---
title: How do I open the preview to see the app my agent is building?
question: open the preview browser
area: browser
order: 1
surface: desktop
audience: beginner
keywords: [preview, web button, test browser, built-in browser, globe icon, open browser, see my app, live preview, watch the app, localhost, adjacent slot, hide browser]
related: [the-built-in-browser, why-does-my-preview-say-no-url, what-is-a-quadrant]
updated: 2026-07-21
---

# How do I open the preview to see the app my agent is building?

Click the **web** button on the quadrant's button row. It is the small globe (🌐) with the word **web** next to it, sitting between **plan** and the settings tools icon. codus opens a built-in browser in the slot next to your quadrant and loads whatever your app is serving, so you watch the live page change as the agent edits the code. Click **web** again to hide it.

The preview is a real browser built into codus. You do not need Chrome or Safari open to use it, and unlike the terminal, your mouse works inside it: you can click links, scroll, and fill in forms just as you would in any browser.

## Before you start

Two things have to be true for the preview to show anything:

- **The neighbouring slot has to be switched on.** The preview opens in the slot next to your quadrant, so that slot needs to be available. If it is off, clicking **web** does nothing and shows no error.
- **There has to be an address to load.** codus reads that from a running dev server on its own, or you set it by hand. If there is no address yet, the preview opens but reads "No URL". See [Why does my preview say "No URL"?](why-does-my-preview-say-no-url.md) for how to fix that.

## Steps

1. Find the button row along the top of your quadrant.
2. Click the **web** button (the globe, 🌐). Hover it first if you are not sure: it reads **Open test browser in the adjacent slot**.
3. The preview opens in the slot beside your quadrant and loads your page.
4. To put it away, click **web** again. Its label now reads **Hide test browser**.

## How do I know it worked?

A browser panel appears next to your quadrant with a toolbar across the top and your page underneath. The **web** button stays highlighted while the preview is open.

## Common problems

**Clicking web does nothing.** The slot next to your quadrant is switched off, so there is nowhere for the preview to open. Turn that neighbouring quadrant on, then click **web** again.

**The preview opens but says "No URL".** Nothing is being served yet. Start your dev server, or set an address by hand. The full fix is in [Why does my preview say "No URL"?](why-does-my-preview-say-no-url.md).

**The preview is covering another panel.** The preview and the Brain panel can share the same space. See [The browser is covering the Brain panel](../troubleshooting/the-browser-is-covering-the-brain-panel.md).