Modernizing complex legacy code with AI agents.

Legacy scientific codebases accumulate over decades, and when original authors leave, the knowledge embedded in the code becomes hard to recover. Moreover, using languages with no active developer ecosystem means missing out on the opportunity to build on top of others’ work. Mistral helped a European energy operator migrate 40,000 lines of Fortran 77 to C++, a physics-intensive reservoir simulator with no test suite and no centralized documentation.

Moving beyond code translation in legacy code modernization.

Translating syntax from one language to another is a largely solved task. Asking any recent model to translate a snippet from a non-completely-obscure language to another, will likely converge to an acceptable outcome in few iterations. However, migrating a full system from a procedural language to object-oriented C++ creates the need for architectural refactors that make the task non-trivial.

Fortran 77 was standardized in 1977, as the name may suggest, and code written in it reflects those constraints directly: no modules, no namespaces, no structured types. State lives in COMMON blocks—global memory shared across the entire program. Variables are implicitly typed by their first letter, so a misspelled name silently creates a new variable instead of raising a compiler error.

For a simple but illustrative example, take the following implementation of a first-degree Taylor expansion. Inputs and outputs are globals in a COMMON block, and IC is an integer only because its name starts with a letter between I and N. Variable names are quite cryptic, as their length is capped to 6 characters.

In C++, one will be able to leverage explicit types, write object-oriented code, and return a value returned instead of a global write:

The scattered COMMON arrays become a single GasProperties parameter and the grid loop moved out to the caller, so there's no line-for-line correspondence to check, which is what makes verifying the migration hard.

We added: