AI agents can now rewrite code far faster than engineers can review it, shifting the central challenge of large-scale modernization from generating code to establishing trust in its correctness. Microsoft describes using agents, static analysis, testing, and formal verification on projects including adapting a codebase of hundreds of millions of lines for Clang/LLVM, fixing analyzer findings, and moving services from x86 to ARM. Its approach concentrates responsibility in a small expert team: agents learn proposed fixes from sampled examples, automated reviewers assess them, and experts approve reusable guidance rather than inspect every edit. Feedback and grouped reviews help direct human attention to ambiguous or novel cases.
For C and C++ modernization, the aim is to translate legacy software into Rust without changing its behavior. The process has progressed from producing code that would not compile to automating much larger migrations; the remaining challenge is proving the translations are sound. Teams organize work around user-visible features, validate them with scenario-based and differential tests, and increasingly use specifications and tools such as Verus to prove properties. More broadly, agents can infer specifications that guide testing and future changes, while engineers approve the properties those specifications should capture. This raises unresolved questions: whether AI-derived specifications reflect intended behavior, how to verify security and reliability as well as functionality, whether engineers will still need to read code, and how responsibility for code changes when agents can modify entire systems.
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