Paste a CheckoutRequest or CheckoutResponse payload and validate it against the Agentic Commerce Protocol (ACP) — the OpenAI/Stripe open standard for AI-agent-initiated checkout flows. Checks required fields, Shared Payment Token (SPT) structure, amount precision, currency codes, agent identifier fields, and signature format. Live standard: adopted by Etsy, Shopify (~1M Shopify-network merchants). Chains into ART-01. Client-side. Zero PII.