x402 lets an API charge per request over HTTP with a stablecoin payment; MCP (Model Context Protocol) lets AI agents discover and call tools. FastAF supports both.

*   **x402**: an open payment protocol that uses HTTP status 402 so a client can pay for a request and retry it with proof of payment. FastAF supports x402 v2.
*   **HTTP 402 Payment Required**: the status code an x402 server returns with payment requirements (price, asset, network, payTo).
*   **Facilitator**: a service that verifies and settles x402 payments on-chain for the server.
*   **USDC on Base**: the stablecoin and network FastAF accepts for x402 payments.
*   **Bazaar**: x402 discovery metadata that lets agents find paid endpoints and their prices; FastAF publishes it.
*   **Pay-per-call**: paying for each request individually, without an account or subscription.
*   **MCP (Model Context Protocol)**: an open protocol for connecting AI clients to tools and data.
*   **MCP server**: a service exposing tools over MCP. FastAF’s is at [https://api.fastaf.co/mcp](https://api.fastaf.co/mcp) with 23 tools.
*   **Streamable HTTP**: the remote MCP transport FastAF uses.
*   **MCP tool**: a named function with an input schema, e.g. `amazon_product`, `scrape_url`.
*   **MCP registry**: a directory of MCP servers; FastAF is listed in the official MCP Registry.
*   **OpenAPI**: a machine-readable API description; FastAF’s is [https://api.fastaf.co/openapi.json](https://api.fastaf.co/openapi.json).
*   **SERP**: search engine results page.

[

## x402

x402 is a protocol for paying for HTTP resources using a payment challenge and a signed retry.

](/glossary/x402/)[

## MCP

Model Context Protocol (MCP) standardizes how AI applications discover and invoke tools and access context.

](/glossary/mcp/)[

## HTTP 402

HTTP 402 Payment Required is an HTTP status code indicating that payment is required for a resource.

](/glossary/http-402/)[

## Pay-per-call API

A pay-per-call API charges for individual API requests rather than requiring a recurring subscription.

](/glossary/pay-per-call-api/)

## How do you make a real request?

This example calls [Google SERP API](/apis/google-serp-api/) using its documented input. Save the API key from the signup email, confirm your email and set FASTAF\_API\_KEY in your server environment.

```
curl --fail-with-body -H "Authorization: Bearer $FASTAF_API_KEY" 'https://api.fastaf.co/v1/serp?query=best+espresso+machine+for+home&country=us&language=en'
```

```
# pip install fastaf-sdk
import os
from fastaf import FastAF

with FastAF(os.environ["FASTAF_API_KEY"]) as client:
    data = client.request("serp", {"query":"best espresso machine for home","country":"us","language":"en"})
    print(data)
```

Google SERP JSON. organic\_results contains ranked title, link and snippet fields; search\_information describes the query. Other Google result blocks may also appear.

The example costs 5 credits after the trial. Alternatively, this endpoint costs $0.003 per successful x402 call at the default configuration; FastAF x402 calls start at $0.003. These are separate billing modes.

For x402, request the same parameters under `/x402/v1/serp` without an API key, validate the live PAYMENT-REQUIRED challenge, then sign and retry with a payment-aware client. A bare curl request returns HTTP 402 and does not pay; unsupported higher-cost options require API-key billing.

## Frequently asked questions

### Do I need a crypto wallet to use FastAF?

Only for x402. Otherwise create an account and get 500 free credits every month after email verification.

### Which x402 versions are supported?

v2: sign the PAYMENT-REQUIRED header and retry with PAYMENT-SIGNATURE.

### Which MCP clients work?

Clients that support remote Streamable HTTP MCP servers can connect to FastAF. Tool calls require an API key or OAuth token.

### Where are prices listed?

In the OpenAPI file and the live 402 response of each endpoint.

### How much does the example cost?

The API-key example uses 5 credits per successful call after the trial. This endpoint has a default x402 price of $0.003 per successful call in USDC on Base; the live payment challenge is authoritative.

### Does the SDK automatically pay an HTTP 402?

No. The API-key SDK does not sign wallet payments; x402 requires an explicitly configured payment-aware client and a funded wallet.

## Try it on your own request

500 free credits every month after email verification, no card.

[Start free](/signup/)

## Explore this catalog

*   [x402](/glossary/x402/)
*   [MCP](/glossary/mcp/)
*   [HTTP 402](/glossary/http-402/)
*   [Pay-per-call API](/glossary/pay-per-call-api/)