Documentation

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Developer documentation for API20 — an experimental discovery and testing layer for x402-powered APIs on Base.

● Early experimental version

Introduction

API20 is an experimental discovery and testing layer built around HTTP 402 Payment Required flows on Base. It helps developers scan, inspect, and document x402-powered APIs — including metadata that may later support MCP-ready agent workflows.

The CLI package is planned, not live. The current version focuses on website documentation, registry preview, terminal simulation, and schema exploration.

What problem API20 is trying to solve

As APIs adopt x402 payment models, developers and AI agents need a reliable way to:

  • Discover which endpoints require payment before access
  • Understand payment requirements (network, asset, amount, scheme)
  • Preview the full request → 402 → pay → retry flow without guesswork
  • Find x402-compatible APIs in a structured registry

API20 consolidates these workflows into a single terminal-inspired interface designed for developer productivity and agent automation.

Example commands

These commands represent the planned CLI behavior. See the full command reference for details.

api20 scan https://server.api20.xyz/data
# Probe endpoint and detect 402 Payment Required

api20 inspect https://server.api20.xyz/data
# Parse payment requirement headers and payload

api20 registry
# Browse indexed x402-compatible APIs

api20 mcp "Weather API"
# Preview MCP-ready metadata (research/planned)

api20 builder-code "Weather API"
# Preview Builder Code attribution metadata (research/planned)

api20 demo
# Run a simulated x402 payment flow (no real payment)

Example 402 response

When an x402-compatible API requires payment, the server responds with HTTP 402 and payment metadata:

HTTP/1.1 402 Payment Required
Content-Type: application/json

{
  "x402Version": 1,
  "accepts": [{
    "scheme": "exact",
    "network": "base",
    "maxAmountRequired": "1000",
    "resource": "https://server.api20.xyz/data",
    "payTo": "0x...",
    "asset": "USDC"
  }]
}

Amount values are typically expressed in the smallest unit (e.g. 1000 = 0.001 USDC with 6 decimals). Live demo endpoints are available at server.api20.xyz.

MCP-ready agent workflows

MCP (Model Context Protocol) is a standard for connecting AI agents to tools and external services. API20 is designed to help developers discover and document APIs that may later be exposed to AI agents through MCP servers.

MCP support is research/planned. API20 does not currently operate an official MCP server. See MCP-ready Workflows for details.

Builder Codes / ERC-8021 attribution research

Builder Codes introduce an attribution layer for x402 payments on Base using ERC-8021 Schema 2. API20 tracks Builder Codes as future registry metadata — not as live settlement integration. See Builder Codes Research.

MPP / Tempo research

API20 currently focuses on x402-style API discovery and HTTP 402 inspection on Base. MPP / Tempo compatibility is listed as future research, not a live feature. See MPP / Tempo Research.

Planned CLI behavior

  • scan — HTTP probe with 402 detection and summary output
  • inspect — Full parse of x402 payment requirement fields
  • registry — Query the public x402 API registry
  • mcp — Preview MCP-ready metadata for an API (research/planned)
  • builder-code — Preview Builder Code attribution metadata (research/planned)
  • demo — Step-through simulation of the payment retry flow

Future versions may add payment execution research, agent mode, and registry submission backend. No timeline is guaranteed — this is an early-stage experimental project.

Hosting status

This site is the early experimental version of API20. It provides:

  • Project overview and feature documentation
  • Interactive terminal UI preview
  • Registry preview with MCP-ready metadata fields
  • Registry schema preview and GitHub documentation
  • MCP-ready workflow and MPP/Tempo research pages
  • Public roadmap with 8 development phases

The installable CLI package has not been published yet. Follow the roadmap for updates.

Disclaimer: API20 is an independent experimental project. It is not affiliated with Coinbase, Base, Coinbase Developer Platform, Tempo, Stripe, or any official x402, MCP, or MPP organization. References are for ecosystem compatibility, research, and educational purposes only.