> ## 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.

# Modular wallets

> Understand how modular wallets work, when to use them, and how Circle enables passkey-based smart accounts with gasless transactions, batch execution, and optional modules.

Modular wallets are user-controlled smart accounts built on
[Modular Smart Contract Accounts (MSCA)](/wallets/account-types#modular-smart-contract-accounts-msca).
Users sign with
<Tooltip tip="A cryptographic key pair stored on the user's device (phone, laptop, or password manager). Signing uses biometrics or device unlock; no seed phrase or password required.">passkeys</Tooltip>
, and you integrate using Circle SDKs for web, iOS, and Android. You can sponsor
gas, run batch or parallel transactions, and extend behavior with modules. For a
comparison of all wallet products, see
[Choose your wallet product and account types](/wallets/account-types).

## Key features

<CardGroup cols={2}>
  <Card title="User retains custody" icon="key">
    Users control their keys with passkeys (WebAuthn). Keys stay on the user's
    device; your app never holds them. Optional
    [recovery](/wallets/modular/set-up-passkey-recovery) lets users restore
    access with a recovery key.
  </Card>

  <Card title="Gasless transactions" icon="gas-pump">
    Use [Gas Station](/wallets/gas-station) to sponsor network fees so users
    don't need native tokens. Set paymaster policies in the Circle Console and
    pass `paymaster: true` when sending user operations.
  </Card>

  <Card title="Batch and parallel execution" icon="layer-group">
    Send multiple operations in one user op for better UX and efficiency. Use 2D
    nonces to run independent transactions in parallel. See [Batch and parallel
    user operations](/wallets/modular/batch-and-parallel-user-operations) for
    details.
  </Card>

  <Card title="Modules and SDKs" icon="cube">
    Extend accounts with
    [modules](/wallets/modular/how-passkeys-and-modules-work#modules) (for
    example allowlists, automation). Build with the
    [Web](/sdks/modular/web-sdk), [iOS](/sdks/modular/ios-sdk), or
    [Android](/sdks/modular/android-sdk) SDK.
  </Card>
</CardGroup>

## What you can build

Use modular wallets when you want user-owned keys with a simple onboarding flow.
Common use cases:

<AccordionGroup>
  <Accordion title="Consumer apps with passkeys" icon="fingerprint">
    Onboard users with biometrics or device unlock, with no seed phrases or
    passwords. Users sign transactions from your app; you orchestrate flows with
    the modular wallets SDK and never hold keys.
  </Accordion>

  <Accordion title="Gasless and batch flows" icon="arrow-right-arrow-left">
    Sponsor gas so users don't need native tokens. Combine multiple actions (for
    example approve and transfer) in one user operation, or run independent
    operations in parallel with 2D nonces.
  </Accordion>

  <Accordion title="Programmability through modules" icon="puzzle-piece">
    Add [modules](/wallets/modular/how-passkeys-and-modules-work#modules) to
    smart accounts for allowlisted transfers or custom logic.
  </Accordion>

  <Accordion title="Third-party wallet connection" icon="link">
    Expose modular wallets as an EIP-1193 provider so other Web3 SDKs (for
    example viem or ethers) can connect. See [Use as an EIP-1193
    provider](/wallets/modular/use-as-eip-1193-provider) for the pattern.
  </Accordion>
</AccordionGroup>

<Note>
  Modular wallets submit user operations directly to the bundler using JSON-RPC.
  Your application is responsible for monitoring user operation status and
  resubmitting failed operations. Use `getUserOperationReceipt` from the
  [bundler client](/sdks/modular/web-sdk#function-createbundlerclient-viem) to
  check status after submission.
</Note>

## Get started

<CardGroup cols={2}>
  <Card title="How passkeys and modules work" icon="circle-info" href="/wallets/modular/how-passkeys-and-modules-work">
    Understand the passkey signer and modules before you run the quickstart.
  </Card>

  <Card title="Create a modular wallet app" icon="wallet" href="/wallets/modular/create-a-modular-wallet">
    Set up the SDK, register with a passkey, and create a Circle Smart Account
    for your user.
  </Card>
</CardGroup>