Crossline

Crossline is a decentralized trading interface designed to enable gasless, cross-chain limit orders across EVM-compatible blockchains — starting with Ethereum Sepolia and Monad testnets.

The core idea is to allow users to place limit orders to trade tokens across chains (e.g., sell WETH on Ethereum and receive USDC on Monad) without paying gas and without having to execute the transaction themselves. Instead, the system uses off-chain order books, signed messages, and a backend matching engine that automatically identifies matching orders, then executes the trade across chains using LayerZero for messaging.

Here’s what makes Crossline unique:

Zero gas cost for users: Users don’t need to hold native tokens for gas or sign transactions manually. They just sign an EIP-712 typed message once, and the system handles the rest.

Cross-chain execution: The system utilizes LayerZero to pass matched order data and execution confirmation between chains, allowing settlement across different chains without user intervention.

Off-chain matching engine: All limit orders are collected and matched off-chain in real time. This design prevents MEV attacks and front-running, which are common in on-chain order books.

Secure by design: Trades are only executed if both parties sign their intent off-chain. On-chain settlement is trustless and executed via smart contracts once a valid match is found.

Modular architecture: The backend handles matching, order management, and execution triggers. The frontend is a fully responsive React app with WebSocket-driven updates for real-time feedback on trade status (e.g., "matching", "executing", "cross-chain", "completed").

Seamless UX: Through wallet integrations (RainbowKit and Wagmi), users can easily switch networks and place trades with one signature. A centralized dashboard helps track order history and live order status.

Ultimately, Crossline is built to show what a next-generation DEX could look like — one that:

removes the technical friction of gas and manual execution,

protects users from MEV,

and scales trustlessly across multiple chains.

This project is not just a demo — it lays the foundation for scalable, efficient, and accessible decentralized finance across ecosystems.

Core Architecture Crossline is a full-stack, production-ready decentralized limit order protocol with real cross-chain settlement. It supports gasless order creation, off-chain matching, and cross-chain execution using LayerZero.

Technology Stack Frontend Next.js 14 (TypeScript)

RainbowKit + Wagmi for wallet support

Viem as Ethereum client

Tailwind CSS (custom design system)

Socket.io-client for real-time updates

Recharts for ETH price charts

Backend Express.js with helmet, compression, rate limiting

MongoDB + Mongoose for persistent storage

Socket.io for WebSocket communication

node-cron for 5-second matching intervals

EIP-712 signature verification

Ethers.js for contract interaction

Smart Contracts Solidity (Hardhat)

EIP-712 typed data signing

LayerZero + Axelar for cross-chain messaging

CrossChainManager abstracts bridges

Cross-Chain Architecture Flow User signs EIP-712 order (gasless)

Backend matches compatible orders

Cross-chain message sent (LayerZero/Axelar)

Contracts settle trade on both chains

Matching Engine Runs every 5 seconds via cron

Matches orders using price-time priority

Uses MongoDB aggregation for matching logic

Emits WebSocket events on status change

Gasless Trading (EIP-712) Users sign structured order data (no gas)

Backend verifies signature, stores order

Only matched orders are broadcast on-chain Deployment Sepolia testnet: contracts deployed, LayerZero + Axelar live

Monad testnet: EVM-compatible, custom adapter used

Notable Implementations Contract detection by chain ID

WebSocket-backed demo mode for hackathon

Price precision calculation based on ETH value

Graceful MongoDB connection fallback

Production Readiness MongoDB persistence

Real signature verification

Rate limiting, error handling

Cross-chain message handling

WebSocket-driven updates

Multi-network support