> ## 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.

# cirBTC overview

> Circle Wrapped Bitcoin (cirBTC) is a 1:1 BTC-backed token issued by Circle on Arc and Ethereum with real-time onchain reserve verification through Chainlink Proof of Reserves.

cirBTC is Circle Wrapped Bitcoin, a tokenized representation of Bitcoin (BTC)
issued by Circle on Arc and Ethereum. Every cirBTC is backed 1:1 by native BTC
held at a regulated entity in the Circle group of companies. The BTC is held for
the exclusive benefit of cirBTC holders and segregated from Circle's corporate
assets.

## Key benefits

Bitcoin is the most widely held digital asset, yet it cannot natively interact
with smart contracts, limiting its use in lending, borrowing, or complex trading
strategies. cirBTC solves this by creating a tokenized version of BTC that
operates on smart contract blockchains.

* **1:1 backed by native BTC**: every cirBTC is redeemable for the underlying
  Bitcoin.
* **Real-time proof of reserves**: onchain verification through Chainlink rather
  than periodic attestations.
* **Neutral issuer**: Circle does not operate a competing exchange or lending
  protocol.

Paired with USDC, cirBTC enables institutional workflows such as posting cirBTC
as collateral to borrow USDC and access capital without selling the underlying
Bitcoin position.

## Access and availability

<Note>
  cirBTC is available to qualified businesses through [Circle Mint](/circle-mint).
  [Contact Circle](https://www.circle.com/mint-contact) to learn more.
</Note>

Mint and redeem cirBTC using the same API endpoints as USDC and EURC, with
`CIRBTC` as the currency code. For step-by-step instructions, see the
[Mint and redeem cirBTC quickstart](/circle-mint/quickstarts/mint-and-redeem-cirbtc).

cirBTC is available on the following blockchains:

* **Arc**: mainnet and testnet
* **Ethereum**: mainnet and Sepolia testnet

Developers can test cirBTC integrations using the
[Circle faucet](https://faucet.circle.com/) to obtain testnet cirBTC, and the
[sandbox environment](https://app-smokebox.circle.com) to test API interactions.

## cirBTC in the API

cirBTC uses the currency code `CIRBTC` in all Circle Mint API calls. You
interact with it the same way you use `USD` (for USDC) or `EUR` (for EURC) when
creating transfers, deposit addresses, and other operations.

For example, to create a deposit address for cirBTC on Ethereum Sepolia:

```bash theme={null}
curl -X POST https://api-sandbox.circle.com/v1/businessAccount/wallets/addresses/deposit \
  -H "Authorization: Bearer ${YOUR_API_KEY}" \
  -H "Content-Type: application/json" \
  -d '{"idempotencyKey":"unique-uuid","currency":"CIRBTC","chain":"ETH"}'
```

For supported blockchains and blockchain codes, see
[Supported chains and currencies](/circle-mint/references/supported-chains-and-currencies).
To get started with your first mint or redemption, see the
[Mint and redeem cirBTC quickstart](/circle-mint/quickstarts/mint-and-redeem-cirbtc).