For a Microsoft-stack enterprise in 2026, Microsoft 365 Copilot Agent Mode is the lower-friction agent-platform choice if the workflow already lives in Microsoft Graph; it is structurally weaker on multi-vendor deployment, model-portability, and platform-independence than dedicated agent platforms (Anthropic Managed Agents, OpenAI Agents SDK, Vertex AI Agent Builder). The procurement decision turns on three questions: (1) is the workflow Microsoft-resident, (2) is multi-vendor model selection a hard requirement, (3) is the agent's primary surface productivity-suite or workflow-orchestration.

Microsoft-stack reader piece. No prior dedicated coverage of M365 Copilot Agent Mode in the corpus despite ~60% of target audience running Microsoft-heavy stacks. Cadence 60-day. Trigger conditions: Microsoft expanding Foundry Agent Service capabilities materially; pricing changes to Copilot for M365 (currently $30/user/month); BAA/data-residency posture shifts; new MCP-server integration that closes the multi-vendor gap. Sister claims: AM-141 (Agent Mode pillar), AM-003 (GPT-5 Pro), AM-125 (ServiceNow Now Assist). Inline sources: Microsoft 365 Copilot pricing page, MS Learn agents documentation, EU Data Boundary.