I put an MCP server behind HTTP 402. Here's the exact setup.

AI agents are hitting APIs thousands of times a day and paying nothing. I wanted to know if the "pay per call" story is real or slideware, so I built the smallest possible version: my own knowledge base, sold per query, settled on-chain. It works. Here's everything, including the transaction.

What it is

Five MCP tools wrapping things I actually know — ComfyUI API-prompt fixes, node wiring recipes, verified dead ends (the kind of knowledge that only exists because I burned GPU hours finding out). Each tool costs $0.005 per call, paid in USDC on Base. Two more tools are free: ping, and list_topics so an agent can see what's for sale before spending anything. Discovery free, answers paid.

The stack (all stock, nothing custom)

• @x402/mcp — Coinbase's x402 MCP package. The key primitive is createPaymentWrapper: you wrap each paid tool's handler and it handles the 402 challenge/response at the MCP protocol level. Paid tools and free tools live side by side.
• @modelcontextprotocol/sdk — the standard MCP server, StreamableHTTP transport via Express (plain SSE stalls behind tunnels — learned the hard way).
• Facilitator: PayAI's public facilitator for Base mainnet (no key needed on the free tier). Note: the x402.org facilitator is testnet-only despite what the docs imply — it errors on mainnet routes.
• Chain: Base. Testnet (Sepolia) for iteration, mainnet for the proof below.

The whole server is ~120 lines. The payment wrapper is the product; everything else is plumbing.

The payment flow, measured

1. Agent calls a paid tool → server answers 402 with payment requirements ($0.005 USDC, my address, Base).
2. Agent signs an EIP-3009 authorization. The buyer pays zero gas — signing is free, no ETH needed. This is the property the whole design rests on.
3. Facilitator verifies the signature and submits the USDC transfer on-chain, paying the gas itself (~$0.001–0.05 on Base).
4. Server gets the settlement confirmation and returns the answer.

Measured end to end on testnet: https://sepolia.basescan.org/tx/0x5b0b2205606f13d7197a5c8d7556d413723d1ae84c9732eac1b3a4b1850fa2cb (success — $0.005 USDC moved). Then one real mainnet settlement: https://basescan.org/tx/0xb762d7856c03fc009b4743859850e075b997ea90d36a68681348e9634b0634f0 — $0.005 moved on Base, verifiable by anyone.

The gas math nobody puts in the pitch deck

At $0.005 per call, gas is not a footnote — it's the margin. If the facilitator pays $0.002 to settle your half-cent payment, 40% of revenue is gone before you start. Per-call micro-pricing only survives with batching or a facilitator that sponsors gas. Price accordingly, and read the facilitator's terms before you set prices. (This is also why "millions of micro-settlements with zero friction" is a fantasy unless someone is batching underneath you.)

What it costs to run

• Server: a Google Cloud e2-micro free-tier VM (static IP, systemd service). CPU-only — no GPU involved.
• Per sale: the facilitator's gas, ~fractions of a cent.
• My time to build it: an afternoon, most of it reading the x402 repo's own examples.

What I actually think

The rail is real and the code is boring in the best way — that's the compliment. What's not real yet is the demand: nobody's agents are lining up to pay half a cent for answers. The people who will pay real money are businesses whose APIs are being scraped for free right now. That's who I'm building this for next.

If that's you — one endpoint behind 402, live in under a week, flat fee. Try the demo: https://demo.quinnsconveyance.com (/health is free, POST /mcp is the paid endpoint). Reach me at [e-mail] — more of my work at github.com/jkomalleyjr.