Nitro's moduleSideEffects defaults override Rolldown settings, causing aggressive tree-shaking #4085

Activity

1. Teages

   added

   pending triage

   on Mar 9, 2026
2. Teages

   changed the title [-]`moduleSideEffects` defaults override Rolldown settings, causing aggressive tree-shaking[/-] [+]Nitro's `moduleSideEffects` defaults override Rolldown settings, causing aggressive tree-shaking[/+] on Mar 9, 2026
3. idirdev commented on Mar 11, 2026

   

   idirdev

   on Mar 11, 2026

   Last edited by idirdev

   More actions

   This is a real problem. Nitro's custom moduleSideEffects config overrides Rolldown's default (which is true, meaning "treat all modules as having side effects"). With the override, Rolldown aggressively tree-shakes modules that register themselves through side effects.

   This is especially bad with plugin-pattern libraries like Pothos where you import a plugin purely for its side effect of registering it with the builder:

   import '@pothos/plugin-prisma';
   // this module's side effect registers the plugin
   // tree-shaking removes it because theres no explicit usage

   Dev mode works (no tree-shaking), production build removes the import and breaks the feature.

   Workaround: mark specific imports as side-effectful in your Nitro config:

   export default defineNitroConfig({
     rollupConfig: {
       treeshake: {
         moduleSideEffects: (id) => {
           if (id.includes('@pothos/')) return true;
           return false;
         },
       },
     },
   });

   The real fix should be on Nitro's side though. Overriding moduleSideEffects to false is too aggressive as a default. Either remove the override (let Rolldown use its default true) or only apply it to known-safe modules.
4. pi0

   added a commit that references this issue on Mar 29, 2026

   fix(build): stop aggressive tree-shaking of side-effect imports

   ...

   This issue will close once commit ebb9329 is merged into the 'main' branch.

   ebb9329
5. pi0

   closed this as completedin #4164on Mar 29, 2026