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Eliza MCP Gateway

A production-ready MCP (Model Context Protocol) gateway that aggregates multiple MCP servers into a unified interface with complete x402 blockchain payment support. Enables AI agents to autonomously pay for tools and services while allowing gateway operators to monetize MCP tool access.

🚀 Core Features

Gateway Capabilities

  • 🔄 Multi-Server Aggregation: Connect to unlimited MCP servers simultaneously
  • 🏷️ Automatic Namespacing: Prevents tool/resource/prompt name conflicts
  • 🚀 Multi-Transport Support: STDIO, HTTP, SSE, WebSocket - mix and match
  • 💪 Health Monitoring: Automatic reconnection and health checks
  • 🛡️ Conflict Resolution: Intelligent handling of duplicate tool names
  • 🧩 Resource & Prompt Resolution: Optional conflict resolution for resources and prompts
  • ⚡ Dynamic Discovery: Real-time capability updates and registry refresh
  • 📋 Configuration-Based: YAML/JSON configuration for easy management
  • 🔐 Env Var Interpolation: Use ${VAR} and ${VAR:-default} in config files

Payment & Monetization

  • 💰 x402 Blockchain Payments: Full EIP-712 signature validation with USDC
  • 🔑 API Key Authentication: Tiered access with custom pricing
  • 🌐 HTTP/SSE Transport: Proper 402 status codes for payment challenges
  • ✅ Facilitator Integration: Official x402.org verification and settlement
  • 💸 Three Payment Modes: Passthrough, Markup, Absorb
  • 📊 Flexible Pricing: Per-tool, per-server, or global defaults
  • 🧪 Testnet Ready: Base Sepolia support for safe testing
  • ↗️ Outbound x402 Payments: Automatic downstream payments via x402-axios
  • 👛 Server Wallet Overrides: Per-server paymentWallet setting for fine control

💰 How MCP Monetization Works

The gateway enables monetization of MCP tools using blockchain payments and API keys. Here's how it works:

Architecture Overview

┌──────────────────┐
│   AI Agent       │  Anthropic Claude, OpenAI, etc.
│  (with wallet)   │  Uses x402-fetch or x402-axios
│  or API key      │  Sends Authorization/X-ELIZA-API-KEY headers
└────────┬─────────┘
         │ MCP Protocol
         │ Auto-pays on HTTP 402 or API key auth
         ↓
┌──────────────────┐
│  HTTP Wrapper    │  Port 8000 (optional, for payments)
│  402 Gateway     │  • Returns HTTP 402 (x402 payments)
└────────┬─────────┘  • Validates API keys (tiered pricing)
         │ OR         • Verifies payments with facilitator
┌──────────────────┐  • Settles blockchain transactions
│  STDIO Gateway   │  Direct MCP (no payment/API key support)
└────────┬─────────┘
         │
         │ MCP Protocol (authenticated)
         ↓
┌──────────────────┐
│  Gateway Core    │  • Payment verification (x402 + API keys)
│                  │  • Tool routing
│                  │  • Namespace management
└────────┬─────────┘
         │
         ├─→ Free MCP Server (CoinGecko, filesystem, etc.)
         ├─→ Paid MCP Server (MCPay, premium APIs)
         └─→ Your Custom MCP Server

Payment Flow (x402)

Step 1: Client Requests Tool

POST http://localhost:8000/message
Content-Type: application/json

{"method": "tools/call", "params": {"name": "crypto:get_search"}}

Step 2: Gateway Returns 402 (Payment Required)

HTTP/1.1 402 Payment Required
Content-Type: application/json
X-Accept-Payment: {...}

{
  "x402Version": 1,
  "accepts": [{
    "scheme": "exact",
    "network": "base-sepolia",
    "maxAmountRequired": "10000",
    "resource": "http://localhost:8000/message",
    "payTo": "0xYourAddress",
    "asset": "0x036CbD...USDC",
    "extra": {"name": "USDC", "version": "2"}
  }]
}

Step 3: Agent Creates Payment (Automatic)

  • Agent's x402-fetch/x402-axios intercepts 402
  • Creates EIP-712 signature with wallet
  • Signs USDC transfer authorization
  • Encodes payment as base64

