> ## 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.

# Set up your account and API key

> Create a sandbox account, generate an API key, and make your first Circle Mint API request.

This guide walks you through creating a sandbox account, generating an API key,
and verifying that you can connect to the Circle Mint API.

## Prerequisites

Before you begin, ensure you have:

* A valid email address to register for a Circle Mint sandbox account
* [curl](https://curl.se/) or another tool for making HTTP requests

## Step 1: Create a sandbox account

The sandbox environment lets you test API integrations without processing real
transactions. For more details on sandbox versus production environments, see
[Sandbox to Production](/circle-mint/references/sandbox-and-testing).

<Steps>
  <Step title="Sign up for a sandbox account">
    Go to [app-sandbox.circle.com/signup](https://app-sandbox.circle.com/signup)
    and complete the registration form.
  </Step>

  <Step title="Verify your email address">
    Check your inbox and confirm your email to activate your account.
  </Step>

  <Step title="Log in to the sandbox">
    After activation, log in at `https://app-sandbox.circle.com`.
  </Step>
</Steps>

## Step 2: Generate an API key

Circle Mint uses API keys to authenticate all requests. Create and manage keys
in the [Mint Console](https://app-sandbox.circle.com).

<Steps>
  <Step title="Open the Mint Console">
    Go to [app-sandbox.circle.com](https://app-sandbox.circle.com) and open the
    Developer section.
  </Step>

  <Step title="Select the API Keys tab" />

  <Step title="Click Create an API key" />

  <Step title="Enter a name for your key" />

  <Step title="(Optional) Add IP addresses to the allowlist">
    Restrict where the key can be used from.
  </Step>

  <Step title="Click Create API key" />

  <Step title="Copy the key and store it securely" />
</Steps>

<Warning>
  API keys grant access to privileged operations on Circle APIs. Store your API
  key securely and never expose it in client-side code, public repositories, or
  other publicly accessible locations. All API requests must be made over HTTPS.
  You can create a maximum of 10 API keys per environment.
</Warning>

You can also harden API access with a transport-layer certificate check.

<Note>
  You can add mutual TLS (mTLS) on top of API key authentication for an extra
  layer of transport-level security. Opting in is optional for most customers
  and required for entities operating under the EU Markets in Crypto-Assets
  (MiCA) regulation. mTLS changes how you generate and rotate API keys. See [How
  mTLS authentication works](/circle-mint/mtls-authentication).
</Note>

## Step 3: Test connectivity

Test raw connectivity by calling the `/ping` endpoint. This endpoint does not
require authentication, so it confirms that your application can reach the API.

```bash theme={null}
curl -s https://api-sandbox.circle.com/ping
```

If your application reached the API, you see the following response:

```json theme={null}
{ "message": "pong" }
```

## Step 4: Verify your API key

Circle Mint uses Bearer token authentication. Include your API key in the
`Authorization` header of every request using the format `Bearer YOUR_API_KEY`.

Call the `/v1/configuration` endpoint to confirm that your API key is valid and
properly configured:

```bash theme={null}
curl -s https://api-sandbox.circle.com/v1/configuration \
  -H "Authorization: Bearer ${YOUR_API_KEY}"
```

A successful response returns your account configuration, including your
`masterWalletId`. The `masterWalletId` is the identifier for the primary wallet
associated with your Circle Mint account, used as the source or destination in
transfer and payout operations.

```json theme={null}
{ "data": { "payments": { "masterWalletId": "1234567890" } } }
```

If the API key is missing or malformed, you receive a `401 Unauthorized` error:

```json theme={null}
{
  "code": 401,
  "message": "Malformed authorization. Are the credentials properly encoded?"
}
```

If you see this error, verify that your `Authorization` header uses the `Bearer`
prefix and that you copied the full API key from the Mint Console.

## Use the TypeScript SDK (optional)

Circle publishes a TypeScript SDK that wraps the Circle Mint API. Install it
from the npm registry:

```bash theme={null}
npm install @circle-fin/circle-sdk
```

See the
[`circle-nodejs-sdk` repository](https://github.com/circlefin/circle-nodejs-sdk)
for usage examples, type definitions, and instructions for filing issues or
contributing.