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

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 Enter fullscreen mode Exit fullscreen mode 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 mcp-publisher publish Enter fullscreen mode Exit fullscreen mode 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"}] } Enter fullscreen mode Exit fullscreen mode Ugly namespace, granted. It's mine, it verifies, and it cost exactly $0. What I learned 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. Free handshake, paid execution is the only viable MCP monetization pattern today — and it maps cleanly onto x402's 402 challenge. 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.