Step 4: Agent Retries with Payment

POST http://localhost:8000/message
X-PAYMENT: <base64-encoded-payment-signature>

{"method": "tools/call", "params": {"name": "crypto:get_search"}}

Step 5: Gateway Verifies & Settles

  • Decodes payment signature
  • Verifies with x402 facilitator
  • Settles transaction on blockchain
  • Forwards request to downstream MCP server

Step 6: Tool Executes

  • MCP server processes request
  • Returns result to gateway
  • Gateway forwards to client

Payment Modes

1. Passthrough Mode 🔄

Use Case: Aggregate paid MCP servers, earn discovery fees

servers:
  - name: "mcpay-server"
    transport:
      type: "http"
      url: "https://paid-api.com"
    paymentMode: "passthrough"

Flow: Client Payment → Gateway (takes fee) → Downstream Server Revenue: Discovery/routing fees, no downstream costs

2. Markup Mode 💰

Use Case: Resell paid APIs with margin

payment:
  enabled: true
  recipient: "0xYourWallet"
  outboundWallet: "0xYourPrivateKey"

servers:
  - name: "premium-api"
    paymentMode: "markup"
    markup: "20%"

Flow: Client pays (downstream + 20%) → Gateway verifies → Gateway pays downstream → Earn 20% margin Revenue: Percentage markup on every transaction

3. Absorb Mode 🎁

Use Case: Subscription/membership model

payment:
  enabled: true
  outboundWallet: "0xYourPrivateKey"
  apiKeys:
    - key: "premium_subscriber"
      tier: "premium"

servers:
  - name: "expensive-api"
    paymentMode: "absorb"
    defaultPricing:
      apiKeyTiers:
        premium: "free"
      x402: "$1.00"

Flow: Client authenticated with API key → Gateway pays all downstream costs → Subscription revenue Revenue: Monthly/annual subscriptions, unlimited usage

Pricing Models

Per-Tool Pricing

servers:
  - name: "myserver"
    tools:
      - name: "basic-tool"
        pricing:
          free: true
      - name: "premium-tool"
        pricing:
          x402: "$0.10"
          apiKeyTiers:
            basic: "$0.05"
            premium: "free"

Server-Wide Default Pricing

servers:
  - name: "myserver"
    defaultPricing:
      x402: "$0.01"

Mixed Strategy

servers:
  - name: "freemium-server"
    defaultPricing:
      x402: "$0.01"
    tools:
      - name: "premium-feature"
        pricing:
          x402: "$1.00"
      - name: "free-sample"
        pricing:
          free: true

📦 Installation

npm install -g @elizaos/mcp-gateway

Or run directly with npx:

npx @elizaos/mcp-gateway --config=config.yaml

With x402 Support (HTTP/SSE Mode)

# Install dependencies
bun install

# Start HTTP wrapper with x402 support
bun run src/transports/http-wrapper.ts --config=your-config.yaml --port=8000

This exposes:

  • SSE endpoint: http://localhost:8000/sse
  • Message endpoint: http://localhost:8000/message
  • Supports: HTTP 402 status codes, x402 payments, API keys

🔧 Configuration

Basic Configuration (Free)

name: "Development MCP Gateway"
version: "1.0.0"

servers:
  # Local STDIO server
  - name: "filesystem"
    transport:
      type: "stdio"
      command: "npx"
      args: ["-y", "@modelcontextprotocol/server-filesystem", "/tmp"]
    namespace: "fs"
    enabled: true

  # Remote SSE server  
  - name: "coingecko"
    transport:
      type: "sse"
      sseUrl: "https://mcp.api.coingecko.com/sse"
      postUrl: "https://mcp.api.coingecko.com/message"
    namespace: "crypto"
    enabled: true

settings:
  enableToolConflictResolution: true
  enableResourceConflictResolution: true
  enablePromptConflictResolution: true
  logLevel: "info"
  healthCheckInterval: 60000
  maxConcurrentConnections: 10

With Payment Gating (Monetization)

name: "Paid MCP Gateway"
version: "1.0.0"

# Payment Configuration
payment:
  enabled: true
  recipient: "0xYourEthereumAddress"
  network: "base-sepolia"  # or "base" for mainnet
  facilitator: "https://x402.org/facilitator"
  
