x4PayAPI Market

Pay-per-call MCP tools for AI agents

A pay-per-call MCP tool lets an agent call a tool and pay for that call, with no API key. The unpaid response states the price before any payment is made. On this AI agent API marketplace the catalogue MCP is free. Paying happens on the provider endpoint, in USDC on Base, Solana or Algorand where the listing offers it.

Last updated: 30 October 2026

What is a pay-per-call MCP tool?

A pay-per-call MCP tool lets an agent call a tool and pay for that call, with no API key. On this AI agent API marketplace the catalogue at https://payapi.market/mcp is free and never returns a 402. Paying happens on the provider endpoint. The unpaid challenge states the price first.

The paid call is the provider's own endpoint. The agent requests it unpaid, reads the HTTP 402, and pays that challenge. Some listings also publish an mcp_endpoint. That value is supplied by the provider. We do not probe it, so a non-empty value is a claim, not a guarantee. The settlement-verified badge covers the route we paid, which is often the HTTP route rather than the MCP URL.

There is a separate first-party server for our own UK property tools. Its tools/call is paid, and it does not expose the marketplace catalogue. Do not point a client at it if you wanted the directory.

Can an agent see the price before paying?

Yes. For an HTTP route, the unpaid 402 states the price, the network and the payTo address before any payment is made. get_api returns price_min and price_max from that live 402. The x402 MCP spec uses the same idea: the unpaid tool result carries the payment requirement before payment is attached.

The x402 MCP transport, written up in the Coinbase spec, uses the same idea inside a tool call. An unpaid call comes back as an error result that carries the payment requirement. The client retries with a payment payload. The server settles and returns the tool result. The requirement is visible before the payment is attached. Paying without an API key is the HTTP version of that sequence, on Base, Solana or Algorand.

How does an agent connect to the catalogue?

Point Claude Desktop, Cursor or any streamable HTTP client at https://payapi.market/mcp. Discovery spends nothing. To pay a provider, the agent needs USDC on the network the 402 names and a client that can sign. Fund a small balance and set a per-call cap. The agent page has the config.

When get_api includes an MCP endpoint, read it as the provider's advertised server. Confirm the price on the unpaid response from that server before you let the agent pay it. Prefer listings whose paid route is settlement-verified if you want a receipt you can open.

The same catalogue query that writes llms.txt currently counts 1,134 settlement-verified APIs, 2,331 listed.

Which other public pay-per-call MCP servers exist?

These are other public MCP servers an agent can pay through x402. Each row links to a page we opened on the date in the last column. They are alternatives for the tool call itself, not mirrors of this catalogue. The transport rules are in the x402 MCP spec.

Public pay-per-call MCP servers
ServerWhat the public page saysSourceChecked on
PayAPI Market catalogueFree discovery MCP for this x402 marketplace. Paying happens on the provider endpoint. The unpaid 402 states the price, network and payTo address.PayAPI MCP30 October 2026
CDP Bazaar MCPCoinbase documents an MCP server for Bazaar search and a proxy that can call a discovered paid endpoint, with payment handling in the x402 MCP client.CDP buyer discovery docs30 October 2026
x402 MCP transportThe spec says an unpaid tool call returns the payment requirement, and the client retries with the payment attached. The price is in that requirement before payment is sent.x402 MCP transport spec30 October 2026
Agent402A hosted MCP server and an npm server for pay-per-call tools. The docs say an unpaid call receives HTTP 402 with the price, then a client can sign a USDC payment and retry. Some tools also offer a proof-of-work path.Agent402 docs30 October 2026
ToolboothOne MCP server of pay-per-call tools. The homepage says an unpaid call is answered with an x402 payment request in USDC on Solana, and that the price is in that request.Toolbooth30 October 2026

Which PayAPI Market listings publish an MCP endpoint?

mcp_endpoint is supplied by the provider. We do not probe it, so a non-empty value is a claim, not a guarantee. The settlement-verified badge covers the route we paid, which is often the HTTP route rather than the MCP URL. Names below come from the same catalogue as llms.txt.

234 live listings publish an MCP endpoint. 231 of those are settlement-verified. The endpoint URL is the provider's own claim: we do not probe it, and the badge covers the route we paid.

Frequently asked questions

Are PayAPI Market MCP tools free?

The catalogue server at https://payapi.market/mcp is free. search_apis, list_apis and get_api never return a 402. Paying happens when the agent calls a provider endpoint and settles that provider's own HTTP 402.

Can an agent see the price before paying for a tool?

Yes. The unpaid 402 response states the price, the network and the payTo address before any payment is made. On the MCP transport, the unpaid tool result carries the same kind of payment requirement before the client attaches a payment.

Does an MCP endpoint on a listing mean we tested that server?

No. mcp_endpoint is provider-supplied and we do not probe it. Settlement-verified means we paid one named route, usually the HTTP route on the listing, and linked the receipt. Read the listing page for which route that was.

Which networks can a paid MCP-backed API use?

Listings accept USDC on Base. Some also accept USDC on Solana or Algorand. The unpaid challenge names the network. Match it to the wallet the agent holds.

How does a provider list a pay-per-call API?

Submit the paid route at https://payapi.market/list. Listing is free and providers keep 100% of each paid call. An MCP URL is optional. The badge still depends on a real paid call to the route we test.

Sources

Related: marketplace · for agents · FAQ · how we verify

How an AI agent pays for MCP tools per call and reads the price first. Discovery on this x402 marketplace is free. Providers keep 100%.