Stablecoin Payments for AI Agents: The x402 Guide

When an AI agent encounters an HTTP 402 Payment Required response, the x402 protocol kicks in. The agent reads the payment requirements from the response header, constructs a stablecoin payment, and settles the transaction — all without human intervention, credit card networks, or traditional banking rails.

x402 is a payment protocol designed for the agentic web. It leverages the long-dormant HTTP 402 status code to create a native payment layer for AI agents. When a merchant returns HTTP 402, the response includes a PAYMENT-REQUIRED header with structured payment details: wallet address, amount in USDC, network, and expiration. The agent parses this, signs a stablecoin transaction, and returns payment proof in a PAYMENT-SIGNATURE header.

The x402 implementation includes multiple security layers. SSRF protection blocks RFC-1918 private addresses, loopback, and cloud IMDS endpoints from being used as facilitator URLs. All facilitator communication requires HTTPS. A float precision guard enforces MAX_SAFE_CENTS to prevent rounding errors in stablecoin amounts, since USDC uses 6 decimal places and conversion to cents must be exact.

Settlement follows a fire-and-forget pattern via HTTP 202. The merchant initiates verification and returns immediately. Behind the scenes, settlement retries with exponential backoff — 5 attempts at 5s, 10s, 20s, 40s intervals. Pending payments that exceed the expiration window are automatically marked as expired, protecting both merchant and agent from stuck transactions.

To accept x402 payments, a merchant configures a wallet address and supported network in their protocol settings. The platform generates HTTP 402 responses with the correct PAYMENT-REQUIRED header format. On the agent side, the x402 adapter detects the 402 response, normalizes the payment intent from the header (converting USDC smallest units to cents), and handles the signature flow. The entire cycle — detection, normalization, payment, and settlement — is managed by the protocol bridge.