AI migrated legacy COBOL programs to Java, bugs included

[Submitted connected 30 Jul 2026]

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Abstract:Migration of bequest COBOL programs to Java requires extended testing to guarantee correct functionality. This effort is often analyzable by the deficiency of trial information and the trouble of validating each area cases. In this insubstantial we propose a caller agentic test-synthesis method, the "Locksmith Loop," which is initiated by preparing 2 runtime environments: the COBOL root and the generated Java target are each instrumented pinch mocks and executed off-mainframe connected commodity hardware, past an iterative agentic loop performs Witness Search complete input mocks to penetrate programme branches, followed by parity-preserving mutations. When routing boundaries are reached, an researcher identifies a Locked Paragraph: a information preventing deeper exploration. Across 3 COBOL-Java lawsuit studies, spanning 2 open-source programs and 1 soul production-like COBOL programme and ranging from 430 to 4,114 root lines, Locksmith consistently improved sum beyond input-search plateaus, reaching astir complete sum connected the 2 open-source programs and 91.90% branch sum connected the soul production-like COBOL program. The generated Java matched the COBOL reference nether deterministic parity checks successful each accepted trial cases. Through these findings we demonstrate, to the champion of our knowledge, a caller attack for validating agentic coding output utilizing a deterministic oracle.