Card 19 of 44· Domain 2 · Design
Extensibility — custom models, MCP and Computer Use
The three extension routes and the order to prefer them, when to extend Microsoft 365 Copilot rather than build separately, and why Computer Use is the over-engineering distractor.
Three ways to extend beyond what the platform ships with, and a clear order of
preference between them. The card is really about not over-engineering.
The three routes
Custom models in Foundry. For when the task genuinely needs a specific or
tuned model. It brings an evaluation pipeline and its own boundary. Remember the
ladder from card 7 — this is step three or four, never step one.
MCP in Copilot Studio. Expose a capability once and reuse it across agents.
This is the standardised-discovery answer. It is also a live egress path, so it
must be governed like any other connector rather than treated as neutral plumbing.
Computer Use. Automating tasks in apps and websites that have no API. The last
resort when integration is genuinely impossible, and brittle by nature because a
change to someone's interface breaks it.
Choosing between them
There is an API → use a connector, a flow, or MCP. Never reach for Computer
Use when an API exists.
Many agents need the same capability → MCP. Build once and reuse, which beats
duplicating a connector for every agent.
The system is a legacy interface → Computer Use with narrow scope. Plan for
maintenance and treat it as tactical rather than permanent.
Extending Microsoft 365 Copilot
Extend when users live in Office and the need is knowledge or light action
inside their existing flow of work.
Build separately when it must face customers, or it needs its own lifecycle,
channel or model.
And remember, because it recurs throughout the exam: Agent Builder advises, it
does not act. Sharing of Agent Builder agents is an allow-list in the Microsoft
365 admin centre.
Exam trap. Computer Use is the tell-tale over-engineering distractor. It is
correct only when there is genuinely no API or connector. If the scenario
mentions any supported integration path, an option using Computer Use is there
to be eliminated.
The one line to carry: API first, MCP when many agents need the same thing,
Computer Use only when nothing else exists — and it will break when someone
redesigns a screen.
Currency (19 Aug 2026). Computer Use went generally available in May 2026, and
standalone computer-use tools are in preview for modular, reusable UI automation with
its own governance and observability. The Windows 365 for Agents MCP server went GA
in June, giving an agent full operational control of a cloud PC. See card 44.