Don't run next build while next dev is running

Twice in one day I broke my own dev server the same way: ran npm run build in one terminal while next dev was serving in another. Everything looks fine until the next reload, then:

Error: ENOENT: no such file or directory,
open '.next/server/app/page/build-manifest.json'

or just a black page with Internal Server Error.

Why

next dev (turbopack here) and next build (webpack here) both write to the same .next directory with different layouts. The build wipes and rewrites paths the dev server is holding open, and the dev server has no idea. It keeps serving from manifests that no longer exist.

Fix

# stop the dev server first, then:
rm -rf .next
npm run dev

Rule

One .next owner at a time. Stop the dev server before next build, or point the build elsewhere (next build --experimental-build-mode compile setups and distDir tricks exist, but stopping the server is the boring reliable answer).