A METR trial found AI tools made experienced developers 19% slower

In a randomized controlled trial published by METR on July 10, 2025, allowing AI tools increased task completion time for experienced open-source developers by 19 percent - that is, the developers were slower with AI than without it. The trial used 16 developers and 246 tasks in repositories they knew well, with AI sessions relying mainly on Cursor Pro and Claude 3.5 and 3.7 Sonnet. The same developers had predicted AI would make them 24 percent faster, and even after the study still believed it had sped them up by about 20 percent. METR cautioned the result is specific to skilled developers in familiar, complex codebases and does not generalize to all software work.