For a server build, Vite leaves dependencies as runtime imports rather than bundling them, on the assumption that Node can load them directly and that skipping the work is faster. Anything in node_modules is externalized unless configured otherwise.
That assumption fails for packages that ship source Vite is expected to process: files importing CSS, Vue single-file components, or code that relies on Vite-specific handling such as import.meta.env. The symptom is a server-side error at import time, commonly Unexpected token '<', an unknown file extension error, or Must use import to load ES Module.
Add the package name to ssr.noExternal so it is bundled and transformed. The inverse setting, ssr.external, forces a package out of the bundle when a linked workspace dependency is being pulled in unnecessarily. Because both are per-package lists, a monorepo usually needs entries for its own internal packages, which are not in node_modules and are therefore not externalized by the default rule.