The Agent Payment Rails Playbook

Notice: This is an educational guide with illustrative code examples. It does not execute code or install dependencies. All examples use the GreenHelix sandbox (https://sandbox.greenhelix.net) which provides 500 free credits — no API key required to get started.

Referenced credentials (you supply these in your own environment):

• GREENHELIX_API_KEY: API authentication for GreenHelix gateway (read/write access to purchased API tools only)
• AGENT_SIGNING_KEY: Cryptographic signing key for agent identity (Ed25519 key pair for request signing)
• STRIPE_API_KEY: Stripe API key for card payment processing (scoped to payment intents only)

Six protocols now compete to define how AI agents pay for things. On April 2, 2026, x402 joined the Linux Foundation with backing from Google, Stripe, AWS, and Visa. Two weeks earlier, Stripe launched its Model Provider Payments (MPP) suite. OpenAI's Agentic Commerce Protocol (ACP) powers checkout inside ChatGPT for Etsy and Shopify merchants. Google and Shopify's Universal Commerce Protocol (UCP) is in production. Visa's Trusted Agent Protocol (AP2/TAP) introduces Know Your Agent compliance for the first time. And ERC-8183 handles on-chain escrow for agent jobs on Ethereum mainnet. Each protocol solves a different slice of the problem. None solves the whole thing. The agent builder who ships a payment integration today faces a brutal question: which protocols do I wire together, and how? This playbook answers that question with production code. Every chapter contains working Python examples against the GreenHelix A2A Commerce Gateway -- 128 tools accessible at https://api.greenhelix.net/v1 via a single the REST API (POST /v1/{tool}) endpoint. By the end, you will have a multi-rail payment system that routes micropayments over stablecoin rails and high-value transactions over card rails, with spending controls, KYA compliance, dispute resolution, and production monitoring. All of it tested against the live gateway.