How to Build a Pay-Per-Call AI API Market with x402 and MCP Protocol

#ai #web3 #tutorial #api

Why Pay-Per-Call?

Most AI API platforms force you into monthly subscriptions. But what if you only need to call an OCR endpoint once? Or run a single image recognition task?

That's why I built GoldBean — a pay-per-call AI API market where you only pay for what you use ($0.01-$0.03 per call).

Architecture Overview

GoldBean runs on a single VPS with:

Node.js server handling API requests (port 9879)
MCP HTTP bridge for Model Context Protocol compatibility (port 3099)
Nginx reverse proxy with SSL termination
Three payment gateways: PayPal, Alipay, USDC (on-chain via x402)

The x402 Payment Protocol

x402 is the key innovation. Here's how it works:

Client sends an API request with an EIP-3009 signed transfer
Server verifies the signature and checks the payment amount
If valid, server executes the API call and returns the result
The entire flow takes less than 2 seconds with near-zero fees

// Client side: signing the payment
const transfer = {
  to: merchantAddress,
  value: priceInWei,
  validAfter: 0,
  validBefore: Math.floor(Date.now() / 1000) + 3600,
  nonce: randomNonce()
};
const signature = await signer._signTypedData(domain, types, transfer);

MCP Server Integration

GoldBean is fully compatible with the Model Context Protocol. You can plug it directly into Claude, Cursor, or any MCP-capable AI tool:

{
  "mcpServers": {
    "goldbean": {
      "url": "https://goldbean-api.xyz/sse"
    }
  }
}

Once connected, your AI assistant can call 47 different AI endpoints — OCR, image recognition, speech synthesis, LLM chat, image generation — without writing any code.

47 Endpoints

Here are some of the key API categories:

OCR: general text, handwriting, table recognition, medical documents
Image Recognition: object detection, scene analysis, logo recognition
Speech: text-to-speech with multiple voices, speech-to-text
LLM: chat completions, text generation, summarization
Image Generation: AI-powered image creation

All accessible via a single API key with pay-per-call pricing.

Free Tier

Every user gets 50 free API calls per day. No credit card required to start.

DEV Community