> ## 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 a notification signature public key > Get the public key and algorithm used to digitally sign webhook notifications. Verifying the digital signature ensures the notification came from Circle. In the headers of each webhook, you can find - `X-Circle-Signature`: a header containing the digital signature generated by Circle. - `X-Circle-Key-Id`: a header containing the UUID. This is will be used as the `ID` as URL parameter to retrieve the relevant public key. ## OpenAPI ````yaml openapi/stablefx.yaml get /v2/stablefx/notifications/publicKey/{id} 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: /v2/stablefx/notifications/publicKey/{id}: get: tags: - Webhook Subscriptions summary: Get a notification signature public key description: > Get the public key and algorithm used to digitally sign webhook notifications. Verifying the digital signature ensures the notification came from Circle. In the headers of each webhook, you can find - `X-Circle-Signature`: a header containing the digital signature generated by Circle. - `X-Circle-Key-Id`: a header containing the UUID. This is will be used as the `ID` as URL parameter to retrieve the relevant public key. operationId: getNotificationSignature parameters: - $ref: '#/components/parameters/Id-2' - $ref: '#/components/parameters/XRequestId' responses: '200': description: Successfully retrieved a digital notification signature. content: application/json: schema: $ref: '#/components/schemas/NotificationSignaturePublicKey' '400': $ref: '#/components/responses/DefaultError' security: - BearerAuth: [] components: parameters: Id-2: name: id description: The universally unique identifier of the resource. in: path required: true schema: type: string format: uuid example: b3d9d2d5-4c12-4946-a09d-953e82fae2b0 XRequestId: name: X-Request-Id description: >- Developer-provided parameter used to identify this request. Useful when communicating with Circle Support. in: header schema: $ref: '#/components/schemas/XRequestId' schemas: NotificationSignaturePublicKey: title: NotificationSignaturePublicKeyResponse type: object properties: data: type: object required: - id - algorithm - publicKey - createDate properties: id: type: string format: uuid example: 09379bc1-4871-45ad-8b8b-8027795df70d algorithm: type: string example: ECDSA_SHA_256 publicKey: type: string example: >- QFkwewylAoZIzj0CBQYIKoZIzj0DAQcDQgAEEext6d7AimvYFDAKBwsUsGXrhqmRHNPYum7V/BwfvJLmJeSMe2V0b9eWKPuFxAEH8HCY/MSZdBI+q/E6IdPnSg== createDate: type: string example: '2023-06-01T18:18:06.931526Z' XRequestId: type: string description: >- A unique identifier, which can be helpful for identifying a request when communicating with Circle support. example: 2adba88e-9d63-44bc-b975-9b6ae3440dde format: uuid Error: title: Error type: object required: - code - message properties: code: $ref: '#/components/schemas/ErrorCode' message: type: string description: Human-readable message that describes the error. example: Unknown error occurred errors: type: array description: Array of detailed error descriptions items: $ref: '#/components/schemas/DescriptiveError' ErrorCode: title: ErrorCode type: integer description: The code that corresponds to the error. enum: - -1 - 0 - 1 - 2 - 3 - 400 - 401 - 403 - 404 DescriptiveError: title: DescriptiveError type: object required: - error - message properties: error: $ref: '#/components/schemas/DescriptiveErrorType' location: type: - string - 'null' description: The key or path where the error occurred message: type: string description: Detailed description of the error DescriptiveErrorType: title: DescriptiveErrorType type: string enum: - MISSING_OR_INVALID_FIELD description: Type of descriptive error responses: DefaultError: content: application/json: schema: $ref: '#/components/schemas/Error' description: Error response headers: X-Request-Id: $ref: '#/components/headers/XRequestId' headers: XRequestId: description: > 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 requred to make a successful request. ````