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**Help article: Make your app ready for previews**

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Documentation

# Make your app ready for previews

A few habits in your repository let Rondar start a preview of each session quickly and without errors.

Updated October 13, 2026

1

## Understand how a preview starts

Rondar has no list of frameworks or commands. When you open a preview, an agent reads the repository and its documentation, finds how to start the app, and starts it on the branch of the session. Rondar then loads the first page and its assets to make sure that the app answers.

Thus the best help is a repository that tells clearly how to run the app.

2

## Write down how to run the app

Put the commands to install, set up and start the app in the README, in the order in which a new developer runs them. Include the command that prepares the database.

A command that works on a clean machine works in a preview. A step that is only in the memory of the team does not.

3

## Listen on the port that you are given

The preview gives your app a port in the `PORT` environment variable. Start the server on that port, and do not write a fixed port in the code.

```
// Node.js
const port = process.env.PORT || 3000;
app.listen(port, "0.0.0.0");
```

```
# Django
python manage.py runserver 0.0.0.0:$PORT

# Rails
bin/rails server -b 0.0.0.0 -p $PORT
```

4

## Bind to all interfaces

Bind the server to `0.0.0.0`, not to `127.0.0.1` or `localhost`. A server that listens only on `127.0.0.1` in its container cannot answer the public address of the preview. This is the most frequent cause of a preview that starts but does not load.

Front-end development servers need an option for this, for example `--host 0.0.0.0` for Vite and `-H 0.0.0.0` for Next.js.

5

## Do not fix the host name

A preview has its own public address, and each session has a different one. Do not write one host name in the configuration.

- Read the allowed hosts from the environment. Rondar gives the public address in `PREVIEW_URL`, and the host names in `RONDAR_PREVIEW_HOSTS`.

- Make links and asset addresses relative, or build them from the address of the request.

- If the development server refuses unknown hosts, let it accept the hosts of `RONDAR_PREVIEW_HOSTS`.

6

## Give example environment values

Keep an `.env.example` file in the repository with each variable that the app needs. Rondar reads `.env.example`, `.env.sample` or `.env.template` during project setup and lists the variables in the project settings, where you give the values.

Use values for development only. A preview must not use production credentials or production data.

7

## Give seed data

A preview is useful when its pages have content. Keep a seed command that fills a development database with a small, realistic set of data: some products, some users, some orders.

If the app has different kinds of users, seed one of each kind. Rondar prepares a sign-in state for each kind that it finds, and you choose one with **View as**.

8

## Make the first page fast

Rondar checks the first page before it shows the preview. A first page that needs a long build, or that waits for an external service, makes the preview slow to start. Let the development server start without services that it does not need.

9

## Check a preview that fails

Open the Preview pane of the session and read its error and its logs. Most failures have one of these causes:

| What you see | Usual cause |
| --- | --- |
| The preview starts but the page does not load | The server listens on `127.0.0.1`, or on a fixed port |
| "Blocked host" or "Invalid Host header" | The app accepts only one host name |
| The page loads without styles or scripts | Asset addresses use a fixed host |
| The server stops at start | A variable has no value, or the database is not prepared |

Correct the cause in the repository, or tell the session what failed. The agent can correct the start of the app in the same conversation.

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## On this page

[Understand how a preview starts](#step-1)[Write down how to run the app](#step-2)[Listen on the port that you are given](#step-3)[Bind to all interfaces](#step-4)[Do not fix the host name](#step-5)[Give example environment values](#step-6)[Give seed data](#step-7)[Make the first page fast](#step-8)[Check a preview that fails](#step-9)

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