' '500': description: Internal error. content: application/json: schema: $ref: '#/components/schemas/ErrorResponse' security: [] components: schemas: SupportedResponse: type: object required: - x402Version - kinds - signers properties: x402Version: type: integer enum: - 2 description: x402 protocol version. kinds: type: array description: > One profile per enabled `(scheme, network, asset)` triple. An `upto` kind appears only on networks where Facilitator Service settles `upto`. items: type: object required: - scheme - network - asset properties: x402Version: type: integer enum: - 2 description: x402 protocol version for this kind. scheme: type: string enum: - exact - upto example: exact network: type: string description: CAIP-2 network identifier. example: eip155:5042002 asset: type: string description: USDC contract address. example: '0x3600000000000000000000000000000000000000' decimals: type: integer example: 6 extra: type: object description: > EIP-712 domain fields the buyer signs against. An `upto` kind also carries `assetTransferMethod: permit2` and `facilitatorAddress`, the facilitator wallet the buyer signs into `witness.facilitator`. `facilitatorAddress` can change between calls, so read it from `/supported` instead of hardcoding it. additionalProperties: true example: name: USDC version: '2' extensions: type: array description: > x402 extensions Facilitator Service honors: `payment-identifier`, `settlement-status`, and `eip2612GasSponsoring`. `eip2612GasSponsoring` applies only to `upto` payments, even though the list is the same for every network. items: type: string example: - payment-identifier - settlement-status - eip2612GasSponsoring signers: type: object description: > Map of CAIP-2 network patterns to signer addresses. For networks that support `upto`, the key is the exact CAIP-2 network identifier and the value lists every facilitator wallet that can settle on that network. additionalProperties: true example: {} ErrorResponse: type: object required: - code - message properties: code: type: integer description: HTTP status code echoed in the body for programmatic handling. example: 403 message: type: string description: >- Human-readable summary of the error, suitable for display to end users. example: Registration required to continue settling on this address errors: type: array description: Per-issue reason codes. items: type: object required: - reason properties: reason: type: string description: | Machine-readable reason code. enum: - registration_required - authorization_expired - payment_identifier_conflict - credential_bound_elsewhere - credential_payer_conflict - authorization_reused - permit2_allowance_required example: registration_required securitySchemes: CircleApiKey: type: http scheme: bearer description: > Bearer token authentication with a Circle API key. Use this for production settlement. Settling with an API key also binds the `payTo` to your Circle account, which ends the keyless trial allowance. FacilitatorSellerProof: type: apiKey in: header name: Facilitator-Seller-Proof description: > Base64url-encoded envelope carrying an EIP-712 signature that proves the caller controls the `payTo` address. Use this for the keyless trial, before you settle with a Circle API key. Settling an `upto` payment requires a Circle API key. ````