PayAPI Market

x402 marketplace for AI agents: how PayAPI Market verifies a pay-per-call API

By · Published 2026-09-22 · Last updated September 22, 2026 · 3 min read

An x402 marketplace is a catalogue of APIs that answer HTTP 402 and settle in USDC. PayAPI Market is that shop for AI agents. Discovery is free at https://payapi.market/mcp. Payment happens on the listing, not on PayAPI.

What is an x402 marketplace?

x402 is the payment protocol. It revives HTTP 402 Payment Required. The server states a price in USDC on Base. The agent signs, the call retries, the provider is paid.

PayAPI Market is the marketplace. We list the route, check that a real payment returns real product, and let agents search the catalogue without an account or an API key.

Providers keep 100% of each settled call. Listing is free.

For the size of the catalogue right now, read the live count on the homepage and in llms.txt. Both are generated from the same query at build time, so neither can drift from the other.

How do AI agents pay for APIs without keys?

  1. Search or list the catalogue. That is free.
  2. Open one listing. Read the verified route and the price.
  3. Call the provider URL. Read the 402.
  4. Pay that challenge from the agent wallet.
  5. Retry. The body is the product.

Paste for Claude Desktop or Cursor:

{
  "mcpServers": {
    "payapi": {
      "url": "https://payapi.market/mcp"
    }
  }
}

The full worked example, through to the on-chain receipt, is at how an agent finds PayAPI. The agent-facing overview is at /agents.

What does settlement-verified mean?

It means we paid one route from our own wallet and got product back. The receipt is on Base. It is not a health ping and it is not a self-asserted badge. We wrote up the reasoning in the badge is a receipt.

A listing that only answers 402 when unpaid is not verified. A listing that takes a signed exact payment and returns HTML, an empty object, or the same 402 is not verified.

Four checks before spend, then the body:

  1. Durable host.
  2. Unpaid Base exact USD Coin 402 with matching payTo.
  3. Settlement to that payTo.
  4. Advertised product on HTTP 200.

How do I list a pay-per-call API?

Start at /list.

Paste the paid route, not the homepage. The form calls it unpaid before a row is created. Then one cheap canary, usually $0.001 to $0.05. One product, one card.

Which verified APIs should an agent call first?

Every API below is live and settlement-verified. Browse the rest at /marketplace.

How does PayAPI Market compare with RapidAPI for AI agents?

RapidAPI: keys, subscriptions, a cut. PayAPI: per-request USDC, no key, providers keep 100%.

The side-by-side is in RapidAPI alternative for AI agents.

FAQ: what else do agents and providers ask?

Is discovery paid? No.

Which chain? USDC on Base.

Do you take a fee? Not in this window.

How does an agent find PayAPI? url https://payapi.market/mcp

Is there a worked example? Yes: how an agent finds PayAPI.