At Build 2026 in San Francisco, Microsoft put agents at the center of Windows, tools and software distribution. Satya Nadella described agents as first-class citizens across the runtime, tooling and distribution model, but the more consequential developer news was Microsoft giving GitHub Copilot a new default brain.
Project Polaris is Microsofts own coding model, built with a mixture-of-experts architecture and specialized modules for programming languages and frameworks. Starting in August, it is slated to replace GPT-4 Turbo as GitHub Copilots default reasoning engine. Users will be moved automatically, while those who want to stay with GPT-4 get a three-month manual opt-back period.
The shift also moves the workload onto Microsofts Maia AI accelerators. Microsoft says Polaris beats GPT-4 Turbo on HumanEval and MBPP, especially in smaller languages such as Rust and Haskell, and that the Pro tier can handle 100,000 lines of multi-file context while generating tests on its own.
The business meaning is clear: Microsoft wants the model, chip layer, developer tool and Windows agent surface inside its own stack. That weakens Copilots dependence on OpenAI at a moment when the two companies increasingly overlap commercially. Polaris still has to prove developers will accept the switch, but Microsoft has now put a dated product migration behind the strategy.
Sources: Microsoft Build 2026 Recap: Windows Is Now an Agent Platform, and Project Polaris Cuts the OpenAI Cord (ChatForest); Microsoft Opens Build 2026 With Agent Mode as the Default for Office 365 Copilot (Technobezz); CocoLoop; Microsoft Build 2026: Windows becomes the platform for AI agents (Windows News).