> ## 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. # User-controlled wallets client SDKs > Client SDKs for embedding user-controlled wallets in web, iOS, Android, and React Native applications. Circle's User-Controlled Wallets client SDKs let you embed user-controlled wallets directly in your web and mobile applications. The SDKs handle key management, authentication, confirmation UIs, and signing flows so that the user's keyshares never leave the user's device. They pair with a server-side SDK or direct API integration that orchestrates wallet operations from your backend. ## Choose your platform
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Get started
Get started
Get started
## How it works A client SDK call flows through Circle's APIs using a challenge ID that the user authorizes on-device: 1. A user performs a wallet action in your application—for example, initiating a USDC transfer. 2. Your server requests the action through Circle's User-Controlled Wallets REST APIs. 3. The API returns a response containing a `challengeId`. The challenge must be completed before the action executes. 4. Your server passes the `challengeId` to the client SDK. The SDK prompts the user to authorize the action. 5. The user authorizes by entering their PIN, completing biometric verification, or confirming on a Confirmation UI. The SDK encrypts the input and delivers it to Circle. 6. Circle sends a webhook callback (or your server polls the corresponding `GET` endpoint) with the action's status. 7. Your application notifies the end user that the action is complete. ## Security Keyshares remain exclusively on the user's device and are never exposed to your server or to Circle. The SDKs encrypt all request bodies in transit. The user retains full control of their assets—neither your application nor Circle can access funds without the user's authorization. ## UI customization Each platform SDK ships with prebuilt UIs for PIN entry, security questions, transaction confirmation, and message signing. Override colors, copy, layout, and component styling to match your brand: * [Confirmation UIs](/sdks/user-controlled/confirmation-uis) * [Web SDK UI customizations](/sdks/user-controlled/web-sdk-ui-customizations) * [iOS SDK UI customization API](/sdks/user-controlled/ios-sdk-ui-customization-api) * [Android SDK UI customization API](/sdks/user-controlled/android-sdk-ui-customization-api) * [React Native SDK UI customization API](/sdks/user-controlled/react-native-sdk-ui-customization-api) ## Server-side companion The client SDK pairs with a server-side SDK that orchestrates wallet operations from your backend. Use the Node.js or Python server SDK to create challenges, manage wallet sets, and integrate with Circle's APIs: * [Node.js SDK](/sdks/user-controlled-wallets-nodejs-sdk) * [Python SDK](/sdks/user-controlled-wallets-python-sdk)