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AI migrated legacy COBOL programs to Java, bugs included

arxiv.org

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Re: AI migrated legacy COBOL programs to Java, bugs included

#12

Post by toplinesoftsys »

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.

Re: AI migrated legacy COBOL programs to Java, bugs included

#13

Post by fock »

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...

Re: AI migrated legacy COBOL programs to Java, bugs included

#15

Post by Maxion »

tapland wrote: post #8
nicman23 wrote: post #5

Earlier quoted context omitted.

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

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.

Anything running on COBOL to day is a large enterprise system. You'll have reports running in other systems in subsidiary companies that rely on bugs in the upstream cobol code.

Re: AI migrated legacy COBOL programs to Java, bugs included

#17

Post by pinkgolem »

toplinesoftsys wrote: post #12

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 l…

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

Re: AI migrated legacy COBOL programs to Java, bugs included

#19

Post by raverbashing »

anthk wrote: post #18

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.

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 ;) )
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