> ## 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.

# Magic numbers for testing End User Onboarding

> Magic numbers you can use to drive an End User Onboarding application into review, RFI, or denial outcomes in the sandbox

In the sandbox, Know Your Business (KYB) applications auto-approve on submission
so you can build quickly. To test other outcomes—compliance review,
[requests for information (RFIs)](/end-user-onboarding/howtos/handle-rfis), or
denial—use a reserved business name prefix as a magic number.

<Note>
  These values work only in the sandbox (`https://api-sandbox.circle.com`). In
  production, they are normal business names. The application then follows the
  real review process.
</Note>

## How it works

Set the legal business name to a value that starts with one of the following
magic-number prefixes. Then
[submit the application](/end-user-onboarding/howtos/submit-and-track-applications).
When the application moves from `DRAFT` to `SUBMITTED`, the sandbox routes it to
that outcome. It does not auto-approve.

* Matching is case-insensitive and only the prefix is reserved. The rest of the
  business name is free text (for example
  `CIRCLE_SANDBOX_DENY Acme Trading Ltd`).
* Any business name without a recognized prefix follows the default behavior:
  the application is approved.

## Magic numbers

| Business name prefix | Resulting outcome | Webhook |
| - | - | - |
| `CIRCLE_SANDBOX_REVIEW` | application routes to `IN_REVIEW` instead of auto-approving | none on entry |
| `CIRCLE_SANDBOX_DENY` | application is denied (`DENIED`, terminal) | `onboarding_application_denied` |
| *Any other value* | Default sandbox behavior: application is approved (`APPROVED`) | `onboarding_application_approved` |

See
[Application types and states](/end-user-onboarding/references/application-states)
for the full lifecycle and
[Webhook notifications](/end-user-onboarding/references/webhook-notifications)
for the payloads.

<Note>
  `CIRCLE_SANDBOX_REVIEW` moves the application to `IN_REVIEW`. An RFI can only be
  issued after that. To test the full RFI flow, ask your Circle contact to issue
  an RFI on the application. You then get an
  `onboarding_application_rfi_requested` webhook. The application moves to
  `PENDING_CUSTOMER_INFORMATION`.
  [Handle the RFI](/end-user-onboarding/howtos/handle-rfis) and resubmit.

  Partners can't create `PERIODIC_REVIEW` or `INFORMATION_UPDATE` applications. To
  test them in sandbox, send your Circle contact the client's `clientEntityId`.
</Note>

## Example

Submit an application whose business name requests a denial outcome:

```bash theme={null}
curl --request POST \
  --url https://api-sandbox.circle.com/v1/onboarding/partner/applications/${APPLICATION_ID}/submit \
  --header "Authorization: Bearer ${YOUR_API_KEY}"
```

With the business name set to `CIRCLE_SANDBOX_DENY Acme Trading Ltd`, the
application transitions to `DENIED` and your registered endpoint receives an
`onboarding_application_denied`
[webhook](/end-user-onboarding/references/webhook-notifications).