Parity: Proof for AI-Translated COBOL

Banks are using AI like IBM Bob to translate COBOL into Java, but in a 2026 study only 44–53% of plain AI translations behaved like the original. The dangerous differences are invisible in the code: a field with no sign turns a customer's debt into a credit, a field that's too small cuts off leading digits, and COBOL rounds differently from Java. Parity runs the old COBOL and the new Java side by side on thousands of inputs, aimed at exactly where the two handle numbers differently. It shrinks each difference to one simple example and explains it against the COBOL line that causes it. A person then decides: keep the old behaviour, or accept the change. Once every difference is decided, Parity issues a signed certificate for the change board. It runs inside IBM Bob as an MCP server, so Bob translates, Parity checks, you decide, and Bob repairs. On translations built to match COBOL exactly, it raised zero false alarms across 200,000 inputs.