The 'Locksmith Loop' Validates Agent-Migrated COBOL-to-Java Code Against a Deterministic Oracle, Hitting 91.9% Branch Coverage

arXiv·medium signalXBlueskyLinkedInCopy link

This agentic test-synthesis method (arXiv 2607.28271, July 30) instruments both the COBOL source and the generated Java target with mocks, runs both off-mainframe on commodity hardware, then iterates a Witness Search over input mocks to penetrate branches followed by parity-preserving mutations. When search plateaus, an analyzer identifies a 'Locked Paragraph' — the condition blocking deeper exploration. Across three case studies from 430 to 4,114 source lines (two open-source, one internal production-like program) it reached near-complete coverage on the open-source programs and 91.90% branch coverage on the internal one, with generated Java matching the COBOL reference under deterministic parity checks in all accepted cases. The general pattern — validating agent output against a deterministic differential oracle rather than an LLM judge — transfers well beyond mainframe migration.