alt Hacker NewsAI migrated legacy COBOL programs to Java, bugs included
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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.
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.
Why should I convert COBOL to Java? LLM can write COBOL just as fine.
While not everything can be easily converted (IMS, CICS, reports, batch processing etc.), there are many situations where automated tooling can be helpful in migrations.
Related to this, I created a tool for situations where you want to compare COBOL code with Java code. It includes a preprocessing step where IMS etc. calls are converted to mocks that return JSON (from file), and also use JSON for input/output, and GnuCOBOL to run the program. More a proof-of-concept than production, but here is a link if somebody finds it helpful.
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...
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
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
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.
COBOL will never die. Whatever this is will only result in more COBOL being written.
To IBM
As specified, please find 99997 correct parts and the 3 defects (do not use)
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...
When they migrate to Java how are they migrating things like the messaging (send, receive) functionality?
Yeah nah maybe fix the bugs before swapping the average COBOL dev for the average Java dev.
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.
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Before, only the senior cobol programmers at the company understood and knew the codebase.
Now, no one does.
I can see the allure of moving away from a legacy cobol system. But an AI rewrite doesn't actually solve any of the issues with having a legacy cobol codebase. You just have a new system no one knows or understands.