x402-mcp-middleware

Express middleware for X402 payment-enabled MCP (Model Context Protocol) servers.

Overview

x402-mcp-middleware provides drop-in middleware for building payment-enabled MCP servers. v2 introduces multi-network support and MCP-spec compliance, enabling servers to accept payments from multiple blockchains while transmitting payment data via JSON-RPC _meta fields.

Key Features (v2)

• Multi-Network Support: Accept payments from multiple EVM chains simultaneously
• MCP-Spec Compliant: Uses JSON-RPC error code 402 and _meta for payment transmission
• Automatic Network Routing: Routes payments to correct network based on payload
• EIP-3009 Authorization: Gas-efficient, trust-minimized payment signatures
• Simulation Before Settlement: Optional transaction simulation before on-chain execution

Documentation

Quick Start

v2 - Multi-Network (recommended)

import { X402Middleware } from 'x402-mcp-middleware/v2'

Server Configuration (Multi-Network):

const middleware = await X402Middleware.create( {
    x402V2ExactEvmConfiguration: {
        contractCatalog: {
            'usdc-base-sepolia': {
                paymentNetworkId: 'eip155:84532',
                address: '0x036CbD53842c5426634e7929541eC2318f3dCF7e',
                decimals: 6,
                domainName: 'USDC',
                domainVersion: '2'
            },
            'usdc-avalanche-fuji': {
                paymentNetworkId: 'eip155:43113',
                address: '0x5425890298aed601595a70AB815c96711a31Bc65',
                decimals: 6,
                domainName: 'USDC',
                domainVersion: '2'
            }
        },
        paymentOptionCatalog: {
            'base-usdc-10k': { contractId: 'usdc-base-sepolia', amount: '10000', payTo: '{{facilitator}}' },
            'avax-usdc-10k': { contractId: 'usdc-avalanche-fuji', amount: '10000', payTo: '{{facilitator}}' }
        },
        restrictedCalls: [
            {
                method: 'tools/call',
                name: 'premium_tool',
                acceptedPaymentOptionIdList: [ 'base-usdc-10k', 'avax-usdc-10k' ]
            }
        ]
    },
    server: {
        payToAddressMap: { facilitator: '0x...' },
        providerUrlByPaymentNetworkId: {
            'eip155:84532': process.env.BASE_SEPOLIA_RPC,
            'eip155:43113': process.env.AVALANCHE_FUJI_RPC
        },
        facilitatorPrivateKeyByPaymentNetworkId: {
            'eip155:84532': process.env.FACILITATOR_KEY,
            'eip155:43113': process.env.FACILITATOR_KEY
        }
    }
} )

app.use( middleware.mcp() )

MCP Protocol Flow:

Client → tools/call (no payment)
Server → JSON-RPC error 402 + accepts[]

Client → tools/call + _meta["x402/payment"]
Server → Validate → Simulate → Settle
Server → result + _meta["x402/payment-response"]

Legacy v1 (frozen)

import { X402Middleware } from 'x402-mcp-middleware/legacy'

Structure

x402-mcp-middleware/
  src/
    v1/                    # v1 implementation (frozen)
    v2/                    # v2 implementation (multi-network)
    legacy/                # Legacy entry point (re-exports v1)
  docs/
    index.md               # Documentation index
    v1/                    # v1 documentation
    v2/                    # v2 documentation
  tests/
    v1/                    # v1 tests
    v2/                    # v2 tests

Version Support

Testing

# GitHub CI tests (no blockchain)
npm run test:ci

# E2E tests (requires .env)
npm run test:v2:server  # In terminal 1
npm run test:v2:client  # In terminal 2

Contribution

Contributions are welcome! If you encounter bugs, have feature suggestions, or want to improve the module, feel free to open an issue or submit a pull request.

License

This project is licensed under the MIT License. See the LICENSE file for details.