AI migrated legacy COBOL programs to Java, bugs included
This paper introduces Locksmith Loop, an agentic test-synthesis method that deterministically validates AI-generated Java migrations of legacy COBOL code. It runs mock-instrumented COBOL and Java environments on commodity hardware, then iteratively searches for inputs that reach deeper branches and applies parity-preserving mutations. In three case studies ranging from 430 to 4,114 source lines, the method achieved near-complete branch coverage on open-source programs and 91.90% branch coverage on an internal production-like program, while all accepted test cases passed deterministic parity checks.
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[Submitted on 30 Jul 2026]
Title:Agentic Method for Deterministic Validation of Legacy Code Migration
View a PDF of the paper titled Agentic Method for Deterministic Validation of Legacy Code Migration, by Andras Ferenczi and 4 other authors
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Abstract:Migration of legacy COBOL programs to Java requires extensive testing to ensure correct functionality. This effort is often complicated by the lack of test data and the difficulty of validating all corner cases. In this paper we propose a novel agentic test-synthesis method, the "Locksmith Loop," which is initiated by preparing two runtime environments: the COBOL source and the generated Java target are each instrumented with mocks and executed off-mainframe on commodity hardware, then an iterative agentic loop performs Witness Search over input mocks to penetrate program branches, followed by parity-preserving mutations. When routing boundaries are reached, an analyzer identifies a Locked Paragraph: a condition preventing deeper exploration. Across three COBOL-Java case studies, spanning two open-source programs and one internal production-like COBOL program and ranging from 430 to 4,114 source lines, Locksmith consistently improved coverage beyond input-search plateaus, reaching nearly complete coverage on the two open-source programs and 91.90% branch coverage on the internal production-like COBOL program. The generated Java matched the COBOL reference under deterministic parity checks in all accepted test cases. Through these findings we demonstrate, to the best of our knowledge, a novel approach for validating agentic coding output using a deterministic oracle.
Comments: 11 pages, 6 figures
Subjects:
Software Engineering (cs.SE); Artificial Intelligence (cs.AI)
Cite as: arXiv:2607.28271 [cs.SE]
(or arXiv:2607.28271v1 [cs.SE] for this version)
https://doi.org/10.48550/arXiv.2607.28271
arXiv-issued DOI via DataCite (pending registration)
Submission history
From: Andras Ferenczi [view email] [v1] Thu, 30 Jul 2026 14:25:23 UTC (2,963 KB)
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