How to Build a Practical Custom AI Agent Using M365 Agent Builder

A developer on DEV Community has outlined a straightforward approach to building a custom AI agent using Microsoft 365 Agent Builder, which requires an M365 Copilot license but incurs no per-use costs. The author emphasizes starting with a genuine use case rather than building an agent in search of a problem, warning that the latter approach consistently fails. For their own use case, they built an agent that generates single-page HTML technical reports, replacing earlier workflows involving PowerPoint and the Research agent. The agent's design covers key elements such as an instruction file, branching logic, and a context-based skill borrowed from Anthropic to enhance UI quality. The author also flags prompt injection as a security risk when reusing third-party skills, advising developers to review skill files in raw form rather than through a markdown viewer.