> ## 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. # How-to: Add another wallet owner > Register an additional passkey as an authorized owner on your MSCA to extend recovery coverage across additional devices. Register an additional [WebAuthn](https://developer.mozilla.org/en-US/docs/Web/API/Web_Authentication_API) passkey as an owner on your [modular smart contract account (MSCA)](/wallets/account-types#smart-contract-accounts-sca-and-msca). Additional owners serve as recovery keys. They extend recovery coverage across additional devices without changing how routine signing works. See [How Digital Asset-Backed Borrowing works](/circle-mint/concepts/digital-asset-backed-borrowing) for the wallet architecture. ## Prerequisites Before you begin, ensure that you've: * Completed the [Borrow USDC against cirBTC collateral](/circle-mint/quickstarts/borrow-with-cirbtc) quickstart and provisioned a wallet. * Registered a new WebAuthn passkey on the device you want to add as an owner. Capture the P-256 public key and the WebAuthn credential identifier as hex strings. ## Steps Call [`POST /v1/borrow/wallets/owners`](/api-reference/circle-mint/digital-asset-backed-borrowing/add-borrow-wallet-owner) with the new passkey's public key and credential ID. ```bash theme={null} curl -X POST https://api-sandbox.circle.com/v1/borrow/wallets/owners \ -H "Authorization: Bearer $API_KEY" \ -H "Content-Type: application/json" \ -d '{ "idempotencyKey": "f7a8b9c0-d123-4e56-7890-abcdef123456", "ownerKey": "0x04c1d2e3f4051627384950a1b2c3d4e5f60718293a4b5c6d7e8f90a1b2c3d4e5f6", "credentialId": "b2c3d4e5f6071829" }' ``` The endpoint returns an `onchainJob` with `jobType: add_owner`: ```json theme={null} { "data": { "id": "a7b8c9d0-e123-4567-890a-bcdef1234567", "jobType": "add_owner", "status": "received", "createDate": "2026-06-25T11:00:00.000Z", "updateDate": "2026-06-25T11:00:00.000Z" } } ``` Save the job `id`. You poll it in the next step. Poll [`GET /v1/borrow/jobs/{id}`](/api-reference/circle-mint/digital-asset-backed-borrowing/get-borrow-job) until `status` reaches `completed`. ```json theme={null} { "data": { "id": "a7b8c9d0-e123-4567-890a-bcdef1234567", "jobType": "add_owner", "status": "completed", "txHash": "0x4d5e6f70819293a4b5c6d7e8f90a1b2c3d4e5f60718293a4b5c6d7e8f9aabbccdd", "createDate": "2026-06-25T11:00:00.000Z", "updateDate": "2026-06-25T11:01:30.000Z" } } ``` Fetch the wallet and confirm the new owner appears in the `owners` array with `status: active`: ```bash theme={null} curl https://api-sandbox.circle.com/v1/borrow/wallets \ -H "Authorization: Bearer $API_KEY" ``` ```json theme={null} { "data": [ { "id": "9c1b2a3d-4e5f-6071-8293-a4b5c6d7e8f9", "walletAddress": "0x742d35Cc6634C0532925a3b844Bc9e7595f9f68C", "owners": [ { "id": "1f2e3d4c-5b6a-7980-1234-56789abcdef0", "ownerType": "passkey", "publicKey": "0x04b0a1c2d3e4f5061728394a5b6c7d8e9f00112233445566778899aabbccddeeff", "credentialId": "a1b2c3d4e5f60718", "status": "active", "addedDate": "2026-06-20T14:10:00.000Z" }, { "id": "2e3d4c5b-6a79-8012-3456-789abcdef012", "ownerType": "passkey", "publicKey": "0x04c1d2e3f4051627384950a1b2c3d4e5f60718293a4b5c6d7e8f90a1b2c3d4e5f6", "credentialId": "b2c3d4e5f6071829", "status": "active", "addedDate": "2026-06-25T11:01:30.000Z" } ] } ] } ``` The wallet now has two passkey recovery owners. Circle's ECDSA co-signer continues to sign every UserOperation for the wallet. The additional passkey does not change routine signing behavior.