  # Optional: API keys for subscription model
  apiKeys:
    - key: "eliza_premium_abc123"
      tier: "premium"
    - key: "eliza_basic_xyz789"
      tier: "basic"

servers:
  - name: "coingecko"
    transport:
      type: "sse"
      sseUrl: "https://mcp.api.coingecko.com/sse"
      postUrl: "https://mcp.api.coingecko.com/message"
    namespace: "crypto"
    
    # Charge $0.01 per tool call
    defaultPricing:
      x402: "$0.01"
      apiKeyTiers:
        premium: "free"
        basic: "$0.005"
    
    # Or per-tool pricing
    tools:
      - name: "get_search"
        pricing:
          x402: "$0.02"
      - name: "get_simple_price"
        pricing:
          free: true

settings:
  logLevel: "info"

With Downstream Payment (Passthrough/Markup)

payment:
  enabled: true
  recipient: "0xYourWallet"
  network: "base"
  
  # Wallet for paying downstream services
  outboundWallet: "0xYourPrivateKey"

servers:
  # Passthrough mode - forward client payments
  - name: "mcpay-search"
    transport:
      type: "http"
      url: "https://mcpay.tech/search"
    paymentMode: "passthrough"
    
  # Markup mode - add 20% margin
  - name: "premium-api"
    transport:
      type: "http"
      url: "https://premium-api.com"
    paymentMode: "markup"
    markup: "20%"
    # Optional: override wallet just for this server
    paymentWallet: "0xServerSpecificPrivateKey"
    
  # Absorb mode - subscription model
  - name: "subscriber-api"
    transport:
      type: "http"
      url: "https://expensive-api.com"
    paymentMode: "absorb"
    defaultPricing:
      apiKeyTiers:
        premium: "free"

Configuration Reference

Payment Fields:

  • payment.enabled: Enable payment gating (true/false)
  • payment.recipient: Your Ethereum address for receiving payments
  • payment.network: Blockchain network (base-sepolia, base, ethereum)
  • payment.facilitator: x402 facilitator URL (default: https://x402.org/facilitator)
  • payment.outboundWallet: Private key for paying downstream (markup/absorb modes)
  • payment.apiKeys: Array of API keys with tiers

Server Fields:

  • transport.type: stdio, http, sse, or websocket
  • namespace: Prefix for tool names (prevents conflicts)
  • paymentMode: passthrough, markup, or absorb
  • markup: Percentage (20%) or fixed amount ($0.05)
  • defaultPricing: Server-wide pricing defaults
  • tools: Per-tool configuration and pricing
  • paymentWallet: Private key override for outbound payments for this server
  • timeout, retryAttempts, retryDelay: Connection stability controls

Pricing Fields:

  • x402: Price for blockchain payments (e.g., $0.01)
  • apiKeyTiers: Tier-specific pricing ({"premium": "free", "basic": "$0.05"})
  • free: Mark tool as free (true/false)

Environment Variable Interpolation

You can reference environment variables directly inside YAML/JSON configs:

payment:
  enabled: true
  recipient: "${RECIPIENT_ADDRESS}"
  network: "${NETWORK:-base-sepolia}"
servers:
  - name: "filesystem"
    transport:
      type: "stdio"
      command: "${MCP_COMMAND:-npx}"
      args: ["-y", "@modelcontextprotocol/server-filesystem", "${MCP_FS_ROOT:-/tmp}"]

Supported formats: ${VAR} and ${VAR:-default}.

🎯 Usage

1. STDIO Mode (Claude Desktop, Cursor)

Free, no payments:

# Start gateway
mcp-gateway --config=config.yaml

Claude Desktop configuration:

{
  "mcpServers": {
    "eliza-gateway": {
      "command": "npx",
      "args": ["-y", "@elizaos/mcp-gateway", "--config", "/path/to/config.yaml"]
    }
  }
}

2. HTTP/SSE Mode (AI Agents with Payments)

With x402 payment support:

# Start HTTP wrapper with x402
bun run src/transports/http-wrapper.ts --config=paid-config.yaml --port=8000

Agents connect to:

  • SSE endpoint: http://localhost:8000/sse
  • Message endpoint: http://localhost:8000/message

