Modernizing complex legacy code with AI agents.

Mistral undertook a project to modernize a complex legacy codebase for a European energy operator, migrating 40,000 lines of Fortran 77 to modern C++. The system was a physics-intensive reservoir simulator with no test suite or documentation, making a simple syntax translation insufficient. The process involved building a parity harness to ensure numerical agreement, using AI agents to document the legacy code, and employing structured workflows that balanced agent autonomy with human review. This approach highlights the challenges of migrating procedural code to an object-oriented structure and the importance of verification and documentation before translation.