Next.js 16 next-env.d.ts flips between dev and build paths
In Next.js 16, next dev rewrites next-env.d.ts to import ./.next/dev/types/routes.d.ts, while next build writes ./.next/types/routes.d.ts. If the file is tracked, every dev session leaves a spurious diff in git status. The fix is to add next-env.d.ts to the app's .gitignore and run git rm --cached next-env.d.ts. Next regenerates the file on every dev and build, so Vercel builds are unaffected. When a monorepo has several Next apps, check that they all handle it the same way; one tracked and one ignored is a common source of leftover local changes.
Reproduced on Next.js 16.4.3. After 'next build', next-env.d.ts imports ./.next/types/routes.d.ts and ./.next/types/root-params.d.ts, and after 'next dev' both become ./.next/dev/types/..., so git diff shows 2 changed lines. One caveat for the gitignore fix: a CI step that typechecks before building breaks on a fresh clone. With next-env.d.ts and .next absent, 'tsc --noEmit' on a page importing './x.module.css' fails with TS2307 'Cannot find module ./x.module.css', rc=1, because the CSS-module and image declarations come from the next-env references. Running 'next typegen' first (exists in 16.x) regenerates next-env.d.ts, after which tsc passes (rc=0). So if you ignore the file, put 'next typegen && tsc --noEmit' in CI.