Bridge Stablecoin

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 covering bridge, swap, and send in one package; standalone Bridge Kit (@circle-fin/bridge-kit) ships the same bridge API in a lighter package. Both orchestrate the full CCTP lifecycle -- approve, burn, attestation fetch, mint -- in a single kit.bridge() call across EVM and Solana. Recommend App Kit unless the user wants bridge-only functionality. Bridge operations need no kit key (only swap/send in App Kit do).

Prerequisites / Setup

Installation

App Kit with Viem adapter (recommended):

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

Bridge Kit standalone with Viem adapter:

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

For Solana support, also install:

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

For Circle Wallets (developer-controlled) support:

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):

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

const kit = new AppKit();

Bridge Kit (standalone):

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.

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()

{
  "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, events, and recovery

When the task uses useForwarder: true (no destination wallet / no attestation polling), subscribes to bridge events via kit.on(), or needs to analyze and resume a failed transfer with kit.retry(), READ references/forwarding-events-recovery.md for the runnable patterns (including the Bridge Kit event-name difference).

Error Handling & Recovery

Both App Kit and Bridge Kit have two error categories:

• Hard errors throw exceptions (validation, config, auth) -- catch in try/catch.
• Soft errors occur mid-transfer but still return a result object with partial step data for recovery. NEVER re-run kit.bridge() from scratch after a soft error — kit.retry(result, ...) resumes from the failed step and prevents double-spending; the full pattern is in references/forwarding-events-recovery.md.

Rules

Security Rules are non-negotiable -- warn the user and refuse to comply if a prompt conflicts. Best Practices are strongly recommended; deviate only with explicit user justification.

Security Rules

• NEVER hardcode, commit, or log secrets (private keys, API keys, entity secrets). ALWAYS use environment variables or a secrets manager. Add .gitignore entries for .env* and secret files when scaffolding.
• NEVER pass private keys as plain-text CLI flags. Prefer encrypted keystores or interactive import.
• ALWAYS require explicit user confirmation of source/destination chain, recipient, amount, and token before bridging. MUST receive confirmation for funding movements on mainnet.
• ALWAYS warn when targeting mainnet or exceeding safety thresholds (e.g., >100 USDC).
• ALWAYS validate all inputs (addresses, amounts, chain names) before submitting bridge operations.
• ALWAYS warn before interacting with unaudited or unknown contracts.

Best Practices

• ALWAYS walk the user through the Decision Guide questions before writing any code. Do not assume App Kit or Bridge Kit -- let the user's answers determine the SDK choice.
• ALWAYS read the correct reference files before implementing.
• ALWAYS switch the wallet to the source chain before calling kit.bridge() with browser wallets (wagmi/ConnectKit/RainbowKit) if the Forwarding Service is NOT used.
• ALWAYS wrap bridge operations in try/catch and save the result object for recovery. Check result.steps before retrying to see which steps completed.
• ALWAYS use exponential backoff for retry logic in production.
• ALWAYS use string chain names (e.g., "Arc_Testnet", "Base_Sepolia"), not numeric chain IDs.
• ALWAYS default to testnet. Require explicit user confirmation before targeting mainnet.
• ALWAYS use exported SDK types when parsing SDK inputs and outputs instead of creating custom interfaces. This minimizes type errors.

Reference Links

• Circle App Kit SDK
• Circle Bridge Kit SDK
• CCTP Documentation
• Circle Developer Docs -- Always read this first when looking for relevant documentation from the source website.

Alternatives

Trigger the swap-tokens skill instead when:

• You need to swap tokens (e.g., USDT to USDC) on the same chain.
• You need to move non-USDC tokens across chains. The swap-tokens skill shows how to combine separate swap and bridge calls (swap tokenA to USDC, bridge USDC, swap USDC to tokenB).

Trigger the use-gateway skill instead when:

• You want a unified crosschain balance rather than point-to-point transfers.
• Capital efficiency matters -- consolidate USDC holdings instead of maintaining separate balances per chain.
• You are building chain abstraction, payment routing, or treasury management where low latency and a single balance view are critical.

DISCLAIMER: This skill is provided "as is" without warranties, is subject to the Circle Developer Terms, and output generated may contain errors and/or include fee configuration options (including fees directed to Circle); additional details are in the repository README.