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AI migrated legacy COBOL programs to Java, bugs included

Agentic Method for Deterministic Validation of Legacy Code Migration

arxiv.org

August 3, 2026

2 min read

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Summary

The "Locksmith Loop" is a novel agentic test-synthesis method designed for deterministic validation of legacy COBOL code migration to Java. It addresses challenges in testing by generating test cases to ensure correct functionality despite limited test data and complex corner cases.

Key Takeaways

The "Locksmith Loop" method improves testing for legacy COBOL to Java migration by utilizing agentic test synthesis to enhance code coverage.
The method achieved nearly complete coverage on two open-source COBOL programs and 91.90% branch coverage on an internal production-like COBOL program.
The generated Java code matched the COBOL reference under deterministic parity checks in all accepted test cases.
The approach demonstrates a novel way to validate agentic coding output using a deterministic oracle.

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Coverage & discussions

Hacker News · arxiv.org

AI migrated legacy COBOL programs to Java, bugs included ↗

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Community Sentiment

Negative

Positives

Using AI to translate COBOL brings extra understanding, which could help demystify legacy systems that are often seen as black boxes.
There’s a chance that AI could write tests and prompt a better architecture, potentially making it easier for teams to work on legacy codebases.

Concerns

The idea that a one-shot migration from COBOL to Java won't introduce a massive number of bugs is naive; AI's non-determinism could lead to a plethora of new issues.
An AI rewrite doesn’t solve the fundamental problems of legacy systems; it just creates a new one that no one understands.
Rewriting COBOL with AI doesn't address the complexities of job control and other mainframe dependencies that simply don't translate to Java.