Modernizing complex legacy code with AI agents.

Mistral helped a European energy operator migrate 40,000 lines of Fortran 77 to C++ for a physics-intensive reservoir simulator that had no test suite or centralized documentation. The team built a numerical parity harness before migrating, used agents to document the codebase via a caller-callee tree, and settled on a human-supervised workflow of coder, tester and reviewer agents migrating module by module. The first sprint covered 40,000 of 300,000 lines, and the authors note that migrations depending on external systems or undocumented physics would face additional challenges.