> ## 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. # Transaction signing and authorization > How signing and authorization work across Circle Wallets products, including who initiates and approves transactions, where keys live, and the transaction signing lifecycle. How transactions are initiated and authorized depends on the [wallet type you choose](/wallets/account-types). The wallet type determines who controls signing (your backend or your users) and where keys live. ## Who initiates and who approves transactions Every onchain transaction must be initiated and authorized before it is broadcast. Initiating means requesting the transaction. Authorizing means approving the signing. The following table shows who handles each and where keys are held for each wallet type. | Wallet type | Who initiates | Who authorizes | Keys | | - | - | - | - | | Developer-controlled | Your backend (API/SDK). Your users can also initiate a request from your app after you authenticate them and assign a wallet. | Your backend, using the entity secret. | MPC. You hold the entity secret. Circle hosts MPC nodes, or you host them (for example, on-premises). | | User-controlled | Your app calls user APIs or SDKs (client or server) for the signed-in end user. | Your user, by authenticating in your app. Circle cannot sign without your user's approval. | MPC. Only your user can complete signing. | | Modular | Your app (SDK), on behalf of your user. | Your user (passkey). Circle cannot sign without your user's approval. The passkey never leaves your user's device. | Passkey on your user's device (or cloud backup). | For details on how keys are secured (MPC and passkeys), see [Key management](/wallets/key-management). ## Transaction signing lifecycle 1. **Initiate:** A client (your backend or your app) calls Circle APIs to create a transaction signing request (for example, transfer, contract call). 2. **Authorize:** The party that controls the key approves the signing. In developer-controlled wallets, your backend approves using the entity secret. In user-controlled and modular wallets, your user authenticates using a confirmation UI, PIN, or biometrics, and approves in your app. 3. **Sign:** Circle (with MPC or passkey participation) produces the signature. 4. **Broadcast:** The signed transaction is sent to the blockchain. For supported blockchains, Circle can handle broadcasting, or you can choose to use the [signing APIs](/wallets/signing-apis) with your own node. **Submitted vs finalized** Sending a transaction to the blockchain (submitted) and the blockchain processing it to finality (complete) are two separate events. Circle's API response does not indicate finality. State changes (for example, sent, confirmed, failed) are delivered asynchronously. Set up [Webhook notifications](/api-reference/webhooks) to receive them.