2026-W40 market review — 2 draft protocols fleshed out with sources

Weekly automated review. Changes this run:

• ACP (draft updated): Fleshed out with verified sources (OpenAI blog, Stripe newsroom, Stripe blog, GitHub repo), howItWorks describing the checkout session lifecycle and Shared Payment Tokens, tradeoffs, and security notes. Still draft — human review needed before promoting.
• UCP (draft updated): Fleshed out with verified sources (Google blog, GitHub spec repo, ucp.dev), howItWorks covering the full shopping journey (catalog, cart, identity linking, checkout, order management), tradeoffs, and security notes. Updated specUrl to ucp.dev. Still draft — human review needed before promoting.

Also noted this week:

• A2A x402 extension v0.2 published September 22 by Google (google-agentic-commerce/a2a-x402), formalizing pay-per-call monetization for A2A agents via x402 payment flows. Not a new protocol but a significant interop milestone between two tracked protocols.
• Google auto-enrolled eligible Shopify merchants in UCP native checkout for AI Mode and Gemini (September 22), inverting the opt-in model for hundreds of thousands of merchants.
• Meta Muse shopping integrations: Shopify enabled Shop Pay agentic checkout across all Shopify stores for Meta's Muse AI agent (September 21–22); PayPal followed with Muse checkout support across its global merchant network (September 24). These integrations use platform-specific checkout (Shop Pay, PayPal, Stripe Link) rather than a tracked protocol.