> ## 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: Create a custodial account

> Create a custodial account for an onboarded end customer from your backend

A custodial account holds an end customer's funds in Circle's custody. Circle
creates the first custodial account for an end customer when it approves their
Know Your Business (KYB). Create more from your backend when an end customer
needs them, for example to keep funds for different purposes apart. For
background, see
[Custodial accounts](https://developers.circle.com/digital-asset-accounts/concepts/custody-model.md).

## Prerequisites

Before you begin, ensure that you've:

* Obtained an API key for Digital Asset Accounts from your Circle
  representative.
* Onboarded the end customer and waited until KYB is approved. See
  [Onboard customers](https://developers.circle.com/digital-asset-accounts/quickstarts/onboard-customers.md).
  Keep their `clientEntityId`.
* Installed cURL on your development machine.

## Create the account

Send a create request from your backend with the end customer's
`clientEntityId`. Generate the idempotency key first, so that you can reuse it
if you retry:

```shell theme={null}
IDEMPOTENCY_KEY=$(uuidgen)

curl --request POST \
  --url https://api-sandbox.circle.com/v1/accounts \
  --header 'Accept: application/json' \
  --header 'Authorization: Bearer YOUR_API_KEY' \
  --header 'Content-Type: application/json' \
  --data '{
    "idempotencyKey": "'"$IDEMPOTENCY_KEY"'",
    "clientEntityId": "{clientEntityId}",
    "type": "third_party",
    "purpose": "custody",
    "description": "{description}"
  }'
```

* `type` and `purpose`: must be `third_party` and `custody` for an end
  customer's account.
* `description`: required, up to 255 characters. A label for the account, which
  you can show to the end customer. It must be unique, ignoring case, among the
  active accounts of all your end customers. A duplicate returns `409`.
* `idempotencyKey`: a UUID v4. Store it before you send the request, and reuse
  it if you retry.

The response returns the new account:

```json theme={null}
{
  "data": {
    "accountId": "1000662322",
    "entityId": "a49f9b1d-75e0-44a9-b8d2-4293b3f11ebd",
    "clientEntityId": "a3f1b2c4-d5e6-7890-abcd-ef1234567890",
    "custodyType": "omnibus",
    "type": "third_party",
    "purpose": "custody",
    "status": "active",
    "description": "Operating account",
    "balances": []
  }
}
```

Save the `accountId`. `custodyType` is `omnibus` for a custodial account. Circle
creates a custodial account with `status` `active`, so it can receive funds
right away. For every field, see
[Create an account](https://developers.circle.com/api-reference/digital-asset-accounts/all/create-account.md).

## See also

* To fund the account with a wire, see
  [Wire deposits and withdrawals](https://developers.circle.com/digital-asset-accounts/howtos/wire-deposits-and-withdrawals.md).
* To receive stablecoins onchain, see
  [Create deposit addresses](https://developers.circle.com/digital-asset-accounts/howtos/create-deposit-addresses.md).