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