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CastPay

CastPay enables seamless, ETH-free USDC transfers via social usernames, reducing friction in crypto payments on social platforms.

Overview

CastPay allows Farcaster users to send USDC payments using simple commands like @username /pay 5 USDC without needing to hold ETH for gas fees. The system uses a Stylus-based Paymaster contract on Arbitrum to sponsor gas fees, making crypto payments as easy as sending a message.

Problem and Solution

The Problem

Traditional crypto payments on social platforms face several barriers:

  • Gas Fee Friction: Users need to hold native tokens (ETH) to pay gas fees, creating a poor UX
  • Complex Addresses: Wallet addresses are hard to remember and type, especially on mobile
  • Multiple Steps: Current payment flows require multiple app switches and confirmations
  • Technical Barriers: Users need deep crypto knowledge to navigate gas prices and network fees

Our Solution

CastPay eliminates these barriers through:

  • Account Abstraction: ERC-4337 compatible smart accounts remove gas fee requirements
  • Social Identity: Pay using familiar @usernames instead of complex wallet addresses
  • Gasless Transactions: Stylus Paymaster sponsors all gas fees from USDC balance
  • Native Integration: Seamless Farcaster Mini App experience without leaving the social platform
  • Instant Settlement: Sub-5 second transaction completion with immediate confirmation

This creates a Web2-like payment experience while maintaining full crypto ownership and transparency.

Features

  • Gasless Payments: Send USDC without holding ETH
  • Username-based Transfers: Pay using @usernames instead of wallet addresses
  • Instant Transactions: Complete payments in under 5 seconds
  • Farcaster Integration: Native mini app integration with Warpcast
  • ERC-4337 Compatible: Built on Account Abstraction standards
  • Stylus Paymaster: Efficient Rust-based smart contracts on Arbitrum

Quick Start

Prerequisites

  • Node.js 16+ and pnpm
  • Warpcast account with linked wallet
  • Arbitrum testnet ETH for initial setup

Installation

git clone https://github.com/your-org/castpay
cd castpay
pnpm install

Development

  1. Start the frontend:

    cd frontend
pnpm dev

  2. Start the relayer service:

    cd backend
pnpm start

  3. Deploy contracts:

    cd contracts/template
cargo build --release
# Deploy using your preferred method

Usage

Sending a Payment

  1. Enable CastPay Mini App in Warpcast
  2. Type @username /pay [amount] USDC in a cast or DM
  3. Confirm the payment in the popup modal
  4. Payment completes instantly without ETH fees

Example

@alice /pay 10 USDC

This sends 10 USDC to the user @alice.

Architecture


  Farcaster               CastPay              Arbitrum     
  Mini App                 Relayer          Blockchain    
  (Frontend)              (Backend)            (Contracts)

Components

  • Frontend: React-based Farcaster Mini App for user interface
  • Backend: Node.js relayer service for transaction bundling
  • Contracts: Stylus Paymaster contract written in Rust
  • Integrations: Farcaster API for username resolution

Project Structure

castpay/
    frontend/           # Farcaster Mini App UI
        src/
            App.tsx
            components/
        package.json
    backend/            # Relayer service
        src/
            relayer.js
        package.json
    contracts/          # Smart contracts
        template/
            src/
                lib.rs
            Cargo.toml
    scripts/           # Deployment scripts

Configuration

Environment Variables

Create .env files in the respective directories:

Frontend (.env):

VITE_RELAYER_URL=http://localhost:3001
VITE_CHAIN_ID=421614

Backend (.env):

PRIVATE_KEY=your_private_key
ARBITRUM_RPC_URL=https://sepolia-rollup.arbitrum.io/rpc
FARCASTER_API_KEY=your_api_key

Smart Contracts

The Paymaster contract is built using Arbitrum Stylus (Rust/WASM) and implements ERC-4337 standards:

  • Validates user operations
  • Sponsors gas fees by deducting from USDC balance
  • Includes ~10% surcharge for sustainability
  • Deployed on Arbitrum testnet

API Reference

Relayer Endpoints

  • POST /send-payment - Submit a payment operation
  • GET /payment-status/:txHash - Check payment status
  • GET /user-balance/:address - Get user USDC balance

Payment Format

{
  "recipient": "@username",
  "amount": "10.00",
  "currency": "USDC",
  "signature": "0x..."
}

Security

  • No private keys stored by the application
  • Rate limiting to prevent abuse
  • Signature validation for all transactions
  • Paymaster validates operations before sponsoring

Contributors

Contributing

  1. Fork the repository
  2. Create a feature branch
  3. Make your changes
  4. Add tests
  5. Submit a pull request

License

MIT License - see LICENSE for details