> ## 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. # Sandbox to production > Transition your Circle Mint integration from the sandbox environment to production. Circle provides a sandbox environment at `https://api-sandbox.circle.com` for prototyping and integration testing. Sandbox APIs match production APIs, so you can develop and test without generating real financial transactions. Simulated transactions use [test networks only](/stablecoins/usdc-contract-addresses#testnet) and do not move real funds. For details on idempotent requests, pagination, and date filtering, see the [API reference](/api-reference/idempotent-requests). After completing your integration in the sandbox, follow these steps to move to production. If you have questions, contact your Circle solutions engineer or [customer support](mailto:customer-support@circle.com). Update all API base URLs from `https://api-sandbox.circle.com` to `https://api.circle.com`. Use the [Mint Console](https://app.circle.com/) to create a production API key. Production API keys allow you to work with real funds. Treat production keys with appropriate security protocols. Consult your security team for key management best practices. If you configure an IP allowlist, Circle only accepts requests using your production API key that originate from an IP address on that list. Verify that the static addresses in your system match the ones you registered with Circle. In the sandbox, your API key provides access to all endpoints. In production, access is restricted to the APIs your entity is authorized to use. * Test each API call to confirm it works in production. For example, verify that your [list all balances](/api-reference/circle-mint/account/list-business-balances) call returns results. * If you receive `403` responses or need additional capabilities, contact your Circle representative. Sandbox settlement times are kept short for testing convenience. Production settlement times reflect real-world processing and are longer. For onchain transfers, the time to finality depends on the number of [blockchain confirmations](/circle-mint/references/blockchain-confirmations) required for each blockchain. * Test actual settlement times in production. * Decide whether to perform actions (such as releasing goods) after confirmation or after settlement. Sandbox fees may not reflect your client agreement with Circle. * Review your contract with Circle to understand production fees. * Test transactions in production to determine actual fees. * Update your interface if you pass fees along to your customers. ## Debug requests with API Logs Circle stores every API request and response your account makes and surfaces them on the API Logs page of the [Mint Console](https://app-sandbox.circle.com) (Developer Tab → Logs). Logs are retained for seven days and include the HTTP status, path, request ID, idempotency key, user agent, origin, timestamp, and full request and response bodies. You can filter by request ID, resource ID, idempotency key, date range, status, HTTP method, and path. Sensitive values—payment methods and personally identifiable information—appear as `[redacted]` in stored payloads. Contact your Circle account manager if you need access to redacted data for a specific debugging session.