Implementing HTTP 402 Micropayments for Agent-to-Agent APIs

Set up an HTTP 402 Payment Required paywall that accepts instant USDC payments from autonomous AI bots without requiring credit card subscriptions or manual API key generation.

Prerequisites

Basic Express.js or Next.js API knowledge
Base or Solana RPC endpoint
USDC receiving address

Step 1: The x402 Architecture & Protocol Flow

Understand the x402 handshake: When a consumer agent requests a premium resource without a valid settlement proof, the server replies with 402 Payment Required and structured payment terms in the WWW-Authenticate header.

Step 2: Install Middleware & Verification Libraries

Install the HTTP server, the chain SDKs used to verify settlement proofs, and dotenv for credentials.

Step 3: Create the Express x402 Server Middleware

This middleware checks incoming requests for the Authorization: L402/x402 signature header. If missing, it returns a 402 response with price parameters and a signed challenge.

Step 4: Configure Client Agent to Handle 402 Paywalls Automatically

Equip your client agent with an automatic retry interceptor that intercepts 402 responses, signs the micropayment transaction, and re-submits the request with the proof header.

Key Takeaway

Your APIs can now be monetized directly by AI agents worldwide with instantaneous micro-settlement and zero chargeback friction.