A randomised trial finds experienced developers slower with AI assistance

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METR published a randomised controlled trial in which sixteen experienced open-source developers worked on 246 real issues in repositories they knew well — about five years of prior experience with the projects on average — using AI tools available between February and June 2025. They took 19% longer with the tools than without, and estimated afterwards that the tools had made them about 20% faster. The gap between the measured and the perceived effect is the finding that generalises furthest, because self-reported productivity gains are the evidence base for a very large share of public claims about this technology.

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