Client integration:

import { createMCPClient } from 'ai';
import { SSEClientTransport } from '@modelcontextprotocol/sdk/client/sse.js';
import { wrapFetchWithPayment } from 'x402-fetch';
import { privateKeyToAccount } from 'viem/accounts';

// Setup wallet for payments
const account = privateKeyToAccount('0xYourPrivateKey');
const fetchWithPayment = wrapFetchWithPayment(fetch, account);

// Create MCP client with payment support
const mcpClient = await createMCPClient({
  transport: new SSEClientTransport(
    new URL('http://localhost:8000/sse'),
    { fetch: fetchWithPayment }
  )
});

3. Environment Variables

  • MCP_GATEWAY_NAME - Gateway name (default: "Eliza MCP Gateway")
  • MCP_GATEWAY_VERSION - Gateway version (default: "1.0.0")
  • MCP_GATEWAY_DESCRIPTION - Description shown to clients
  • MCP_LOG_LEVEL - Log level: error, warn, info, debug (default: info)
  • MCP_SERVERS - Semicolon-separated server specs
  • MCP_ENABLE_TOOL_CONFLICT_RESOLUTION - Enable tool conflict resolution (default: true)
  • MCP_ENABLE_RESOURCE_CONFLICT_RESOLUTION - Enable resource conflict resolution (default: true)
  • MCP_ENABLE_PROMPT_CONFLICT_RESOLUTION - Enable prompt conflict resolution (default: true)
  • MCP_MAX_CONCURRENT_CONNECTIONS - Max concurrent server connections (default: 10)
  • MCP_HEALTH_CHECK_INTERVAL - Health check interval in ms (default: 60000)

🚀 Transport Support

Fully Supported Transports:

  • 📟 STDIO Transport - Local MCP servers via stdin/stdout

    • Perfect for command-line MCP servers, Claude Desktop, Cursor
    • Configuration: command, args, env, cwd

  • 🌐 HTTP Transport - Remote MCP servers via HTTP/HTTPS

    • For web-based MCP servers and API integrations
    • Configuration: url, apiKey, headers

  • 📡 SSE Transport - Server-Sent Events for streaming

    • Real-time streaming capabilities
    • Configuration: sseUrl, postUrl, apiKey, headers

  • 🔌 WebSocket Transport - Real-time bidirectional communication

    • Low-latency, persistent connections
    • Configuration: url, apiKey, headers
# Modern transport configuration
servers:
  # Local STDIO server
  - name: "local-server"
    transport:
      type: "stdio"
      command: "npx"
      args: ["-y", "user-review-mcp"]
    
  # Remote HTTP server
  - name: "remote-server"
    transport:
      type: "http"
      url: "https://mcp.example.com/api"
      apiKey: "YOUR_API_KEY"
    
  # Legacy format (still supported)
  - name: "legacy-server"
    command: "npx"
    args: ["-y", "@upstash/context7-mcp"]

Working Examples:

See the examples/ directory for complete, tested configurations:

🧪 Testing

The gateway includes a comprehensive End-to-End test suite that validates all functionality across different configurations and transport types.

Quick Testing (Recommended)

# Run essential tests (fastest)
bun run test:quick

Full Test Suite

# Run all tests including payment tests
bun run test:all

# Or run test suites individually:
bun run test:quick   # Quick smoke tests (~45s)
bun run test         # Full E2E tests (~180s)
bun run test:payment # Payment E2E tests (~31s)
bun run test:passthrough # Passthrough/Markup/Absorb E2E tests

Test Specific Configurations

# Test with specific config
bun run start --config=tests/configs/basic.yaml

# Available test configs:
# Core functionality:
# - basic.yaml - Single server, STDIO transport
# - basic.json - Same as basic.yaml but JSON format
# - namespaced.yaml - Single server with namespace
# - multi-server.yaml - Multiple servers, different namespaces
# - invalid.yaml - Invalid config for error testing
# - failing-server.yaml - Server connection failure testing
#
# Payment features:
# - paid-free-tools.yaml - Mix of free and paid tools
# - paid-api-key-only.yaml - API key authentication only
# - paid-x402-only.yaml - x402 blockchain payments only
# - paid-default-pricing.yaml - Server-wide default pricing
# - paid-disabled.yaml - Payment disabled (backward compatibility)

