---
title: "Init Scripts & Extensions"
description: "Browser automation CLI for AI agents"
canonical_url: "https://agent-browser.dev/init-scripts"
---

# Init Scripts & Extensions

Init scripts run before page JavaScript. Use them to install hooks, polyfills, instrumentation, or test helpers before the first navigation.

## Launch-time scripts

```bash
agent-browser --init-script ./instrumentation.js open https://example.com
agent-browser --init-script ./a.js --init-script ./b.js open https://example.com
```

You can also configure scripts with an environment variable:

```bash
export AGENT_BROWSER_INIT_SCRIPTS="./a.js,./b.js"
agent-browser open https://example.com
```

Launch-time scripts are applied when the browser starts. If the daemon is already running, close it before changing launch-time script options.

## Built-in scripts

```bash
agent-browser --enable react-devtools open http://localhost:3000
```

`react-devtools` is the built-in feature currently available. It installs the React DevTools hook before application code runs and is required for `react tree`, `react inspect`, `react renders`, and `react suspense`.

## Runtime scripts

```bash
agent-browser addinitscript "window.__testMode = true"
# Returns an identifier

agent-browser removeinitscript <identifier>
```

Runtime init scripts are useful after a session has started. They are still init scripts, so they affect future documents and navigations rather than rewriting already-executed page code.

Runtime init-script identifiers are session-wide. `removeinitscript` removes the script from every open tab where it was registered and prevents it from being replayed into tabs opened later.

## Pre-navigation setup

For flows that need routes, cookies, or scripts installed before the first real navigation, start the browser without a URL:

```bash
agent-browser batch \
  '["open"]' \
  '["network","route","*","--abort","--resource-type","script"]' \
  '["cookies","set","--curl","cookies.curl","--domain","localhost"]' \
  '["navigate","http://localhost:3000/target"]'
```

`open` with no URL launches the browser on `about:blank`, leaving room to stage state before the target page loads.

## Extensions

```bash
agent-browser --extension ./extension open https://example.com
agent-browser --extension ./a --extension ./b open https://example.com
```

Extensions are launch-time options. They require a local Chromium-based browser and are not supported with CDP connections, cloud providers, or Lightpanda.

## Safety

Init scripts and extensions run with high privilege in the page context. Only load scripts and extension directories from trusted local paths.