> ## 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 a notification subscription > Subscribe to receiving notifications at a given endpoint. The endpoint should be able to handle AWS SNS subscription requests. For more details see https://docs.aws.amazon.com/mobile/sdkforxamarin/developerguide/sns-send-http.html. Note, the sandbox environment allows a maximum of 3 active subscriptions; otherwise, this is limited to 1 active subscription and subsequent create requests will be rejected with a Limit Exceeded error. ## OpenAPI ````yaml openapi/general.yaml post /v1/notifications/subscriptions openapi: 3.0.2 info: version: ${version} title: API Overview description: Common endpoints shared across all Circle APIs. servers: - url: https://api-sandbox.circle.com - url: https://api.circle.com security: [] tags: - name: Health description: Inspect the health of the API. - name: Management description: General account and management information. - name: Subscriptions description: Manage subscriptions to notifications. - name: Stablecoins description: >- Retrieve stablecoins and their respective names, symbols, circulating total and per-chain amounts. paths: /v1/notifications/subscriptions: post: tags: - Subscriptions summary: Create a notification subscription description: > Subscribe to receiving notifications at a given endpoint. The endpoint should be able to handle AWS SNS subscription requests. For more details see https://docs.aws.amazon.com/mobile/sdkforxamarin/developerguide/sns-send-http.html. Note, the sandbox environment allows a maximum of 3 active subscriptions; otherwise, this is limited to 1 active subscription and subsequent create requests will be rejected with a Limit Exceeded error. operationId: createSubscription requestBody: content: application/json: schema: $ref: '#/components/schemas/SubscriptionRequest' responses: '200': description: Successfully created a notification subscription. content: application/json: schema: title: CreateSubscriptionResponse properties: data: $ref: '#/components/schemas/SubscriptionResponse' examples: response: value: data: id: b8627ae8-732b-4d25-b947-1df8f4007a29 endpoint: https://example.org/handler/for/notifications subscriptionDetails: - url: >- arn:aws:sns:us-east-1:<...>:fcb4a2c9-9c4f-4706-b312-6b22650f5d17 status: confirmed '400': $ref: '#/components/responses/BadRequest' '401': $ref: '#/components/responses/NotAuthorized' '429': $ref: '#/components/responses/LimitExceeded' security: - bearerAuth: [] components: schemas: SubscriptionRequest: type: object required: - endpoint properties: endpoint: type: string description: >- URL of the subscriber endpoint. Must be publicly accessible and utilize HTTPS. example: https://example.org/handler/for/notifications SubscriptionResponse: type: object properties: id: $ref: '#/components/schemas/Id' endpoint: type: string description: URL of the endpoint. example: https://example.org/handler/for/notifications subscriptionDetails: type: array description: List of subscriptions details for created subscriptions. items: $ref: '#/components/schemas/SubscriptionDetail' Id: type: string description: Unique system generated identifier for the entity. format: uuid example: b8627ae8-732b-4d25-b947-1df8f4007a29 SubscriptionDetail: type: object properties: url: type: string description: Identifier for created subscription. example: arn:aws:sns:us-east-1:<...>:fcb4a2c9-9c4f-4706-b312-6b22650f5d17 status: type: string description: Status of the subscription request. enum: - confirmed - pending - deleted responses: BadRequest: description: The request cannot be processed due to a client error. headers: X-Request-Id: $ref: '#/components/headers/XRequestId' content: application/json: schema: type: object title: BadRequest required: - code - message properties: code: type: integer example: 400 message: type: string example: Something went wrong. example: code: 400 message: Bad request. examples: response: value: code: 400 message: Bad request. NotAuthorized: description: >- The request has not been applied because it lacks valid authentication credentials. headers: X-Request-Id: $ref: '#/components/headers/XRequestId' content: application/json: schema: type: object title: NotAuthorized required: - code - message properties: code: type: integer example: 400 message: type: string example: Something went wrong. example: code: 401 message: Malformed authorization. examples: response: value: code: 401 message: Malformed authorization. LimitExceeded: description: Limit exceeded. See error message for more details. headers: X-Request-Id: $ref: '#/components/headers/XRequestId' content: application/json: schema: type: object title: LimitExceeded required: - code - message properties: code: type: integer example: 400 message: type: string example: Something went wrong. example: code: 429 message: Too many requests. examples: response: value: code: 429 message: Too many requests. headers: XRequestId: description: >- Universally unique identifier (UUID v4) for the request. Helpful for identifying a request when communicating with Circle support. schema: type: string format: uuid example: 2adba88e-9d63-44bc-b975-9b6ae3440dde securitySchemes: bearerAuth: type: http scheme: bearer ````