> ## 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. # Register a trade signature > Registers a signed EIP-712 payload from the trader that confirms trade intent. ## OpenAPI ````yaml openapi/stablefx.yaml post /v1/exchange/stablefx/signatures openapi: 3.1.0 info: title: StableFX API description: > The StableFX API provides endpoints for trading stablecoins. ## Authentication All API requests require authentication using an API key. ## Base URL The base URL for all API endpoints is: `https://api.circle.com/v1/exchange/stablefx` version: 1.0.0 servers: - url: https://api.circle.com description: StableFX API server security: - BearerAuth: [] tags: - name: Quotes description: Endpoints for creating and managing quotes - name: Trades description: Endpoints for creating and managing trades - name: Signatures description: Endpoints for retrieving presign typed data and registering signatures - name: Fees description: Endpoints for retrieving fees - name: Funding description: Endpoints for funding trades - name: Webhook Subscriptions description: Manage subscriptions to notifications - name: Settlement Advance description: Request, track, and repay settlement advances. paths: /v1/exchange/stablefx/signatures: post: tags: - Signatures summary: Register a trade signature description: >- Registers a signed EIP-712 payload from the trader that confirms trade intent. operationId: registerTradeSignature requestBody: content: application/json: schema: $ref: '#/components/schemas/RegisterTradeSignatureRequest' responses: '200': description: Trade signature registered successfully content: application/json: schema: $ref: '#/components/schemas/RegisterTradeSignatureResponse' security: - BearerAuth: [] components: schemas: RegisterTradeSignatureRequest: type: object description: Request body for submitting a maker trade signature required: - tradeId - address - details - signature properties: tradeId: $ref: '#/components/schemas/Id' address: type: string description: The wallet address signing the trade presign data example: '0x1234567890abcdef1234567890abcdef12345678' details: $ref: '#/components/schemas/MakerPermit2Message' signature: type: string description: The signature generated from signing the trade presign data RegisterTradeSignatureResponse: type: object description: Response from registering a trade signature required: - tradeId - createDate - updateDate properties: tradeId: $ref: '#/components/schemas/Id' createDate: $ref: '#/components/schemas/CreateDate' updateDate: $ref: '#/components/schemas/UpdateDate' Id: type: string format: uuid description: System-generated unique identifier of the resource. example: c4d1da72-111e-4d52-bdbf-2e74a2d803d5 MakerPermit2Message: type: object description: >- The maker Permit2 message for signature registration. Contains the signed Permit2 witness transfer data including maker-specific witness details with consideration and fee. required: - permitted - spender - nonce - deadline - witness properties: permitted: $ref: '#/components/schemas/TokenPermissions' spender: type: string description: The address of the StableFX contract example: '0xa8f94168b4981840ba27d423f4ad6332bedee006' nonce: type: string description: >- A unique nonce for the signature to prevent replay attacks, as a string representation of a uint256 example: '309585810' deadline: type: string format: epoch description: >- The validity period for the signature in epoch seconds, as a string representation of a uint256 example: '1770302983' witness: type: object description: Witness data containing maker trade details required: - consideration - fee properties: consideration: $ref: '#/components/schemas/TradeSignatureConsideration' fee: type: string description: The fee for the trade, as a string representation of a uint256 example: '80000' CreateDate: type: string format: date-time description: Date and time when the resource was created example: '2023-01-01T12:04:05Z' UpdateDate: type: string format: date-time description: Date and time when the resource was last updated example: '2023-01-01T12:04:05Z' TokenPermissions: type: object description: Token permissions for Permit2 required: - token - amount properties: token: type: string description: The token contract address example: 0xTOKEN amount: type: string description: >- The amount of tokens permitted, as a string representation of a uint256. example: '1000' TradeSignatureConsideration: type: object description: EIP-712 typed data for a StableFX trade created from a quote. properties: quoteId: type: string base: type: string quote: type: string baseAmount: type: string quoteAmount: type: string maturity: type: string format: epoch description: >- The settlement maturity timestamp for the trade, as a string representation of a uint256 example: '1716153600' securitySchemes: BearerAuth: type: http scheme: bearer bearerFormat: PREFIX:ID:SECRET description: >- Circle's API Keys are formatted in the following structure "PREFIX:ID:SECRET". All three parts are requred to make a successful request. ````