x402-sdk-go

Go SDK for the x402 payment protocol. Implements the full HTTP 402 Payment Required flow — client signing, server paywall middleware, and facilitator settlement.

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What is x402?

x402 is an open standard for internet-native payments using the HTTP 402 status code. This SDK provides the Go implementation for integrating x402 into your services.

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Architecture

The SDK has three roles:

Role	Description	Key Files
Client	Signs EIP-712 payment permits, handles 402 retry	client.go, http_client.go
Server	Protects endpoints with paywall middleware, returns 402	server.go, middleware/
Facilitator	Verifies signatures, settles on-chain via Permit402 contract	facilitator.go, facilitator_client.go

Payment Flow

Client Request → Server returns 402 + PaymentRequired
       ↓
Client signs EIP-712 permit → Retries with PAYMENT-SIGNATURE header
       ↓
Server → Facilitator verifies signature + calls Permit402.permitTransferFrom()
       ↓
Settlement confirmed → Server returns resource

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Installation

go get github.com/springmint/x402-sdk-go

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Quick Start

Server (Paywall Middleware)

import (
    x402 "github.com/springmint/x402-sdk-go"
    "github.com/springmint/x402-sdk-go/middleware"
)

// Gin example
router.Use(ginmw.PaywallMiddleware(middleware.PaywallConfig{
    Scheme:          "permit402",
    Network:         x402.BSCMainnet,
    PayToAddress:    "0xYourAddress",
    Amount:          "1000000000000000000", // 1 USDT (18 decimals on BSC)
    Asset:           "USDT",
    FacilitatorURL:  "https://cppay.finance/api/x402/facilitator",
}))

Client (Auto 402 Handling)

import x402 "github.com/springmint/x402-sdk-go"

client := x402.NewX402HTTPClient(privateKey, rpcURL)
resp, err := client.Get("https://api.example.com/protected-resource")

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Supported Networks

Network	Identifier	Permit402 Address
TRON Mainnet	tron:mainnet	—
TRON Nile	tron:nile	—
TRON Shasta	tron:shasta	—
BSC Mainnet	eip155:56	0x105a6f4613a1d1c17ef35d4d5f053fa2e659a958

Supported Tokens

• USDT — TRON, BSC
• USDC — BSC, Ethereum, Base

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Project Structure

├── abi/              # Contract ABI definitions (Permit402, ERC-20, EIP-712)
├── mechanisms/       # Payment scheme implementations (permit402, transfer_auth)
├── middleware/       # HTTP paywall middleware (net/http, Gin, GoFrame)
├── signers/          # EIP-712 signing and verification
├── tokens/           # Token registry (symbol → address per chain)
├── utils/            # EIP-712 hashing, provider resolution
├── setup/            # Convenience setup helpers
├── client.go         # X402Client — client-side payment orchestration
├── server.go         # X402Server — server-side payment validation
├── facilitator.go    # FacilitatorClient interface
├── config.go         # Network configs (chain IDs, contract addresses, RPCs)
├── types.go          # Core data structures
└── http_client.go    # HTTP client with automatic 402 retry

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License

MIT