GPT-6.1 Sol Reaches 98 Percent COBOL Behavior Preservation at Higher Reasoning Effort

OpenAI reported on October 26, 2026 that GPT-6.1 Sol reached 98 percent behavior preservation on COBOL migrations at higher reasoning effort, up from 87 percent at low effort. COBOL is a decades-old language still running many banking systems. At low effort, GPT-6 Sol scored 21 percent and GPT-5.6 Sol 5 percent. Success requires every test to pass.