The Biggest Risk in AI-Assisted Coding Might Be Your Own Brain
The hardest bugs in AI-assisted development are in the human.
In 2025, METR ran a randomized controlled trial with 16 experienced open-source developers working on 246 real tasks in their own mature codebases. Each task was randomly assigned to allow or forbid AI tools. Developers forecast a 24% speedup. After finishing, they still believed AI had saved them about 20%. The measured result was that they took 19% longer with AI.
The speed question is still open. The gap between feeling and measurement is the part worth studying, and psychology has been explaining it for decades.
Follow-up Trial Update for 2026
2025 trial2026 follow-upDesignRandomized, 16 developers, 246 tasksLarger cohort, newer toolsResult19% slower with AILeaned toward a speedup, wide uncertaintyCaveatSmall sample, early-2025 toolsMETR calls the signal unreliable (participants opted out of no-AI work)PerceptionBelieved ~20% fasterPerception gap remained...
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