What happened
Next.js 16, released 21 October 2025, promotes Turbopack — Vercel's Rust-based bundler — to stable for both `next dev` and `next build`, and makes it the default bundler for all new projects. Vercel reports up to 10× faster Fast Refresh and 2–5× faster production builds, and says over 50% of dev sessions and 20% of production builds on 15.3+ were already on Turbopack. Custom webpack setups must now opt out with `next build --webpack`. The release also stabilizes React Compiler support, adds Cache Components (a `use cache` model built on Partial Prerendering), and raises the floor to Node.js 20.9 and React 19.2.
Why it matters
The default bundler for the most-used React meta-framework has switched from webpack to a Rust-based engine — the culmination of the industry's move to native-speed tooling. For most teams the win is real (faster CI and feedback loops), but 'default' flips the burden of proof: staying on webpack is now the deliberate, documented choice, and any custom webpack config or plugin becomes a migration item.
This is a decision about the supply chain of your build, not just a speed bump. The Staff work is auditing what your webpack config actually does — loaders, plugins, aliases, custom chunking — and confirming Turbopack covers it before an upgrade forces the switch. Where it doesn't yet, `--webpack` buys time, but treat that as debt with a deadline: webpack is the path being deprecated, not invested in.
Your team is on Next.js and the default bundler just changed from webpack to Turbopack. How would you plan and de-risk that migration across a large application?
Probes build-system reasoning and migration risk, not framework trivia.