AI migrated legacy COBOL programs to Java, bugs included

AI migrated legacy COBOL programs to Java, bugs included(arxiv.org)

31 points by felineflock 4 hours ago | 27 comments

The biggest problem is not that bugs are migrated with COBOL, but that lots of new bugs are going to be introduced. AI is not deterministic, it will be making tons of mistakes. The only realistic low-error approach is incremental step-by-step migration using Cursor or similar tools. However, it requires much more time as each step must be prompted, tested and committed manually. Any hope that one-shot migraton of a large code base will not introduce enormous number of bugs is very naive. LLM is very bad on handling long context - it is their nature unfortunately. There is no answer to this problem yet.

selcuka 38 minutes ago | |

> AI is not deterministic, it will be making tons of mistakes.

From the paper:

> The COBOL source is passed through an internal deterministic Migrator to produce a generated Java target.

Also, humans are not deterministic either. Give the same COBOL -> Java translation to multiple developers and each will come up with a different solution. Heck, even the same developer will produce a different output for the same task, depending on the day of the week.

maoberlehner 12 minutes ago | | |

> Also, humans are not deterministic either.

Indeed, and that's why so few dare to migrate them, and so many who do fail or blow the budget many times over.

I think that was the implicit point of the comment: don't expect that with AI, suddenly we can convert all those COBOL apps with a single prompt.

red75prime 37 minutes ago | |

> AI is not deterministic, it will be making tons of mistakes.

Just set the sampling temperature to zero and remove any unintended non-determinism during the parallel computation of the token probability distribution. The problem is solved? Of course, not. Non-determinism has little to do with LLMs' mistakes.

dragonwriter 23 minutes ago | |

> The only realistic low-error approach is incremental step-by-step migration using Cursor or similar tools.

Correction: AI is not deterministic, the only realistic low-error solution is not a more complex use of non-deterministic AI, but deterministic transpilation.

The problem is that this results in COBOL-in-Java which runs correctly but it is a nightmare to maintain.

JV00 36 minutes ago | |

You don’t need the entire codebase in context in every moment to migrate it. Also AI being non deterministic does not prevent it from one-shotting perfect solutions 100% of the time for simple enough problems. And every model generation brings this bar higher. So that’s really not a fundamental problem. And we can also implement llm inference deterministically if we want, it’s just that it’s not worth the loss in performance to do it.

pinkgolem 1 hour ago | |

Does it? I found llms to be great for straight conversions

At least if tescoverage is good, but well... That's something llms can also be used for

pjmlp 32 minutes ago |

Stuff like this are already a commercial product, it isn't only Zig to Rust rewrites going around.

https://www.pega.com/insights/resources/break-free-legacy-ma...

https://www.ibm.com/products/watsonx-code-assistant-z

https://global.fujitsu/en-global/pr/news/2026/03/30-01

https://www.rocketsoftware.com/en-us/insights/ai-powered-cob...

pacaro 1 hour ago |

The largest test case was 4kloc.

There are hundreds of billions of lines of cobol in production.

The IRS alone has approx 160 cobol programs, averaging 230kloc each.

madduci 1 hour ago |

I remember a similar story shared this year at JAX2026 from the Sparkasse Group, they said they were using AI to migrate from COBOL, but they still were in the middle of the migration. Maybe they faced the same issues / problems? It seemed pretty zealous to me, that everything was working smoothly, but this article highlights the limitations

Surac 45 minutes ago |

How does ai implement all the not Cobol parts a Cobol program rely on? Job Contol, CICS, sort processors? Cobol and mainframe technologies are non existant in java on any modern machine

p_l 10 minutes ago | |

Batch job control is handled by alternative systems (already often triggering the work on mainframes anyway) like Control-M

CICS is the big issue but AFAIK there were attempts. Everything else, including sort and VSAM, has various options provided usually by COBOL compiler vendors.

aldente0630 41 minutes ago |

As I understand it, the translation isn't done by an LLM but by a deterministic AST-based migrator. Also, carrying over the bugs is the stated goal.

bigbuppo 1 hour ago |

COBOL will never die. Whatever this is will only result in more COBOL being written.

hexasquid 1 hour ago |

To IBM

As specified, please find 99997 correct parts and the 3 defects (do not use)

fock 1 hour ago |

300 to 4000 lines of "production like" (whatever that is) cobol code which is easily ported to a non-mainframe env. Our's sometimes uses assembler in its innards, so good luck with real legacy code spanning a dozen files and 50k loc...

I recently threw in (want to check those intelligence metrics!) some real production code into a non-agentic system (just to get a feel how things perform without a custom harness) and results where ... interesting. The particular program uses some preprocessor no LLM we have access to (newest was GPT 5.5) has any clue about - so they confabulate what it could do (Gemini 2.5 didn't even notice there was a preprocessor...). This is expected of course but it somehow seems the problem of this technology that unless you feed it masses of data or mechanically break up the tasks in rote subunits, it just doesn't do anything sensible still...

BigJono 1 hour ago |

Yeah nah maybe fix the bugs before swapping the average COBOL dev for the average Java dev.

nicman23 1 hour ago | |

sometimes if the bug exists for enough time it is intended behavior

tapland 1 hour ago | | |

Absolutely, and you'll have all kinds of fixes for the symptoms throughout the project.

COBOL isn't hard, the tooling around it on old systems are a pain though.

surfingdino 1 hour ago | | |

Indeed, there may even be a whole lot of code that depends on it.

vrighter 44 minutes ago | |

hyrum's law applies here

anthk 54 minutes ago |

COBOL is not about the language, it's about the whole environment in the mainframe. LLM fanboys won't understand this. You need something like a mainframe with a 99.99% uptime no matter what happens in hardware, with live CPU swapping and such.

jjmarr 30 minutes ago | |

Erlang/Elixir/OTP has a resilient environment with hot swapping. Is that truly problematic?

michaelmrose 16 minutes ago | |

Can you explain why you need one machine never falling over ever instead of a cluster of machines never falling over at once collectively?

raverbashing 49 minutes ago | |

Yes and that mainframe runs several other technologies besides COBOL, and more importantly, that don't depend on it (COBOL fanboys won't understand this ;) )