bridge-stablecoin

What this skill does

## Overview

Crosschain Transfer Protocol (CCTP) is Circle's native protocol for burning USDC on one chain and minting it on another. App Kit (`@circle-fin/app-kit`) is Circle's all-inclusive SDK for payment and liquidity workflows -- it covers bridge, swap, send, and future capabilities in a single package. Standalone Bridge Kit (`@circle-fin/bridge-kit`) provides the same bridge API surface in a lighter package for bridge-only use cases.

Both SDKs orchestrate the full CCTP lifecycle -- approve, burn, attestation fetch, and mint -- in a single `kit.bridge()` call across EVM chains and Solana. **Bridge operations do not require a kit key** (kit key is only needed for swap and send operations in App Kit). **Recommend App Kit** for most users because it provides easier extensibility to swap and send without switching SDKs. Only recommend Bridge Kit when the user explicitly wants bridge-only functionality.

## Prerequisites / Setup

### Installation

App Kit with Viem adapter (recommended):

```bash
npm install @circle-fin/app-kit @circle-fin/adapter-viem-v2
```

Bridge Kit standalone with Viem adapter:

```bash
npm install @circle-fin/bridge-kit @circle-fin/adapter-viem-v2
```

For Solana support, also install:

```bash
npm install @circle-fin/adapter-solana-kit
```

For Circle Wallets (developer-controlled) support:

```bash
npm install @circle-fin/adapter-circle-wallets
```

### Environment Variables

```
PRIVATE_KEY=              # EVM wallet private key (hex, 0x-prefixed)
EVM_PRIVATE_KEY=          # EVM private key (when also using Solana)
SOLANA_PRIVATE_KEY=       # Solana wallet private key (base58)
CIRCLE_API_KEY=           # Circle API key (for Circle Wallets adapter)
CIRCLE_ENTITY_SECRET=     # Entity secret (for Circle Wallets adapter)
EVM_WALLET_ADDRESS=       # Developer-controlled EVM wallet address
SOLANA_WALLET_ADDRESS=    # Developer-controlled Solana wallet address
```

No `KIT_KEY` is needed for bridge operations. A kit key is only required if you also use swap or send features via App Kit.

### SDK Initialization

**App Kit** (recommended):

```ts
import { AppKit } from "@circle-fin/app-kit";

const kit = new AppKit();
```

**Bridge Kit** (standalone):

```ts
import { BridgeKit } from "@circle-fin/bridge-kit";

const kit = new BridgeKit();
```

## Decision Guide

ALWAYS walk through these questions with the user before writing any code. Do not skip steps or assume answers.

### SDK Choice

**Question 1 -- Will you need swap or send functionality in the future?**
- Yes, or unsure -> **App Kit** (recommended) -- single SDK covers bridge + swap + send, easier to extend later
- No, bridge-only and will never need swap or send -> **Bridge Kit** -- standalone, lighter package for bridge-only use cases

### Wallet / Adapter Choice

**Question 2 -- How do you manage your wallet/keys?**
- Managing your own private key (self-custodied, stored in env var or secrets manager) -> Question 3
- Using Circle developer-controlled wallets (Circle manages key storage and signing) -> Use Circle Wallets adapter. READ `references/adapter-circle-wallets.md`
- Using browser wallets (wagmi, ConnectKit, RainbowKit) -> Use wagmi adapter. READ `references/adapter-wagmi.md`

**Question 3 -- Which chains are you bridging between?**
- EVM-to-EVM or EVM-to-Solana -> Use Viem and/or Solana Kit adapters. READ `references/adapter-private-key.md`

