Serving from a sub-path

The client's asset URLs are written into index.html at build time; the server learns its mount point at runtime. A sub-path is therefore two variables, and they have to carry the same value.

VITE_BASE_PATH=/app/ npm run build
BASE_PATH=/app npm start

That serves the app at https://example.com/app/.

What each one does

VITE_BASE_PATH is Vite's base. It rewrites the asset URLs in index.html, and the router reads it back, so every address the app builds sits below it.

BASE_PATH tells the server which prefix is the app. Anything outside it is a 404. A missing trailing slash is normalized, so /app and /app/ are the same value to the server, and /app — the mount point without its slash — is the app's own root.

The Chatfuel proxy routes (/chatfuel/*) stay where they are, above the mount point.

When the two disagree

A client built for /app/ and served at the root asks for /app/assets/..., which is not there: the page arrives, every script 404s, and the app never starts. The other way round is the same failure from the other side — a client built for the root, served under BASE_PATH=/app, asks for /assets/..., which is outside the mount point.

You cannot fix either at runtime. The URLs are already in the bundle, so the answer is a rebuild with the value the server is running with.

Not on Vercel

Vercel serves the app from the domain root and supports no sub-path at all. If you need one, run the bundled Node server yourself — on a box you own or in Docker.