SplitChain

Problem Statement

Splitchain is a Web3-powered expense-sharing application that revolutionizes how friends, roommates, and crypto enthusiasts manage and settle shared costs. Unlike traditional expense-tracking apps like Splitwise that only record debts without facilitating actual payments, Splitchain enables instant, onchain settlement of expenses through blockchain technology.Built on Coinbase Developer Platform (CDP), Splitchain leverages cutting-edge Web3 infrastructure to deliver a seamless user experience. The app utilizes CDP Embedded Wallets for frictionless onboarding—automatically creating wallets for users without requiring seed phrases or complex crypto knowledge. Users can create expenses, automatically calculate fair splits among group members, and trigger instant settlements.Key features include Coinbase Pay integration for easy fiat-to-crypto wallet top-ups, the CDP Trade API for automatic token swaps ensuring seamless transactions regardless of which tokens users hold, and an intuitive interface that makes Web3 accessible to mainstream users. The platform provides transparency through basic analytics while maintaining the social aspects of expense sharing.Splitchain addresses critical pain points in the current expense-sharing landscape: the friction of chasing payments, delays in settlement, trust issues between parties, and the complexity of managing expenses across different currencies and platforms. By combining the familiarity of traditional expense-splitting apps with the power of blockchain technology, Splitchain creates a future where settling shared bills is as simple as creating them—instant, transparent, and trustless.

Solution

Splitchain was built as a full-stack Web3 application leveraging Coinbase Developer Platform (CDP) technologies to create a seamless expense-sharing experience. Here's the technical breakdown:Frontend Architecture:Built with TypeScript and modern web frameworks, the UI prioritizes simplicity and accessibility for both crypto-native and mainstream users. The interface handles expense creation, group management, and real-time settlement tracking with a clean, intuitive design that abstracts away blockchain complexity.Coinbase Developer Platform Integration:The core of Splitchain relies heavily on CDP's suite of tools:CDP Embedded Wallets: Implemented for passwordless, seedless wallet creation. This was crucial for onboarding users who've never touched crypto—wallets are auto-generated on signup, eliminating the biggest barrier to Web3 adoption. Users don't need to understand private keys or manage seed phrases.CDP AgentKit & X402 Payments: Integrated for instant, onchain settlement. When a user confirms their share of an expense, X402 triggers real-time micropayments with immediate confirmations. This creates the "instant settlement" magic that differentiates us from traditional expense apps.Coinbase Pay: Embedded for frictionless fiat-to-crypto onramps. Users can top up their wallets directly within the app using credit/debit cards, making it as easy as any traditional payment app.CDP Trade API: Powers automatic token swaps in the background. If User A wants to pay in USDC but User B prefers ETH, the Trade API handles the conversion seamlessly without users needing to think about it.Smart Contract Layer:Custom smart contracts deployed on Base (Ethereum L2) handle the expense logic, settlement distribution, and group management. Base was chosen for its low transaction fees and fast finality, making microtransactions economically viable.Notable Hacks & Technical Highlights:Optimistic Settlement UI: To combat blockchain latency, we implemented optimistic UI updates that assume transaction success while confirming in the background. Users see instant feedback while the onchain settlement completes asynchronously.Gasless Transactions: Integrated meta-transactions so users don't need to hold native tokens for gas fees. The app sponsors small gas costs for micro-settlements, removing another friction point for Web3 newcomers.Group Wallet Abstraction: Created a lightweight smart contract wallet system where each "group" (e.g., roommates, trip buddies) gets its own multisig-style wallet for transparent fund management without the complexity of traditional multisig UX.Real-time State Sync: Combined WebSocket connections with onchain event listeners to provide instant updates when group members add expenses or make payments, creating a social, collaborative feel.Hackathon Speed-Run:The entire MVP was built within the hackathon timeframe by leveraging CDP's SDK documentation and pre-built components. The Embedded Wallets SDK saved days of authentication work, while the Trade API's simple REST interface allowed quick integration without deep DeFi protocol knowledge. The most challenging part was balancing Web3's asynchronous nature with the real-time expectations of expense-sharing UX—solved through the optimistic UI pattern mentioned above.Tech Stack Summary:Frontend: TypeScript, React/Next.js, CSS3Backend: Node.js, ExpressBlockchain: Base (Ethereum L2), Solidity smart contractsCDP Technologies: Embedded Wallets, Coinbase PayDatabase: PostgreSQL for off-chain expense metadata and user preferencesReal-time: WebSocket for live updatesThis architecture demonstrates how CDP's tools can transform a simple expense-tracking app into a true Web3 application that delivers real value—instant, trustless settlement—while maintaining the UX standards users expect from modern consumer apps.