onUI
onUI is a lightweight browser extension for Chrome, Edge, and Firefox that lets developers annotate any UI for AI agents without modifying application code. It provides a local MCP bridge for annotation-first UI pair programming, enabling AI tools to understand and act on UI elements. It is designed for developers and teams working with AI coding assistants.
✨ Key features
- No integration into app code required
- Per-tab ON/OFF control, off by default
- In-page annotation dialog with intent and severity
- Draw mode for region annotations (rectangle and ellipse)
- Export outputs in compact, standard, detailed, or forensic formats
- Local MCP server with native bridge, no cloud backend
🎯 Use cases
- Annotate UI elements for AI agents to understand and interact with
- Provide context to AI coding assistants during pair programming
- Create region-based annotations for visual testing or documentation
- Export structured annotations for debugging or analysis
📦 Installation
🧰 Requirements: Requires a Chromium browser (Chrome or Edge) or Firefox, and Node.js 20+ for MCP setup. No API keys or accounts required.
Option A: Browser Extension Stores (recommended)
Chrome Web Store: https://chromewebstore.google.com/detail/onui/hllgijkdhegkpooopdhbfdjialkhlkan?authuser=0&hl=en-GB
Microsoft Edge Add-ons: https://microsoftedge.microsoft.com/addons/detail/onui/fkcmlckehjhcicihbnmhkadfhjhfnond
Option B: One-command installer from latest GitHub release
macOS/Linux:
curl -fsSL https://github.com/onllm-dev/onUI/releases/latest/download/install.sh | bash
macOS/Linux (Firefox artifact):
curl -fsSL https://github.com/onllm-dev/onUI/releases/latest/download/install.sh | bash -s -- --firefox
Windows (PowerShell):
irm https://github.com/onllm-dev/onUI/releases/latest/download/install.ps1 | iex
Windows (PowerShell, Firefox artifact):
iwr https://github.com/onllm-dev/onUI/releases/latest/download/install.ps1 -OutFile install.ps1; .\install.ps1 -Firefox
The installer handles extension install and can set up MCP in the same run.
When prompted with Set up local MCP bridge now? [y/N], enter y to enable MCP.
Then load it in Chrome or Edge:
- Open
chrome://extensionsoredge://extensions - Enable Developer mode
- Cl
🚀 Usage
- Open any supported website tab.
- Click the onUI extension icon.
- Enable
This Tab. - Use the on-page launcher to open the compact toolbar.
- Toggle Annotate mode for element targeting or Draw mode for region targeting.
- Hold
Shiftand click multiple elements to batch-select targets. - Release
Shiftto open a shared annotation dialog for selected targets. - Save once to create one annotation per selected element (or one region annotation in draw flow).
- Open toolbar Settings to choose output level and configure Clear on copy.
- Copy exported output from the toolbar.
⚠️ Good to know
The project is stable and production-ready, but browser support is limited to Chrome stable, Edge stable, and Firefox stable (unpacked).
❓ FAQ
Does onUI require modifying my application code?
No, onUI is designed to annotate any UI without integration into app code.
How do I set up the local MCP bridge?
Run the installer and answer 'y' when prompted, or use the --mcp flag on macOS/Linux or set ONUI_INSTALL_MCP=1 on Windows.
Can I use onUI with Firefox?
Yes, onUI supports Firefox stable, but you need to install it manually as a temporary add-on from the release artifact.
What output formats are available for annotations?
You can export annotations in compact, standard, detailed, or forensic formats.
📊 Repository
🤖 Overview, features, install steps and FAQ were generated from the project's README on Sep 4, 2026. Always check the original source before running commands.