Interactive Testing

# Manual testing with MCP Inspector
npx @modelcontextprotocol/inspector bun run src/index.ts --config=examples/config.yaml

Test Coverage

The E2E test suite validates:

  • ✅ Configuration Loading: YAML, JSON, and environment variables
  • ✅ All Transport Types: STDIO, HTTP, SSE, WebSocket
  • ✅ Multi-Server Support: Multiple servers with namespace handling
  • ✅ Payment Features: x402 verification, API keys, tiered pricing
  • ✅ Error Handling: Invalid configs, connection failures, graceful degradation
  • ✅ Tool Execution: MCP protocol communication and tool calls
  • ✅ Process Management: Startup, shutdown, cleanup

Latest Test Results:

  • Quick E2E: ✅ 11/11 passed (45s)
  • Full E2E: ✅ 28/28 passed (180s)
  • Payment E2E: ✅ 15/15 passed (31s)
  • Total: ✅ 54/54 tests (100% success rate)

CI/CD Integration

# For continuous integration pipelines
bun run test:quick  # Fast, essential tests only
bun run test:all    # Complete test suite

See tests/README.md for the detailed testing guide.

📊 Example Output

When running with multiple servers, you'll see:

[INFO] Starting Eliza MCP Gateway Server: Development MCP Gateway v1.0.0
[INFO] Initializing 2 servers...
[INFO] Successfully initialized server context7
[INFO] Successfully initialized server user-review  
[INFO] Initialized 2/2 servers successfully
[INFO] Registry refreshed: 3 tools, 0 resources, 0 prompts
[INFO] === Eliza MCP Gateway Status ===
[INFO] Server Connections: 2/2 active
[INFO] Tools by Server:
[INFO]   - context7: 2 tools
[INFO]   - user-review: 1 tools

Available tools with namespacing:

  • docs:resolve-library-id (Context7)
  • docs:get-library-docs (Context7)
  • review:get-user-review (User Review MCP)

🏗️ Architecture

High-Level System Architecture

┌─────────────────────────────────────────────────────────┐
│                     AI Agents / Clients                  │
│  (Claude Desktop, Cursor, Custom Agents, AI Frameworks) │
└──────────────────────┬──────────────────────────────────┘
                       │
        ┌──────────────┴──────────────┐
        │                             │
   STDIO Mode                    HTTP/SSE Mode
  (No Payments)                (With x402 Payments)
        │                             │
        │                     ┌───────┴────────┐
        │                     │  HTTP Wrapper  │
        │                     │  Port 8000     │
        │                     │  • Returns 402 │
        │                     │  • Verifies x402
        │                     │  • Settles txn │
        │                     └───────┬────────┘
        │                             │
        └──────────────┬──────────────┘
                       │
              ┌────────┴────────┐
              │  Gateway Core   │
              │  • Registry     │
              │  • Routing      │
              │  • Namespacing  │
              └────────┬────────┘
                       │
        ┌──────────────┼──────────────┐
        │              │              │
   ┌────┴────┐   ┌────┴────┐   ┌────┴────┐
   │ STDIO   │   │  HTTP   │   │   SSE   │
   │ Server  │   │ Server  │   │  Server │
   └─────────┘   └─────────┘   └─────────┘
     Local         Remote        Streaming

Transport Wrapper Options

  • HTTP Wrapper (src/transports/http-wrapper.ts): Spawns the stdio gateway per SSE session, handles HTTP/SSE endpoints, returns HTTP 402 for payments, and injects request headers into MCP messages for verification.
  • SSE Server Transport (src/transports/sse-server.ts): Alternative built-in SSE/HTTP transport that can host the MCP server directly with CORS and header handling. Useful when you don’t want a subprocess per session.

Both wrappers inject inbound HTTP headers into MCP request params as metadata so the gateway core can verify payments.

Header Propagation and Verification

  1. Client sends HTTP request with headers (e.g., X-PAYMENT, X-ELIZA-API-KEY).
  2. Wrapper injects headers into MCP params as _meta.headers.
  3. Gateway core retrieves headers from request.params._meta.headers.
  4. PaymentMiddleware.verifyPayment() authorizes via API key or x402, or returns requirements for a 402 challenge.
  5. On success, request routes to the correct downstream MCP server.

