Legacy code is every developer's nightmare. Legacy code in COBOL is that nightmare, time 10. Undocumented business logic. Cryptic variable names. GOTOs nobody knows where they go to. Code that takes a hundred lines to do what modern languages do in five. It is the code equivalent of a haunted house, and someone has to be brave enough to go in.

Then, LLMs took over the software engineering world. Suddenly, migrating COBOL to C# didn't sound impossible anymore. Coding agents that can read, understand, and rewrite decades-old mainframe code, sounded like the perfect solution. What could go wrong?

As it turns out, quite a lot. But fortunately also, some thing went right.

In this talk, I will share the story of a real migration project that took a COBOL system and moved it to C#, using LLMs and Microsoft's Agent Framework. I will dive in the process: the groundwork we forgot, the do's and don'ts we only learned by getting them wrong first, and the quick wins that made the whole thing worth it.

And, because agents are only confident, and not always right, I will show the garbage they produced along the way, and what it took to catch it and change it.

Let’s skip the polished happy-flow demo, and dive into the real-world rollercoaster that AI-assisted legacy migration is. Are you in for the ride?