A 2,430-run study says Claude Code builds, it doesn't buy.

Amplifying ran Claude Code across 4 greenfield repos with 100 open-ended prompts, 20 tool categories, and three models. "Custom/DIY" was the single most common choice, topping 12 of 20 categories, it hand-rolls JWT+bcrypt auth and feature flags before reaching for Auth0 (Amplifying). But when it does adopt, it's decisive: GitHub Actions 94%, Stripe 91%, shadcn/ui 90%. The lesson is direct. Agents default to reinventing infrastructure, so if you want a specific library used, you have to name it explicitly in CLAUDE.md or the prompt. Don't assume it'll reach for the obvious dependency.