Agent-Ready Commerce, Part 5: Keeping ACP, MCP, and AP2 Adapters Thin

Protocol adapters are one of the easiest places for agent-commerce architecture to drift. An adapter begins with the narrow responsibility of translating an external protocol request into something the commerce platform understands. For example, an MCP-style tool may ask for return terms, an ACP-style interaction may ask whether checkout can be prepared, an AP2-related flow may carry payment authority information, and an internal feed may publish product capabilities. Those are adapter concerns at the boundary. The problem starts when the adapter does more than translate. It checks product availability from catalog fields. It interprets policy text. It decides whether checkout is ready. It treats a payment artifact as authority. It turns a domain blocker into a softer protocol response. Each shortcut may solve an integration problem locally, but it also creates a second place where commercial meaning is decided. When several adapters exist, those local decisions begin to diverge. The MCP tool may block return-policy quotation, the ACP adapter may expose the product as purchasable, the feed may publish it as checkout-ready, and the AP2-related flow may reject delegated payment. At that point, the platform does not only have multiple integrations. It has multiple interpretations of the same commercial state. This is the adapter problem in agent-ready commerce: semantic drift at the protocol boundary. The adapter should know how to speak the protocol. It should not decide product truth, policy meaning, eligibility, checkout validity, or payment authority. Those decisions belong inside the commerce platform, where they can be shared, tested, evidenced, and audited. This is the fifth article in the Agent-Ready Commerce series. Part 1 introduced the broader architecture model: Facts → Eligibility → Authority → State transition → Evidence → Audit Part 2 focused on commercial truth. It argued that catalog data is not enough. A platform needs source-backed, freshness-aware p