> ## 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. # Settle an x402 payment > Settles an x402 payment by submitting the EIP-3009 authorization. The authorization will be verified, the sender's balance locked, and the transaction queued for batch processing. ## OpenAPI ````yaml openapi/gateway-nanopayments.yaml post /v1/x402/settle openapi: 3.0.3 info: title: Gateway Nanopayments version: 1.0.0 description: > Gateway Nanopayments settles gasless USDC payments for the x402 protocol using Circle Gateway's batched settlement infrastructure. Sellers verify buyer authorizations and settle net positions in bulk onchain. servers: - url: https://gateway-api-testnet.circle.com - url: https://gateway-api.circle.com security: [] tags: - name: Settlement description: >- Verify buyer-signed x402 authorizations and submit them for batched settlement through Circle Gateway. - name: Batched transfers description: >- Query the batched USDC transfers Gateway settles from accepted x402 authorizations. - name: Supported payment kinds description: Discover the x402 payment schemes and networks this facilitator supports. paths: /v1/x402/settle: post: tags: - Settlement summary: Settle an x402 payment description: | Settles an x402 payment by submitting the EIP-3009 authorization. The authorization will be verified, the sender's balance locked, and the transaction queued for batch processing. operationId: SettleX402Payment requestBody: required: true content: application/json: schema: type: object properties: paymentPayload: $ref: '#/components/schemas/PaymentPayload' paymentRequirements: $ref: '#/components/schemas/PaymentRequirements' required: - paymentPayload - paymentRequirements responses: '200': description: Settlement result. Check the success field for outcome. content: application/json: schema: type: object required: - success - transaction - network properties: success: type: boolean description: Whether the settlement was successful. errorReason: type: string description: Error code. Present when success is false. enum: - unsupported_scheme - unsupported_network - unsupported_asset - invalid_payload - address_mismatch - amount_mismatch - invalid_signature - authorization_not_yet_valid - authorization_expired - authorization_validity_too_short - self_transfer - insufficient_balance - nonce_already_used - unsupported_domain - wallet_not_found payer: type: string description: >- The sender address (present on success or when identifiable). transaction: type: string description: Transaction UUID on success, empty string on failure. network: type: string description: CAIP-2 network identifier. '400': description: Invalid or malformed request body '500': description: Unexpected infrastructure error content: application/json: schema: type: object required: - success - errorReason - transaction - network properties: success: type: boolean example: false errorReason: type: string example: unexpected_error transaction: type: string example: '' network: type: string components: schemas: PaymentPayload: type: object description: x402 payment payload containing the payment authorization and metadata. properties: x402Version: type: integer description: Version of the x402 protocol. example: 1 resource: $ref: '#/components/schemas/ResourceInfo' description: Information about the resource being paid for. accepted: $ref: '#/components/schemas/PaymentRequirements' description: The payment requirements that were accepted by the client. payload: type: object description: Scheme-specific payment data (e.g., EIP-3009 authorization). additionalProperties: true extensions: type: object description: Optional protocol extensions. additionalProperties: true required: - x402Version - accepted - payload PaymentRequirements: type: object description: Defines an acceptable way to pay for a resource using the x402 protocol. properties: scheme: type: string description: Payment scheme identifier (e.g., "exact"). example: exact network: type: string description: Network identifier (e.g., "base-sepolia", "base"). example: base-sepolia asset: type: string description: Token contract address or symbol. example: '0x036CbD53842c5426634e7929541eC2318f3dCF7e' amount: type: string description: >- Payment amount in atomic units (e.g., wei). For USDC, 1000000 = 1 USDC. example: '1000000' payTo: type: string description: Address to receive the payment. example: '0x1234567890abcdef1234567890abcdef12345678' maxTimeoutSeconds: type: integer description: Maximum time in seconds for the payment to be valid. example: 3600 extra: type: object description: Scheme-specific additional parameters. additionalProperties: true required: - scheme - network - asset - amount - payTo - maxTimeoutSeconds ResourceInfo: type: object description: Information about the resource being paid for in an x402 payment. properties: url: type: string description: URL of the resource. description: type: string description: Human-readable description of the resource. mimeType: type: string description: Expected MIME type of the response. required: - url ````