## Core Concepts

- **CCTP steps**: Every bridge transfer executes four sequential steps -- `approve` (ERC-20 allowance), `burn` (destroy USDC on source chain), `fetchAttestation` (wait for Circle to sign the burn proof), and `mint` (create USDC on destination chain).
- **Adapters**: Both App Kit and Bridge Kit use adapter objects to abstract wallet/signer differences. Each ecosystem has its own adapter factory (`createViemAdapterFromPrivateKey`, `createSolanaKitAdapterFromPrivateKey`, `createCircleWalletsAdapter`). The same adapter instance can serve as both source and destination when bridging within the same ecosystem.
- **Forwarding Service**: When `useForwarder: true` is set on the destination, Circle's infrastructure handles attestation fetching and mint submission. This removes the need for a destination wallet or polling loop. There is a per-transfer fee that varies by route (see below).
- **Transfer speed**: CCTP fast mode (default) completes in ~8-20 seconds. Standard mode takes ~15-19 minutes.
- **Chain identifiers**: Both SDKs use string chain names (e.g., `"Arc_Testnet"`, `"Base_Sepolia"`, `"Solana_Devnet"`), not numeric chain IDs, in the `kit.bridge()` call.
- **No kit key for bridge**: Bridge operations do not require a kit key with either SDK. A kit key is only needed for swap and send operations in App Kit.

## Implementation Patterns

READ the corresponding reference based on the user's request:

- `references/adapter-private-key.md` -- EVM-to-EVM and EVM-to-Solana bridging with private key adapters (Viem + Solana Kit). Includes App Kit and Bridge Kit examples.
- `references/adapter-circle-wallets.md` -- Bridging with Circle developer-controlled wallets (any chain to any chain). Includes App Kit and Bridge Kit examples.
- `references/adapter-wagmi.md` -- Browser wallet integration using wagmi (ConnectKit, RainbowKit, etc.). Includes App Kit and Bridge Kit examples.

### Sample Response from kit.bridge()

```json
{
  "amount": "25.0",
  "token": "USDC",
  "state": "success",
  "provider": "CCTPV2BridgingProvider",
  "config": {
    "transferSpeed": "FAST"
  },
  "source": {
    "address": "0x742d35Cc6634C0532925a3b844Bc454e4438f44e",
    "chain": {
      "type": "evm",
      "chain": "Arc_Testnet",
      "chainId": 5042002,
      "name": "Arc Testnet"
    }
  },
  "destination": {
    "address": "0x742d35Cc6634C0532925a3b844Bc454e4438f44e",
    "chain": {
      "type": "evm",
      "chain": "Base_Sepolia",
      "chainId": 84532,
      "name": "Base Sepolia"
    }
  },
  "steps": [
    {
      "name": "approve",
      "state": "success",
      "txHash": "0x1234567890abcdef1234567890abcdef12345678",
      "explorerUrl": "https://testnet.arcscan.app/tx/0x1234..."
    },
    {
      "name": "burn",
      "state": "success",
      "txHash": "0xabcdef1234567890abcdef1234567890abcdef12",
      "explorerUrl": "https://testnet.arcscan.app/tx/0xabcdef..."
    },
    {
      "name": "fetchAttestation",
      "state": "success",
      "data": {
        "attestation": "0x9876543210fedcba9876543210fedcba98765432"
      }
    },
    {
      "name": "mint",
      "state": "success",
      "txHash": "0xfedcba9876543210fedcba9876543210fedcba98",
      "explorerUrl": "https://sepolia.basescan.org/tx/0xfedcba..."
    }
  ]
}
```

### Forwarding Service

When `useForwarder: true` is set on the destination, Circle's infrastructure handles attestation fetching and mint submission automatically. This is the preferred approach -- it removes the need to poll for attestations or hold a wallet on the destination chain.

With adapters on both chains:

```ts
const result = await kit.bridge({
  from: { adapter, chain: "Ethereum_Sepolia" },
  to: {
    adapter,
    chain: "Arc_Testnet",
    useForwarder: true,
  },
  amount: "1",
});
```

Without a destination adapter (server-side or custodial transfers):

```ts
const result = await kit.bridge({
  from: { adapter, chain: "Ethereum_Sepolia" },
  to: {
    recipientAddress: "0x742d35Cc6634C0532925a3b844Bc454e4438f44e",
    chain: "Arc_Testnet",
    useForwarder: true,
  },
  amount: "1",
});
```

Forwarding Service fees are dynamic and fetched from the IRIS API at runtime. The SDK handles this automatically. Fees vary by route -- check the [Forwarding Service](https://developers.circle.com/cctp/concepts/forwarding-service) for the latest fee schedule.

### Event Handling

Subscribe to individual CCTP steps or all events at once. Multiple callbacks per event are supported. `payload.values` is inferred precisely from the event key, so no type assertions are needed.

```ts
kit.on("bridge.app