> ## 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. # Execute a query function on a contract > Query the state of a contract by providing the address and blockchain. ## OpenAPI ````yaml openapi/smart-contract-platform.yaml post /v1/w3s/contracts/query openapi: 3.0.3 info: description: This is the Circle Contracts API documentation. title: Circle Contracts version: '1.0' servers: - url: https://api.circle.com security: - BearerAuth: [] tags: - name: Templates - name: View / Update - name: Interact - name: Deploy / Import - name: Event Monitors paths: /v1/w3s/contracts/query: post: tags: - Interact summary: Execute a query function on a contract description: Query the state of a contract by providing the address and blockchain. operationId: queryContract parameters: - $ref: '#/components/parameters/XRequestId' requestBody: $ref: '#/components/requestBodies/ReadContractState' responses: '200': content: application/json: schema: $ref: '#/components/schemas/ReadContractState' description: Contract read successfully. headers: X-Request-Id: $ref: '#/components/headers/XRequestId' '400': content: application/json: schema: $ref: '#/components/schemas/Error' description: Error response for the request. headers: X-Request-Id: $ref: '#/components/headers/XRequestId' security: - BearerAuth: [] components: parameters: XRequestId: name: X-Request-Id in: header description: > Developer-provided identifier for this request, used for tracing requests in Wallets API logs and the Developer Console, and when communicating with Circle Support. **Must be a UUID to appear in logs.** Non-UUID values are accepted by the API but are ignored by logging and tracing systems. schema: $ref: '#/components/schemas/XRequestId' requestBodies: ReadContractState: content: application/json: schema: title: ReadContractStateRequest type: object required: - address - blockchain properties: abiFunctionSignature: $ref: '#/components/schemas/AbiFunctionSignature' abiParameters: $ref: '#/components/schemas/AbiParameters' abiJson: $ref: '#/components/schemas/AbiJson' address: description: Address of the contract to be queried. type: string example: '0x1e124d7384cd34448ea5907bd0052a79355ab5eb' blockchain: $ref: '#/components/schemas/Blockchain' callData: description: CallData is input data that encodes method and parameters. type: string fromAddress: description: >- FromAddress is the address that will populate msg.sender in the contract call. type: string description: Read contract state by executing a read ABI function. required: true schemas: ReadContractState: title: ReadContractStateResponse type: object required: - data properties: data: type: object required: - outputValues - outputData properties: outputValues: type: array description: Output for the ABI interaction. items: $ref: '#/components/schemas/AnyValue' outputData: description: OutputData is output in hex format. format: hex type: string Error: title: Error type: object required: - code - message properties: code: type: integer description: Code that corresponds to the error. message: type: string description: Message that describes the error. XRequestId: type: string format: uuid description: >- A unique identifier, which can be helpful for identifying a request when communicating with Circle support. example: 2adba88e-9d63-44bc-b975-9b6ae3440dde AbiFunctionSignature: title: AbiFunctionSignature type: string description: "The contract ABI function signature or\_`callData`\_field is required for interacting with the smart contract. The ABI function signature cannot be used simultaneously with\_`callData`. e.g. burn(uint256)" example: burn(uint256) AbiParameters: title: AbiParameters type: array items: anyOf: - type: string - type: integer - type: boolean - type: array items: {} description: "The contract ABI function signature parameters for executing the contract interaction. Supported parameter types include string, integer, boolean, and array. These parameters should be used exclusively with the abiFunctionSignature and cannot be used with\_`callData`." example: - '100' - '1' AbiJson: type: string description: The contract's ABI in a JSON stringified format. example: >- [{"inputs": [],"stateMutability": "nonpayable","type": "constructor"},... Blockchain: type: string description: > The blockchain network that the resource is to be created on or is currently on. Required along with `sourceAddress` if you don't provide `walletId`. The `blockchain` and `walletId` fields are mutually exclusive. enum: - ETH - ETH-SEPOLIA - MATIC - MATIC-AMOY - ARB - ARB-SEPOLIA - UNI - UNI-SEPOLIA - BASE - BASE-SEPOLIA - OP - OP-SEPOLIA - AVAX - AVAX-FUJI - ARC - ARC-TESTNET - MONAD - MONAD-TESTNET example: MATIC-AMOY AnyValue: description: Can be any value - string, number, boolean, array or object. headers: XRequestId: description: > Developer-provided header parameter or Circle-generated universally unique identifier (UUID v4). Useful for identifying a specific request when communicating with Circle Support. schema: $ref: '#/components/schemas/XRequestId' 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 required to make a successful request. ````