x402 Protocol — How It Works

Overview

x402 is a pay-per-call micropayment protocol for machine-to-machine API access. An AI agent calls an HTTP endpoint, receives a 402 Payment Required response with pricing, signs a USDC payment on-chain, and resends the request with payment proof. The server verifies payment and returns data.

No API keys. No OAuth. No rate limits. No subscriptions. Just pay-per-call.

Payment Flow

Agent                          x402 Service                    Base L2
  │                                │                              │
  ├── POST /service ──────────────>│                              │
  │                                │                              │
  │<── 402 Payment Required ───────┤                              │
  │    {amount, token, recipient,  │                              │
  │     network, facilitator}      │                              │
  │                                │                              │
  ├── Sign USDC payment ──────────────────────────────────────────>│
  │                                │                              │
  ├── POST /service ──────────────>│                              │
  │    X-PAYMENT: {signature}      │                              │
  │                                ├── Verify payment ────────────>│
  │                                │<── Confirmed ────────────────┤
  │<── 200 OK + data ─────────────┤                              │
  │                                │                              │

The 402 Response

When you call an x402 endpoint without payment, you get:

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

{
  "x402Version": 1,
  "accepts": [{
    "scheme": "exact",
    "network": "base",
    "maxAmountRequired": "1000",
    "resource": "https://x402.bankr.bot/0x.../vision",
    "description": "AI image analysis",
    "mimeType": "application/json",
    "payTo": "0x8f9ec800972258e48d7ebc2640ea0b5e245c2cf5",
    "maxTimeoutSeconds": 300,
    "asset": "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913",
    "extra": {
      "name": "vision",
      "description": "Image analysis via Groq Llama 4 Scout"
    }
  }]
}

Key fields:

Payment Token

All x402 services on Base use USDC:

$0.001 = 1000 units. $1.00 = 1000000 units.

Facilitators

A facilitator is a trusted intermediary that processes x402 payments. The two main facilitators:

  1. Coinbase CDP (cdp.coinbase.com) — Coinbase’s facilitator, auto-lists services in x402 Bazaar
  2. Bankr (api.bankr.bot) — Bankr’s facilitator, used by services deployed via Bankr CLI

The facilitator handles payment verification so the service doesn’t need to run its own verification infrastructure.

Ecosystem Stats (as of April 2026)

Metric Value
Total services indexed 2,060+
Actually working ~14% (288)
Total volume $19.6M USDC
Transactions 5.1M+
Active providers ~6 established
Network Base mainnet
Payment token USDC

Source: ScoutScore

The 86% Problem

86% of indexed x402 services are spam, broken, or abandoned. 64% accept payment and return errors. Before calling an unknown x402 service:

  1. Send the minimum payment first
  2. Verify the response is valid data, not an error
  3. Check the x402 Service Encyclopedia wiki for verified reviews

Resources