OpenAI's GPT-6.1 Sol Achieves 97.9% Accuracy on Legacy Code Migration

OpenAI released GPT-6.1 Sol on September 29, 2026, demonstrating dramatic improvements in converting decades-old codebases to modern programming languages, with accuracy reaching 97.9% on rigorous internal benchmarks. The model achieved 86.6% accuracy on COBOL migration tasks at low reasoning effort, compared with just 5.4% for its predecessor GPT-5.6 Sol—a 16-fold improvement that addresses a critical infrastructure challenge facing financial institutions and government agencies worldwide.