Building an MCP Server for Native macOS App UI Control
Building an MCP Server for Native macOS App UI Control
Most AI agents are stuck in the browser or the terminal. An MCP server that connects to macOS accessibility APIs lets Claude interact with any native app - clicking buttons, reading text fields, filling forms, and traversing the full UI tree.
What the MCP Server Exposes
The server provides tools that map to accessibility API operations:
- Traverse - walk the accessibility tree of any running app and return the UI hierarchy
- Click - click a button, menu item, or any clickable element by reference
- Read - get the value of text fields, labels, and other elements
- Type - enter text into focused fields
- Screenshot - capture the current screen state for visual verification
Each tool takes an element reference from the accessibility tree, so Claude can navigate complex UIs step by step.
Why Accessibility APIs Over Screenshots
Screenshot-based agents use vision models to interpret pixels. This works but is slow, expensive, and unreliable - a button that looks slightly different than expected breaks the flow.
Accessibility APIs give you structured data. You know exactly what every element is, what it does, and where it is. A button labeled "Save" is always identifiable regardless of its visual style, position, or the current theme.
The Implementation Pattern
The MCP server runs as a local process with accessibility permissions. It uses the macOS Accessibility framework to:
- List running apps and their windows
- Build a tree of UI elements for a given app
- Execute actions on specific elements
- Return results including updated state after each action
The key challenge is keeping the element references stable between calls. UI trees change as the app updates, so the server needs to re-traverse and match elements intelligently.
What This Enables
With native app control through MCP, Claude can automate workflows that span multiple desktop apps - not just browser tabs. Data entry, report generation, cross-app data transfer, and testing all become possible without building custom integrations for each app.
- MCP Server for macOS Accessibility and Screen Capture
- MCP Servers Beyond Chat - Desktop Automation
- Native OS APIs vs Terminal for Desktop Agents
Fazm is an open source macOS AI agent. Open source on GitHub.