Comprehension-Performance Gap in GenAI-Assisted Brownfield Programming: A Replication and Extension

Code comprehension is essential for brownfield programming tasks, in which developers maintain and enhance legacy code bases. Generative AI (GenAI) coding assistants such as GitHub Copilot have been shown to improve developer productivity, but their impact on code understanding is less clear. We replicate and extend a previous study by exploring both performance and comprehension in GenAI-assisted brownfield programming tasks. In a within-subjects experimental study, 18 computer science graduate students completed feature implementation tasks with and without Copilot. Results show that Copilot significantly reduced task time and increased the number of test cases passed. However, comprehension scores did not differ across conditions, revealing a comprehension-performance gap: participants passed more test cases with Copilot, but did not demonstrate greater understanding of the legacy codebase. Moreover, we failed to find a correlation between comprehension and task performance. These findings suggest that while GenAI tools can accelerate programming progress in a legacy codebase, such progress may come without an improved understanding of that codebase. We consider the implications of these findings for programming education and GenAI tool design.