Base paths

Your app is served at /apps/<slug> on the apps address, not at the root of a host. Most “it deployed but the page is broken” problems come from this.

What your app is given

Variable	Holds
PORT	The port to listen on.
HOST	The address to bind to.
BASE_PATH	The prefix, e.g. /apps/reports.

For most frameworks the proxy strips the prefix before forwarding, so your routes stay at /. Next.js, Streamlit and marimo are configured with the prefix and receive it unchanged.

Links carry the prefix

Anything the browser resolves — links, form actions, script and stylesheet URLs — must land under the prefix.

<!-- wrong: requests /styles.css, outside your app -->
<link rel="stylesheet" href="/styles.css">

<!-- right -->
<link rel="stylesheet" href="styles.css">
<link rel="stylesheet" href="{{ base_path }}/styles.css">

Relative URLs work everywhere. Use BASE_PATH when you can’t be relative.

Redirects don’t need to

The proxy adds the prefix to an absolute-path Location header, so you can redirect as if your app were at the root, in every framework:

return redirect("/dashboard")               # right
return redirect("/apps/reports/dashboard")  # also right: never prefixed twice

A Location that already carries the prefix, from url_for or a framework’s own helpers, is left as it is. Relative redirects (new/, ?page=2) pass through untouched.

By framework

Framework	What to do
Next.js	Nothing. basePath is set before the build.
Streamlit, marimo	Nothing.
FastAPI	Nothing. root_path is set, so url_for includes the prefix.
Flask	Relative URLs, or wrap with ProxyFix(app.wsgi_app, x_prefix=1) so url_for reads the X-Forwarded-Prefix header.
Node, Go, plain Python	Relative URLs, or prefix with BASE_PATH.
Shiny, plumber	Nothing for routing; keep asset URLs relative.
Static sites	Set your generator’s base option. See Static sites.

Cookies and WebSockets

Cookies your app sets are scoped to its own path. The platform’s sign-in cookies (toft_*) are removed before requests reach your app.

WebSockets pass through. Connect to the same prefixed path the page was served from.