> ## 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. # Create Event Monitor > Create a new event monitor based on the provided blockchain, contract address, and event signature. ## OpenAPI ````yaml openapi/smart-contract-platform.yaml post /v1/w3s/contracts/monitors 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/monitors: post: tags: - Event Monitors summary: Create Event Monitor description: >- Create a new event monitor based on the provided blockchain, contract address, and event signature. operationId: createEventMonitor parameters: - $ref: '#/components/parameters/XRequestId' requestBody: $ref: '#/components/requestBodies/CreateEventMonitor' required: true responses: '200': description: Event monitor already exists. headers: X-Request-Id: $ref: '#/components/headers/XRequestId' content: application/json: schema: $ref: '#/components/schemas/EventMonitorResponse' '201': description: Event monitor created successfully. headers: X-Request-Id: $ref: '#/components/headers/XRequestId' content: application/json: schema: $ref: '#/components/schemas/EventMonitorResponse' '400': content: application/json: schema: $ref: '#/components/schemas/Error' description: Invalid request payload. 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: CreateEventMonitor: content: application/json: schema: title: CreateEventMonitorRequest type: object required: - blockchain - contractAddress - eventSignature - idempotencyKey properties: blockchain: allOf: - $ref: '#/components/schemas/Blockchain' contractAddress: $ref: '#/components/schemas/ContractAddress' eventSignature: type: string description: >- The specific event to which you want to subscribeven. Please ensure no spaces are included. idempotencyKey: $ref: '#/components/schemas/IdempotencyKey' examples: CreateEventMonitorRequest: value: blockchain: ETH contractAddress: '0x6bc50ff08414717f000431558c0b585332c2a53d' eventSignature: Transfer(address,address,uint256) idempotencyKey: a0eebc99-9c0b-4ef8-bb6d-6bb9bd380a11 description: Create Event Monitor Request 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' schemas: EventMonitorResponse: title: EventMonitorResponse type: object required: - data properties: data: type: object required: - eventMonitor properties: eventMonitor: $ref: '#/components/schemas/EventMonitor' 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 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 ContractAddress: type: string description: The on-chain address of this contract. example: '0x1e124d7384cd34448ea5907bd0052a79355ab5eb' IdempotencyKey: type: string description: >- Universally unique identifier (UUID v4) idempotency key. This key is utilized to ensure exactly-once execution of mutating requests. To create a UUIDv4 go to [uuidgenerator.net](https://www.uuidgenerator.net). If the same key is reused, it will be treated as the same request and the original response will be returned. example: a0eebc99-9c0b-4ef8-bb6d-6bb9bd380a11 format: uuid EventMonitor: type: object required: - id - blockchain - contractAddress - eventSignature - eventSignatureHash - isEnabled - createDate - updateDate properties: id: type: string format: uuid example: e3c998a5-bdf1-4f3e-812f-24da238c0fff blockchain: allOf: - $ref: '#/components/schemas/Blockchain' contractAddress: $ref: '#/components/schemas/ContractAddress' eventSignature: type: string example: Transfer(address indexed from, address indexed to, uint256 value) eventSignatureHash: type: string example: '0xd3d3dd4b1fd3e53f94deb24e763485b4c925345c5abfa9ad529c67aa55a3b784' isEnabled: type: boolean example: true 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. ````