What changed
METR reported that 16 experienced open-source developers completed 246 tasks in familiar repositories 19 percent more slowly when allowed to use early-2025 AI coding tools.
Why it matters
The result challenged self-reported productivity expectations and showed why engineering teams should measure completed work and rework instead of assuming tool use saves time.
What to do
Run a preregistered comparison on representative repository tasks, measuring cycle time, accepted changes, review effort, defects, retries, and developer experience before setting policy.
Caveat
The study was small, involved mature repositories and developers unfamiliar with some tools, and does not establish that AI slows all programmers, tasks, or later systems.