Cheap code, precious trust: Code modernization at scale

From the source

What happens when AI agents can rewrite millions of lines of code orders of magnitude faster than humans can review them? At Microsoft, we’re working on exactly that problem. We’re building and running AI agents that transform some of the world’s largest codebases, including Windows and Azure. Our work ranges from migrating a 400-million-line codebase to a new compiler toolchain to modernizing legacy C and C++ code into Rust—increasingly backed by formal verification. Generating code is becoming the easy part. Knowing whether to trust it is the real challenge. When agents make changes across hundreds of millions of lines of code, how do we know they haven’t broken something subtle? How do we preserve behavior, security, and architectural intent? And how can a small team of engineers confidently stand behind changes that AI agents make at that scale? > Since 2024, we’ve been combining AI agents with static analysis, testing, and formal verification to make large-scale software transformation practical and trustworthy. These questions are at the heart of our work. …

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