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b402 is built in three layers. Each layer can be used independently, but they work best together.

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Protocol Layer

The foundation is the b402 Relayer smart contract, deployed on each supported EVM chain. It handles:
  • EIP-712 signature verification for gasless payment authorization
  • On-chain token transfers via transferWithAuthorization()
  • Replay protection using random nonces and time windows (validAfter / validBefore)
  • Token whitelisting for supported assets (USDT, USDC, USD1)
The Relayer enables meta-transactions: users sign off-chain, the facilitator pays gas and executes on-chain. Contracts:
ContractAddressNetwork
B402 RelayerV30xE91b564EB8DFF305Ff8efA332f84c487b9da5171BNB Chain + Base Mainnet
Nexus Account Factory0x0000006648ED9B2B842552BE63Af870bC74af837BNB Chain Mainnet

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Service Layer

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Facilitator

The Facilitator is the API service that bridges HTTP requests and blockchain settlement. It provides: EOA Payments - Verify and settle EIP-712 signed payments for standard wallets. Smart Wallet Payments - Build and submit ERC-4337 UserOperations for Nexus smart wallets. The facilitator injects fees, signs paymaster data, and submits to bundlers. Wallet Deployment - Deterministic smart wallet creation via ERC-7579 Nexus factory. Wallets are computed off-chain and deployed on first use. Incognito Operations - Privacy-preserving payments through the b402 privacy pool. Shield tokens, transact privately, and unshield to recipients. Base URL: https://facilitatorv3.b402.ai

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b402scan

Agent reputation and seller discovery platform. Browse verified sellers, check agent payment history, and discover APIs accepting b402 payments. Currently in development.

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Developer Layer

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Direct API Integration

The Facilitator API is the primary integration point. Any HTTP client can interact with b402: 
// Verify and settle a payment
const verify = await fetch('https://facilitatorv3.b402.ai/verify', {
  method: 'POST',
  headers: { 'Content-Type': 'application/json' },
  body: JSON.stringify({ paymentPayload }),
});
const settle = await fetch('https://facilitatorv3.b402.ai/settle', {
  method: 'POST',
  headers: { 'Content-Type': 'application/json' },
  body: JSON.stringify({ paymentPayload }),
});

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Agent Framework Integration

b402 works with popular AI agent frameworks:
  • MCP Tools — Expose b402 payment capabilities as Model Context Protocol tools for Claude, GPT, and other LLM agents
  • LangChain / CrewAI — Custom tools that wrap the Facilitator API for autonomous agent payments
  • Vercel AI SDK — Server-side payment verification in AI-powered API routes

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EIP-712 Signatures

For buyers, the core integration is signing EIP-712 typed data. See the EIP-712 Signature Guide for manual construction.

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Payment Flow

The full payment lifecycle:
b402 payment flow: Client, Server, Facilitator, and Blockchain interaction sequence

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Smart Wallet Flow

For ERC-7579 Nexus smart wallets, the flow uses UserOperations instead of direct signatures:
  1. Client requests payment with wallet address
  2. Facilitator builds unsigned UserOp with fee injection
  3. Client signs the UserOp hash
  4. Facilitator submits to ERC-4337 bundler with paymaster sponsorship
  5. Bundler executes on-chain (batch call: approve + transfer)
  6. Facilitator returns transaction receipt
This enables gasless, batched operations for AI agents without requiring native gas tokens.