6.2. Prebuilt or Custom: Choosing the Right Agent
💡 First Principle: Prebuilt agents are fixed: you get Microsoft's behavior exactly as designed, and you can't change it. That makes the decision simple. If Microsoft's behavior already fits the job, use it. If the job depends on your content, rules or processes, it needs a custom agent, and the size of the audience and the complexity of the workflow decide which tool should build it.
Prebuilt agents can't be customized. Researcher, for example, is a first-party agent with fixed logic, and it can't be extended in Copilot Studio. So "customize Researcher to use our internal pricing rules" is never the answer. When you need your own knowledge or logic, the choice moves to a custom agent.
Two tools build custom agents, and the exam tests which fits:
| Agent Builder | Copilot Studio | |
|---|---|---|
| Where it lives | Inside the Microsoft Copilot (formerly Microsoft 365 Copilot) app | A separate web portal |
| Built by | Information workers, no code | Makers and developers |
| Audience | You or a small team | A department, the whole organization, or external customers |
| Typical agent | Lightweight Q&A over organizational knowledge: project FAQ, product documentation assistant, onboarding helper | Multi-step workflows with approvals and branching, custom connectors, autonomous actions: support agent that creates tickets and escalates, IT help desk triage, CRM sales assistant |
| Governed in | Microsoft 365 admin center | Power Platform admin center (with dev/test/prod lifecycle management) |
Microsoft's decision factors are audience, deployment scope, functionality and governance needs. An Agent Builder agent can later be copied to Copilot Studio with its core configuration preserved, so starting small doesn't lock you in.
Some "prebuilt" agents are really templates. The Employee Self-Service agent is built on Copilot Studio and designed for organizations to customize, with HR and IT starters that admins deploy. It's a Microsoft-provided starting point, not a fixed prebuilt agent like Researcher.
Licensing and governance at a decision level:
- Both Agent Builder and Copilot Studio are included with a Microsoft Copilot license. Without one, usage is paid through Copilot Credits or pay-as-you-go. Agent Builder is free for agents grounded only on web knowledge.
- Copilot Chat users without a license can use agents grounded in instructions and public websites at no extra cost. Agents that use shared tenant data, such as SharePoint or connectors, are metered and off by default for them.
- No new privileges: custom agents respect existing Microsoft 365 permissions, and audit, DLP and retention apply. An agent can't show anyone content they couldn't already open.
⚠️ Exam Trap: Both directions of the tool mismatch are wrong answers. "Use Copilot Studio for a personal FAQ agent over my project folder" over-engineers a job Agent Builder handles in the Copilot app. "Use Agent Builder for an external customer support agent that creates tickets and escalates to a human" under-powers a job that needs Copilot Studio's workflows, connectors and external publishing.
Reflection Question: Three requests arrive: (1) a deep, cited market report; (2) a Q&A agent over a 12-person team's project site; (3) a customer-facing agent that opens service tickets and hands off to a human. Which option fits each, and what single question about the first request rules out building anything?
