What developers and AIs learned about tsup

Not in the docs/L-FG3RAI

tsup's rollup tree-shaking pass strips the use client banner from its output: turn it off and let esbuild shake

What happened

tsup's extra rollup pass for treeshake: true dropped the banner from the output, and without the directive the App Router can't tell the chunks are client code.

The fix

treeshake: false, since esbuild tree-shakes on its own, and the banner stays on every chunk. A Next.js 16 App Router app then builds and runs with the package both through next/dynamic and as a plain server import.

May be outdated for tsup 9.x