Complete Payment Flow

sequenceDiagram
    participant Agent as AI Agent<br/>(x402-fetch)
    participant Wrapper as HTTP Wrapper<br/>(Port 8000)
    participant PM as Payment<br/>Middleware
    participant Facilitator as x402.org<br/>Facilitator
    participant Gateway as Gateway Core<br/>(stdio)
    participant MCP as Downstream<br/>MCP Server

    Agent->>Wrapper: POST /message (CallTool)
    Wrapper->>PM: Check payment required?
    PM-->>Wrapper: Yes, $0.01 required
    
    Wrapper-->>Agent: 402 Payment Required<br/>+ payment requirements
    
    Note over Agent: x402-fetch intercepts 402<br/>Creates EIP-712 signature<br/>Signs USDC authorization
    
    Agent->>Wrapper: POST /message + X-PAYMENT header
    Wrapper->>PM: Decode & verify payment
    PM->>Facilitator: Verify signature
    Facilitator-->>PM: ✅ Valid, payer verified
    PM->>Facilitator: Settle transaction
    Facilitator-->>PM: ✅ Settlement queued
    PM-->>Wrapper: ✅ Payment authorized
    
    Wrapper->>Gateway: Forward CallTool (stdio)
    Gateway->>MCP: Execute tool
    MCP-->>Gateway: Tool result
    Gateway-->>Wrapper: MCP response
    Wrapper-->>Agent: Tool result (SSE stream)
Loading

Payment Modes at a Glance

  • Passthrough: Client pays upstream provider directly. Wrapper/gateway returns 402 and forwards/accepts x402 headers as needed; pricing is defined by the upstream. Use when aggregating already-paid MCP servers.
  • Markup: Client pays your gateway price; gateway calculates final price and pays the upstream via outbound wallet if needed. Use payment.outboundWallet (global) or paymentWallet (per server) to control the paying account.
  • Absorb: Client authenticates (e.g., API key tier free), and gateway pays all downstream costs using outbound wallet. Useful for subscriptions.

Component Breakdown

1. HTTP Wrapper (src/transports/http-wrapper.ts)

  • Handles HTTP/SSE protocol
  • Returns proper HTTP 402 status codes
  • Integrates x402 SDK for payment verification
  • Manages gateway subprocesses per session
  • Required for x402 payments

2. Gateway Core (src/core/gateway.ts)

  • MCP protocol implementation
  • Tool/resource/prompt routing
  • Namespace management
  • Server connection management
  • Works over stdio (no HTTP needed)

3. Payment Middleware (src/core/payment-middleware.ts)

  • x402 payment verification (EIP-712 signatures)
  • API key tier validation
  • Facilitator integration
  • Payment settlement
  • Pricing calculation (markup, passthrough, absorb)

4. X402 Payment Client (src/core/x402-client.ts)

  • Outbound payments to downstream HTTP/SSE services (markup/absorb)
  • Uses x402-axios for automatic 402 handling and signing
  • Supports per-server wallet override via paymentWallet

5. Server Manager (src/core/server-manager.ts)

  • Multi-transport support (STDIO, HTTP, SSE, WebSocket)
  • Health monitoring
  • Automatic reconnection
  • Connection pooling

6. Gateway Registry (src/core/registry.ts)

  • Capability aggregation
  • Conflict resolution
  • Namespace application
  • Dynamic updates

7. SSE Server Transport (src/transports/sse-server.ts)

  • Hosts SSE and message endpoints directly (no subprocess)
  • Injects headers and supports CORS/keep-alive
  • Can be paired with PaymentMiddleware for inline checks

Conflict Resolution Rules

  • Tools: If enableToolConflictResolution and a duplicate name is detected, the registry appends the server ID (e.g., name → name_serverA). Namespacing (namespace: "docs") prefixes names (e.g., docs:get-library-docs).
  • Resources: If enableResourceConflictResolution, URIs are namespaced (query param when URL-like; otherwise namespace:uri). Duplicates are suffixed with server ID.
  • Prompts: If enablePromptConflictResolution, duplicate prompt names are suffixed with server ID; namespacing applies as with tools.

