Many legacy modernization projects start with a simple question: what does this thing actually do?

In this project, we were modernizing a decades-old Delphi application with limited documentation, no meaningful test coverage, engineers long since moved on, and significant unknowns about the environment in which it operated.

Modernizing legacy systems is challenging, particularly when documentation is limited and system knowledge has been lost over time. When LLMs and AI are applied thoughtfully, they can help teams understand legacy systems faster and reduce modernization risk.

This article focuses on how we used LLMs to understand, document, and de-risk an unfamiliar legacy system before modernization began. Once the application was understood and the architecture was defined, the team leveraged AI-assisted development workflows to accelerate the Delphi-to-.NET rewrite itself. Evan Sanning shares that side of the project in his companion article, How We Used LLMs to Rewrite a Legacy Delphi Application in C#.