bridge-stablecoin by @circlefin

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.

Both SDKs run in browser and server applications. Use browser wallet provider adapters in client code and keep private keys plus Circle Wallets credentials on the server. App Kit >=1.11.0 and Bridge Kit >=1.12.2 bundle for browsers without consumer-provided Node or Buffer polyfills.

Prerequisites / Setup

Installation

Pick one base kit — App Kit (recommended) or the standalone Bridge Kit — then add adapters as needed.

App Kit with Viem adapter (recommended):

npm install @circle-fin/app-kit @circle-fin/adapter-viem-v2
# Optional: Solana support
npm install @circle-fin/adapter-solana-kit
# Optional: Circle Wallets (developer-controlled) support
npm install @circle-fin/adapter-circle-wallets

Or, for bridge-only apps, the standalone Bridge Kit (lighter package) instead of App Kit:

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

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. Browser-wallet integrations need none of the variables above. Never expose a private key, Circle API key, entity secret, or optional App Kit credential to client code.

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 an EVM browser wallet (wagmi, ConnectKit, RainbowKit, or any EIP-1193 provider) -> Use the Viem provider adapter. READ references/adapter-wagmi.md
Using a Solana browser wallet (Wallet Standard provider such as Phantom, Solflare, or Backpack) -> Use the Solana provider adapter. READ references/adapter-browser-wallet.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

Workflow

Walk the Decision Guide -- settle the SDK (App Kit vs Bridge Kit) and the wallet/adapter with the user before writing any code.
Install and initialize -- install the chosen kit plus adapter, then construct the kit and adapter (see Prerequisites / Setup).
Confirm the transfer -- source and destination chains, recipient, and amount. Validate chain names and addresses. Default to testnet; require explicit confirmation before mainnet.
Execute the bridge -- call kit.bridge({ from, to, amount, ... }), only from an explicit user action (never auto-invoke). READ the adapter reference for the exact code.
Track and recover -- inspect result.state / result.steps, subscribe with kit.on(), and on a soft failure resume with kit.retry(result, ...) -- never re-run kit.bridge() from scratch.

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", "Arc_Testnet", "Base_Sepolia", "Solana_Devnet"), not numeric chain IDs, in the kit.bridge() call.
Browser-safe packages: Current App Kit, Bridge Kit, and Solana adapters ship the required browser compatibility internally. Browser requests omit Node-only headers, and Solana operations need no consumer Buffer shim. Upgrade stale packages instead of adding polyfills.

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.
references/adapter-browser-wallet.md -- Solana browser wallet integration using a Wallet Standard provider, with no manual polyfills

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://explorer.testnet.arc.io/tx/0x1234..."
    },
    {
      "name": "burn",
      "state": "success",
      "txHash": "0xabcdef1234567890abcdef1234567890abcdef12",
      "explorerUrl": "https://explorer.testnet.arc.io/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, kit keys). ALWAYS use environment variables or a secrets manager. Add .gitignore entries for .env* and secret files when scaffolding.
NEVER put a private key, Circle API key, entity secret, or kit key in browser code or a public environment variable (VITE_*, NEXT_PUBLIC_*, etc.).
NEVER import the main @circle-fin/adapter-circle-wallets entry in a browser; it requires server-side Circle credentials.
NEVER pass private keys as plain-text CLI flags. Prefer encrypted keystores or interactive import.
ALWAYS surface the source/destination chain, recipient, amount, and token before bridging. In a UI, the user's click on the enabled Bridge button is explicit confirmation; do not add a second confirmation prompt or synthetic confirmation object. In a server helper, export the fund-moving operation without auto-invoking it. NEVER call bridge() automatically after estimation, from an effect, during render, or at module startup. 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.
For browser apps, require browser-safe package versions, use provider-based adapters, and do not add Node/Buffer polyfills.
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", "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 with a direct cross-chain swap route.

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.