Core Concepts

Understanding the key concepts behind T402.

Payment Flow

T402 follows a simple HTTP-based payment flow:

Key Terms

Resource Server

The HTTP server that provides a resource (API, content, file) that requires payment.

Client

The entity making HTTP requests and paying for resources.

Facilitator

A server that handles payment verification and settlement. Can be self-hosted or use a hosted service.

Scheme

A logical way of moving money. T402 supports multiple schemes:

• exact: Pay an exact amount (EIP-3009 for EVM, SPL for Solana)
• exact-legacy: Legacy token transfers (USDT on Ethereum)

Network

The blockchain network where payment occurs:

• eip155:1 - Ethereum Mainnet
• eip155:8453 - Base
• eip155:42161 - Arbitrum
• ton:mainnet - TON
• tron:mainnet - TRON
• solana:mainnet - Solana

Payment Headers

Request Headers

Response Headers

Payment Requirements

When a server responds with 402, it includes payment requirements:

interface PaymentRequirements {
  scheme: 'exact' | 'exact-legacy'
  network: string           // e.g., 'eip155:8453'
  maxAmountRequired: string // e.g., '1000000' (1 USDT)
  resource: string          // Resource identifier
  description?: string      // Human-readable description
  mimeType?: string         // Expected response type
  payTo: string             // Recipient address
  maxTimeoutSeconds?: number
  asset?: string            // Token address
  extra?: Record<string, unknown>
}

Payment Payload

The client creates a payment payload based on the requirements:

interface PaymentPayload {
  t402Version: number
  scheme: string
  network: string
  payload: {
    // Scheme-specific payload
    // For EVM: signature, authorization data
    // For TON: signed BOC
    // For TRON: signed transaction
  }
}

Token Support

USDT0 (Recommended)

The new OFT (Omnichain Fungible Token) version of USDT with:

• EIP-3009 support for gasless transfers
• LayerZero bridging between chains
• Available on Ethereum, Arbitrum, Base, Ink, Berachain, Unichain

USDT (Legacy)

Traditional USDT on various chains:

• Ethereum (ERC-20)
• TRON (TRC-20)
• Polygon, BNB Chain

USDC

Also supported on EVM chains with EIP-3009.

Security Model

T402 is designed to be trust-minimizing:

1. Client signatures: Payments require client authorization
2. Amount limits: Clients specify maximum amounts
3. Time limits: Authorizations expire
4. Nonce tracking: Prevents replay attacks
5. No custody: Facilitators never hold funds

The facilitator can only move funds exactly as authorized by the client.