What changed: Mistral published firsthand evidence that AI agents produce significantly higher-quality output in large-scale legacy code migration when given structured roles rather than full autonomy.
Mistral published a blog post detailing the migration of 40,000 lines of Fortran 77 code to C++ for a European energy operator. The source code belonged to a physics-intensive reservoir simulator with no test suite and no centralized documentation. The structural constraints of Fortran 77—standardized in 1977 and lacking modules, namespaces, or structured data types, with state stored in COMMON blocks shared across the entire program and variables implicitly typed by the first letter of their name—defined the technical difficulty of the project from the outset.
Before agents were deployed, a numerical parity harness was built to verify the equivalence of the two codebases. It consisted of additional Fortran subroutines for state export, a C++ test framework to load checkpoints, and Skill.md files to guide agent behavior. For the documentation phase, over a hundred agents were spawned via Vibe CLI; each could access relevant PDFs through document libraries with Mistral OCR support. A reviewer agent running on a cron schedule monitored newly opened pull requests and scheduled fix tasks as needed.
The project's most significant finding concerns the balance between agent autonomy and agent structure. In the first attempt, one agent was assigned per Fortran subroutine; COMMON blocks were converted one-to-one into global structs, and GOTO-driven control flow was left intact rather than being restructured into loops or early returns. In the second attempt, a team-based approach was introduced: a planner, a coder, a tester, and a code quality reviewer worked together on each module, substantially improving code quality.
Key facts
- Mistral migrated 40,000 lines of Fortran 77 code belonging to a physics-intensive reservoir simulator to C++ for a European energy operator.
- A parity harness built before the migration continuously verified the numerical equivalence of the two codebases.
- Over a hundred agents deployed via Vibe CLI generated comprehensive documentation for the source code with Mistral OCR support.
- After fully autonomous per-subroutine agent assignments proved insufficient in the first attempt, the project switched to a structured multi-agent team model comprising a planner, coder, tester, and reviewer.
- A reviewer agent running on a cron schedule automatically reviewed newly opened pull requests and scheduled fix tasks when necessary.
Why it matters
This is one of the first detailed case studies on agent-assisted migration for energy, defense, and scientific computing sectors facing inevitable transitions away from decades-old Fortran codebases.
Watch next: Whether the second attempt covered all modules and whether an independent outcome report from the client will be published remains unknown.
Source date: 2026-09-09T20:21:04.455Z · Verification confidence: 92/100
