How I Built a 120-Endpoint x402 API Marketplace in a Weekend (And Why It Matters)

Building the "Swiss Army Knife" for AI Agents on the x402 protocol

The Problem

I've been building on the x402 protocol — Coinbase's micropayment standard for AI agents. The idea is beautiful: agents call APIs, pay per request with USDC, no API keys, no subscriptions. It's the HTTP of agent commerce.

But there was a gap. When I looked at the existing services, they were all single-purpose:

• BlockRun: AI chat completions
• OttoAI: AI chat completions
• Aubr: AI search

One API = one task. An agent that needs a price chart, a weather report, a blockchain lookup, and an AI chat — that's four different x402 integrations.

That's when I realized: nobody had built the "everything API" for agents.

The Idea

What if one API could do everything an agent needs?

• Blockchain: 🪙 BTC price, gas fees, wallet balances, ENS lookup, DeFi insights
• AI: 🤖 Chat, code, translation, image generation, video generation
• Weather: 🌤️ Current, forecast, historical
• Social: 🐦 Twitter, Discord, Telegram, Instagram, TikTok
• Search: 🔍 Web search, arXiv, GitHub
• E-commerce: 🛒 Product search, reviews
• Finance: 💱 Forex, stocks, commodities
• Utilities: ✉️ Email verification, IP lookup, domain WHOIS, URL shorten
• Security: 🔐 AML screening, KYC verification, phishing detection
• PI: 🏢 RWA, real estate, credit scoring, bond data

120 endpoints. 20+ categories. One API.

That's GoldBean.

The Tech Stack

The stack is refreshingly simple:

GoldBean = Express.js + x402 micropayments + ChatGPT

Architecture

┌─────────────┐     ┌─────────┐     ┌──────────────┐
│   AI Agent  │────▶│ nginx   │────▶│  GoldBean    │
│ (x402 SDK)  │     │ (SSL)   │     │ (Express.js) │
└─────────────┘     └─────────┘     └──────────────┘
                                            │
                                     ┌──────┴──────┐
                                     │  Base Chain  │
                                     │  (USDC on   │
                                     │  eip155:8453)│
                                     └─────────────┘

The flow is elegant:

1. Agent discovers GoldBean via /.well-known/x402 or /openapi.json
2. Agent calls /paid/llm-chat → gets back HTTP 402 Payment Required
3. 402 response includes Payment-Required header (base64-encoded JSON: amount, network, payTo)
4. Agent pays via x402 → gets back a payment receipt → retries with it
5. GoldBean validates → serves the response

No API keys. No auth. No subscriptions. Just pay and use.

The Hardest Part: Getting Registered on x402scan

This deserves its own section because it took longer than building the API.

What x402scan expects

The x402scan scanner validates four things:

1. /.well-known/x402 — discovery endpoint
2. /openapi.json — the canonical API contract
3. /openapi.json has x-payment-info on paid endpoints
4. Endpoint returns proper HTTP 402 with Payment-Required header

The 402 Header Format (V2)

Here's the format that works (took 6+ iterations to nail):

{
  "x402Version": 2,
  "accepts": [{
    "scheme": "exact",
    "network": "eip155:8453",
    "amount": "40000",
    "asset": "0x833589f...",
    "payTo": "0x7484b0...",
    "maxTimeoutSeconds": 300
  }],
  "resource": {
    "url": "https://goldbean-api.xyz/paid/llm-chat",
    "description": "llm-chat",
    "mimeType": "application/json"
  }
}

Lessons learned:

• x402Version must be a number (not string)
• amount is in atomic units (USDC = 6 decimals, so $0.04 = "40000")
• amount must be a string (not number)
• Body must be empty {} (V2 moves all payment info to headers)
• The response must have Cache-Control: no-store
• Add BOTH Payment-Required AND x-payment-required headers for compatibility

I built an x402-check tool to verify format — it saved me hours of debugging.

The Schema Trap

Getting 31/120 endpoints registered was easy. The remaining 89 all failed with:

Missing input schema

The fix: every endpoint needs a proper input schema in OpenAPI so the scanner can construct a valid test request. A parameter like address for address lookups, prompt for chat endpoints, lat/lon for weather. The scanner reads the OpenAPI, builds a probe request, and expects 402. Without input schemas, it can't even probe.

What Running 120 Endpoints Actually Looks Like

The Good

One server process. One domain. One integration. 120 capabilities.

npx agentcash try https://goldbean-api.xyz

Boom. Your agent can now check BTC price, get a weather forecast, generate an image, analyze sentiment, and chat with an LLM — all through a single API with a single $USDC wallet.

The Reality Check

$0 revenue. 0 buyers. 0 transactions.

Registration doesn't equal usage. The next phase is getting actual agents to call the endpoints. That means:

1. Dev.to / Medium — telling the story
2. Community — showing up on x402 Discord/Twitter
3. Free trials — first 10 calls free to lower the barrier
4. Self-hosting — running automated calls to generate initial tx records (who wants to be the first buyer of a zero-tx service?)

What's Next

Why I'm Building This

The x402 protocol is going to be huge. 480,000+ agents already transacting, $165M+ in agent-to-agent commerce on Base chain.

But the infrastructure is still early. Most x402 services are single-purpose. GoldBean is my bet that agents need a general-purpose API — one they can call for anything, without worrying about "does this endpoint support x402?"

120 endpoints down. 200+ on the roadmap.

Let's see where this goes.

Built with ☕ and x402. Check us out: https://goldbean-api.xyz