Architecture as a Product Decision & Interoperability Protocols
Why technical architecture is a product decision, separating Inference APIs from Interoperability Protocols, and the protocol landscape.
Architecture is a product choice
Day 1 established that an agent is more than a model. It is a model inside a harness that provides context, tools, state, and feedback. Day 2 asks what happens when that harness must reach outside itself.
The tempting answer is to connect everything. Give the agent every API, add several specialist agents, and let it generate any interface it needs. That produces an impressive demo, but it also creates a product with unclear ownership, invisible permission boundaries, and too many ways to fail.
Protocols are useful because they separate these concerns. They do not create user value on their own. They define how a capability is discovered, invoked, governed, and observed. For a Product Manager, the architecture decision starts with the relationship the product needs:
Use one running example throughout Day 2: a vendor-onboarding agent that helps a PM compare analytics providers and start an approved pilot. It must retrieve internal requirements and vendor facts, ask a compliance specialist to review data handling, present a comparison the PM can edit, and-only if the user chooses a vendor-prepare a bounded commercial transaction. The user sees one workflow, but each step transfers a different kind of responsibility.
The model interaction may run through OpenResponses or an Interactions API. MCP lets the agent use a bounded retrieval capability. A2A lets it delegate an open-ended compliance review to a participant that may ask questions. A2UI describes the comparison without sending executable UI code. UCP exchanges catalog and order data with a merchant, while AP2 proves the user's payment authority. The table is a compact map of those relationships.
| Product need | Interface | What it standardizes |
|---|---|---|
| Use an external capability | MCP | Tools, resources, prompts, and connection details |
| Delegate to a specialist | A2A | Agent identity, task state, and collaboration |
| Present an interactive result | A2UI | Declarative UI intent rendered by the host |
| Build a commercial transaction | UCP and AP2 | Merchant interaction, authorization, and payment intent |
The protocol map below is best read from the center outward. The agent remains responsible for understanding the user's goal. Each protocol handles a different boundary around that goal. MCP reaches tools, A2A reaches other agents, A2UI reaches the user through a trusted renderer, and commerce protocols govern transactions.
The Protocol Map Around One Agent
Read from the center outward. The agent owns the user's goal; each protocol governs one boundary around it.
Select a protocol to inspect the boundary it governs
MCP
• Use an external capabilityTools, resources, prompts, and connection details for a bounded call.
Who owns this server, which data leaves the boundary, and what does a denied scope look like?
This distinction prevents a common roadmap mistake: treating every integration as the same kind of work. A database lookup, a remote compliance review, a generated comparison table, and a payment are four different product commitments. Each needs different lifecycle, permission, fallback, and evaluation decisions.