# X_SIDE_EFFECTS_UNDECLARED

## Means

a module runs a statement at import time and its package's `sideEffects` excludes it

## Typical cause

a new `registerErrorCodes()` or `registerBaseCatalog()` call in a module the array does not name — Bun honours the field, so the bundler is told the module is droppable and deletes that call from every app that does not import its bindings

## Fix

add `"./<the module>"` to `"sideEffects"` in that package's `package.json` (`bun run side-effects --explain --json` prints the array this tree measures), or, when the effect only matters once one of the module's own bindings is used, a `SIDE_EFFECTS_BY_USE` row in `scripts/side-effects.ts` saying why

[CLI and verify](/error-codes#cli-and-verify)