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Marcus Chenmember_832ef635
Marcus Chenmember_832ef635

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I Wrapped My x402 Pay-Per-Call APIs in an MCP Server — and Got Into the Official Registry Without GitHub

#mcp #x402 #aiagents #webdev

Three weeks ago I put a QR code generator behind HTTP 402 and called it a business. It made $0.00. Then I added an image toolkit. Same result. The endpoints were listed on every x402 directory I could find, the 402 challenges were textbook-compliant, and still — nothing.

The problem wasn't pricing or discoverability. It was shape: the agents I wanted as customers don't shop for REST endpoints. They shop for tools. MCP tools.

So last night I wrapped all three pay-per-call APIs in a single MCP server (streamable-http), kept the x402 payment gate exactly where the money moves, and published the result to the official MCP Registry — without a GitHub account, without a paid domain, without KYC. Here's the whole trick.

The gate: 402 on tools/call, everything else free

The MCP handshake has to be free, or directory probes and clients can't even list your tools. My server answers initialize, tools/list, ping and notifications for free, and only charges when an agent actually calls a tool:

FREE_METHODS = ("initialize", "notifications/initialized",
                "notifications/cancelled", "tools/list", "ping")

if method != "tools/call":
    ...  # free
# tools/call without a PAYMENT-SIGNATURE header -> HTTP 402
# + a base64 PAYMENT-REQUIRED header describing price in USDC on Base
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One subtlety that cost me an hour: directory validators send empty or unknown-method POSTs to probe your endpoint. If you answer those with a JSON-RPC error (HTTP 200), the validator reports "endpoint returns 200 instead of 402" and rejects you. Any non-free method, identified or not, must get the 402 challenge.

Three tools, settled through the PayAI facilitator (permissionless, no API key, the facilitator pays the gas):

  • generate_qr_code — $0.01
  • process_image (resize/convert/compress/thumbnail) — $0.01
  • analyze_text (summary/sentiment/keywords/translate, LLM-backed) — $0.02

The registry: HTTP auth with a key you host yourself

The official MCP Registry (registry.modelcontextprotocol.io) is what PulseMCP and friends crawl. The docs push the GitHub login flow — my GitHub account was long dead, so I used the other option almost nobody writes about: HTTP authentication.

You generate an Ed25519 key, serve one line of text at /.well-known/mcp-registry-auth on a domain you control, and the registry accepts your reverse-DNS namespace:

openssl genpkey -algorithm Ed25519 -out key.pem
PUBLIC_KEY="$(openssl pkey -in key.pem -pubout -outform DER | tail -c 32 | base64)"
echo "v=MCPv1; k=ed25519; p=${PUBLIC_KEY}" > mcp-registry-auth
# serve it at https://your-domain/.well-known/mcp-registry-auth
mcp-publisher login http --domain your-domain --private-key <hex>
mcp-publisher publish
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My "domain" is a free sslip.io hostname on a $4 VPS. It worked. One gotcha: the registry caps description at 100 characters (422 otherwise) — the schema error tells you the limit, at least.

The server.json for a remote-only server is refreshingly small:

{
  "name": "io.sslip.249.111.77.187/x402-mcp-tools",
  "description": "Pay-per-call MCP tools via x402 (USDC on Base): QR codes, image processing, LLM text analysis.",
  "version": "1.0.0",
  "remotes": [{"type": "streamable-http",
               "url": "https://mcp.187.77.111.249.sslip.io/mcp"}]
}
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Ugly namespace, granted. It's mine, it verifies, and it cost exactly $0.

What I learned

  1. Distribution beats features. The same three APIs sat unbought as REST endpoints. As MCP tools they're one tools/list away from any agent with a wallet.
  2. Free handshake, paid execution is the only viable MCP monetization pattern today — and it maps cleanly onto x402's 402 challenge.
  3. You don't need GitHub for the official registry. The HTTP auth flow is documented but nobody seems to use it. A throwaway domain and one OpenSSL command are enough.

Revenue so far: still $0.00. But the server is now in the registry every major MCP directory crawls, the 402 gate is standard-compliant, and settlement costs me nothing. The infrastructure bet is placed — now we see if agents actually pay for QR codes.

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