> ## 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: Add collateral to a loan

> Post additional cirBTC as collateral to an open loan to lower its LTV and restore its health factor after a margin call.

Post additional cirBTC as collateral against an open loan. Adding collateral
lowers the loan's
[LTV](https://developers.circle.com/circle-mint/concepts/digital-asset-backed-borrowing.md#ltv-health-factor-and-liquidation)
and raises its health factor without changing the outstanding debt. This is the
standard response to a margin call when you want to keep the loan open rather
than repay it.

## Prerequisites

Before you begin, ensure that you've:

* Completed the
  [Borrow USDC against cirBTC collateral](https://developers.circle.com/circle-mint/quickstarts/borrow-with-cirbtc.md)
  quickstart and hold at least one loan in `active` status.
* Held sufficient cirBTC in your Mint balance to cover the additional collateral
  you plan to post.

## Steps

**1. Submit the collateral request**

Call
[`POST /v1/borrow/loans/{id}/collateral`](https://developers.circle.com/api-reference/circle-mint/digital-asset-backed-borrowing/add-borrow-loan-collateral)
with the amount of cirBTC you want to add in raw `uint256` base units.

```bash theme={null}
curl -X POST https://api-sandbox.circle.com/v1/borrow/loans/fc988ed5-c129-4f70-a064-e5beb7eb8e32/collateral \
  -H "Authorization: Bearer $API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "idempotencyKey": "e5c6d7b8-9012-4a3b-cdef-1234567890ab",
    "amount": "2500000"
  }'
```

The endpoint returns an `onchainJob` with `jobType: add_collateral`:

```json theme={null}
{
  "data": {
    "id": "f6a7b8c9-d012-3456-efab-456789012345",
    "jobType": "add_collateral",
    "status": "received",
    "createDate": "2026-06-25T10:15:00.000Z",
    "updateDate": "2026-06-25T10:15:00.000Z"
  }
}
```

Save the job `id`. You poll it in the next step.

**2. Poll the job to completion**

Circle builds, signs, and submits the onchain UserOperation for the added
collateral. The job progresses through `submitted` and `confirmed` to
`completed`. Poll
[`GET /v1/borrow/jobs/{id}`](https://developers.circle.com/api-reference/circle-mint/digital-asset-backed-borrowing/get-borrow-job)
until you see the terminal state.

```json theme={null}
{
  "data": {
    "id": "f6a7b8c9-d012-3456-efab-456789012345",
    "jobType": "add_collateral",
    "status": "completed",
    "amount": "0.025",
    "asset": "CIRBTC",
    "collateralAmount": "0.025",
    "collateralAsset": "CIRBTC",
    "loanId": "fc988ed5-c129-4f70-a064-e5beb7eb8e32",
    "txHash": "0x3c4d5e6f70819293a4b5c6d7e8f90a1b2c3d4e5f60718293a4b5c6d7e8f9aabb",
    "createDate": "2026-06-25T10:15:00.000Z",
    "updateDate": "2026-06-25T10:17:00.000Z"
  }
}
```

**3. Confirm the updated position**

Fetch the loan to confirm the added collateral is reflected and the health
factor has improved:

```bash theme={null}
curl https://api-sandbox.circle.com/v1/borrow/loans/fc988ed5-c129-4f70-a064-e5beb7eb8e32 \
  -H "Authorization: Bearer $API_KEY"
```

`collateralAmount` on the loan should now include the added amount, and
`healthFactor` should be higher than it was before.