Request Lifecycle (HTTP/SSE mode)

  1. Client opens GET /sse to receive streaming events and obtains a per-session POST /message endpoint.
  2. Client posts MCP requests to /message with payment or API key headers.
  3. Wrapper injects headers into _meta.headers and forwards to the gateway stdio server (or inline MCP server for SSE transport).
  4. Gateway resolves the target tool/resource/prompt via the registry.
  5. Payment middleware authorizes or responds with 402 + X-Accept-Payment.
  6. On success, the request is routed to the downstream MCP server; response streams back via SSE and HTTP.
  7. Optional: for markup/absorb flows, outbound payments are executed with the configured wallet.

💼 Real-World Use Cases

1. Monetize Free APIs

Turn free public APIs into paid services:

  • Gateway connects to CoinGecko (free)
  • Charges agents $0.01 per tool call
  • Revenue: 46 tools × $0.01 = $0.46 per full query

2. Aggregate Paid MCP Servers

Become a discovery/routing layer for paid services:

  • Connect to multiple MCPay servers
  • Use passthrough mode (forward payments)
  • Revenue: Discovery fees, routing fees

3. Resell with Margin

Add value and earn markup:

  • Connect to paid APIs
  • Add 20-40% markup
  • Revenue: Margin on every transaction

4. Subscription Gateway

Bundle multiple paid APIs:

  • Absorb downstream costs
  • Charge monthly subscription via API keys
  • Revenue: Predictable recurring income

5. Freemium Platform

Mix free and paid tools:

  • Free tools for discovery
  • Premium tools for revenue
  • Revenue: Conversion from free to paid users

🔐 Security & Production

Testnet (Development)

payment:
  network: "base-sepolia"
  recipient: "0xTestAddress"
  facilitator: "https://x402.org/facilitator"

Mainnet (Production)

payment:
  network: "base"
  recipient: "0xProductionAddress"
  facilitator: "https://x402.org/facilitator"
  • Real USDC transactions
  • Monitor settlement status
  • Secure key management essential

Best Practices

  • ✅ Use checksummed Ethereum addresses
  • ✅ Store private keys in secure vaults
  • ✅ Monitor payment verification logs
  • ✅ Set reasonable timeout values
  • ✅ Test on testnet first
  • ✅ Implement rate limiting
  • ✅ Log all payment attempts

📚 Technical Details

x402 Payment Specs

  • Protocol: EIP-712 typed signatures
  • Asset: USDC (6 decimals)
  • Networks: Base, Base Sepolia, Ethereum, Optimism, Polygon
  • Min Amount: $0.001 (1000 atomic units)
  • Settlement: Gasless via facilitator
  • Timeout: 30-60 seconds

Supported Networks

Network USDC Contract Chain ID Type
base-sepolia 0x036CbD...3dCF7e 84532 Testnet
base 0x833589...02913 8453 Mainnet
ethereum 0xA0b869...eB48 1 Mainnet
optimism 0x0b2C63...Ff85 10 Mainnet
polygon 0x3c499c...c3359 137 Mainnet

Transport Compatibility Matrix

Transport Payment Support Use Case
STDIO ❌ No HTTP headers Claude Desktop, Cursor (free only)
HTTP ✅ Full x402 Remote APIs, agents with wallets
SSE ✅ Full x402 Streaming, real-time agents
WebSocket ⚠️ Experimental Low-latency applications

Performance

  • Connection: ~500ms (SSE handshake)
  • Tool Discovery: ~1s (46 tools)
  • Payment Verification: ~200ms (facilitator)
  • Tool Execution: 1-5s (depends on downstream)
  • Total Latency: 2-7s per paid query

📖 Documentation

  • Examples: See the examples/ directory for ready-to-run configs
  • Testing Guide: See tests/README.md
  • Agents: See example-agents/ for SSE client examples

🤝 Contributing

  1. Fork the repository
  2. Create a feature branch (git checkout -b feature/amazing-feature)
  3. Make your changes with tests
  4. Commit your changes (git commit -m 'feat: Add amazing feature')
  5. Push to the branch (git push origin feature/amazing-feature)
  6. Open a Pull Request

📄 License

MIT License - see LICENSE file for details.

🙏 Acknowledgments

Core Technologies

MCP Servers Tested With

Built with ❤️ for the AI Agent ecosystem

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