Argonaut

Million-Token Mainframe Migration: COBOL to Java, proven byte for byte Banks and insurers still run their core money calculations on decades-old COBOL. Migrating it typically takes years and millions of dollars, and the hardest part isn’t translation but trust. Teams can’t prove the new code behaves exactly like the old, so migrations stall. This project uses Gemini 4 Argon to migrate three interdependent mainframe batch jobs from AWS’s open-source CardDemo credit-card system: posting daily transactions, calculating interest, and generating the transaction report. Argon reads the whole codebase, including programs and shared copybooks, in one context and writes the complete Java system in one long-output pass, keeping shared data layouts consistent across every file. A differential test harness then runs the original COBOL and the new Java on identical data and compares every output file byte for byte, along with console output and return codes. Argon matched all 7 outputs on its first pass with zero fixes, in 11 minutes, for about $1. A Gemini 3.8 Flash baseline translating file by file produced 0/7. It didn’t compile and created conflicting versions of the same data structures, even after a fix round. An engineer reviews the diff and test report before anything merges.