AI made experienced devs 19% slower. Here's the side project trap that created.

METR measured it in 2025 — senior devs with AI coding assistants worked 19% slower and thought they were 43% faster. For side projects, this gap is career-ending for your product. In July 2025, METR ran a controlled trial with experienced software developers using AI coding assistants. The result: they worked 19% slower than without AI tools. Not faster. Worse: they thought they were working 43% faster. A 62-percentage-point gap between perception and reality. I kept thinking about what this means for side projects specifically — because I think it matters more there than anywhere else. The planning fallacy didn't go away Daniel Kahneman named it in 1979: we systematically underestimate how long our own tasks take, and overestimate how much we'll get done. External reference points help. Deadlines help. Someone watching helps. The assumption was: AI will fix this. AI will plan better, build faster, reduce rework. METR's data says otherwise. Not because AI tools are bad — they're genuin