Cognition Labs has rebuilt its AI software engineer Devin for Claude Sonnet 4.5, achieving 2x speed and 12% performance gains—but the upgrade revealed unexpected model behaviors like 'context anxiety' and autonomous note-taking. The overhaul exposed how next-gen LLMs are reshaping agent design paradigms, forcing engineers to abandon prior architectural assumptions. This deep dive explores the surprising lessons from rebuilding one of AI's most advanced coding agents.