Q402 (Sign-to-Pay) Protocol

The Q402 protocol introduces a universal sign-to-pay model for EVM chains. It replaces the traditional “approve → transfer → pay” pattern with a single signature flow that is:

• Gasless for users (facilitator pays gas)

• Permit2-compatible (no ERC-3009 token upgrades needed)

• Policy-aware (spend limits, expiry, and whitelists)

• Fully verifiable (on-chain receipts and logs)

This system forms the execution backbone of Quack AI’s AI Economy — powering both governance automation and day-to-day agent transactions.

For developers, the entire implementation — including core contracts, SDKs, middleware, and facilitator services — is open source here:

🔗 GitHub: github.com/quackai-labs/X402-BNB

Repository Structure

The x402 repository is organized into modular packages for flexible integration:

Directory / Package	Purpose	Tech Stack
/packages/core	Core SDK and client utilities for creating and signing x402 payment headers	TypeScript
/packages/facilitator	Standalone gas relay and settlement service for executing delegated transactions	Node.js / Docker
/packages/middleware-express	Express.js middleware for instant 402 payment validation	TypeScript
/packages/middleware-hono	Lightweight middleware for Hono-based APIs	TypeScript
/examples/bsc-testnet	Demo apps showing how to integrate x402 with APIs or dApps	JavaScript / Express
/docs	Architecture, standards, and deployment guides	Markdown
/specs	Core EIP-7702 implementation logic	Solidity + TS bindings

Each package is published under the @x402-bnb namespace and can be installed independently via pnpm or npm.

Core Developer Flow

Developers can use the x402 stack through three main integration paths:

Flow	Use Case	Entry Point
Client SDK	Apps or wallets generating sign-to-pay transactions	@x402-bnb/core
Server Middleware	APIs requiring payment validation or premium access	@x402-bnb/middleware-express or @x402-bnb/middleware-hono
Facilitator Service	Independent relay handling verification and gas sponsorship	@x402-bnb/facilitator

All flows support EIP-712 + EIP-7702 standards, use short-lived authorization tuples, and output a verifiable X-PAYMENT header that can be traced on-chain.

Example Use Cases

Scenario	Implementation Path
Gasless API access (e.g., premium endpoints)	Express middleware using @x402-bnb/middleware-express
DAO proposal execution with policy checks	Governance API + Facilitator
RWA payouts and treasury automation	Governance API + Q402 SDK
Subscription renewals for data agents	Facilitator + Permit2 integration
Merchant or marketplace settlement	Batch transactions via Facilitator Service

Each transaction includes a receipt hash linking signature → transaction → policy execution — providing a transparent audit trail for compliance and governance monitoring.