# Bridge Stablecoin > Build USDC bridging with Circle App Kit or standalone Bridge Kit SDK and Crosschain Transfer Protocol (CCTP). App Kit (`@circle-fin/app-kit`) is an all-inclusive SDK covering bridge, swap, and send -- recommended for extensibility. Bridge Kit (`@circle-fin/bridge-kit`) is a standalone package for bridge-only use cases. Neither requires a kit key for bridge operations. Supports bridging USDC between EVM chains, between EVM chains and Solana, and between any two chains on Circle Wallets (i.e Developer-Controlled Wallets or Programmable wallets). Use when: bridge USDC, setting up Bridge Kit adapters (Viem, Ethers, Solana Kit, Circle Wallets), handling bridge events, collecting custom fees, configuring transfer speed, or using the Forwarding Service. Triggers on: bridge USDC, CCTP, move USDC between chains, @circle-fin/bridge-kit, @circle-fin/app-kit, forwarding service. - Skill: `circlefin/bridge-stablecoin` (Agent Skill, multi-file: 5 files) - Install (CLI): `npx skillmds@latest add circlefin/bridge-stablecoin` - Raw SKILL.md: https://api.skillmd.com/api/skills/circlefin/bridge-stablecoin/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: Coding & Dev Tools - Author: circlefin (https://skillmd.com/u/circlefin) - Updated: 2026-09-09 - Page: https://skillmd.com/skills/circlefin/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): ```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. ## 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, 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](https://docs.arc.network/app-kit) - [Circle Bridge Kit SDK](https://docs.arc.network/app-kit/bridge) - [CCTP Documentation](https://developers.circle.com/cctp) - [Circle Developer Docs](https://developers.circle.com/llms.txt) -- **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](https://console.circle.com/legal/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](https://github.com/circlefin/skills/blob/master/README.md).