A2UI vs MCP Apps: Choosing the Right Protocol

Introduction

As AI agents become more capable, developers face a new challenge: how should agents present interactive UI to users? Two protocols have emerged as leading solutions: A2UI from Google and MCP Apps (part of the Model Context Protocol ecosystem) backed by Anthropic and OpenAI.

Both protocols solve the same fundamental problem — enabling agents to generate dynamic UI — but they take fundamentally different approaches. This guide will help you understand the trade-offs and choose the right tool for your project.

Quick Comparison

Deep Dive: A2UI

A2UI takes a native-first approach. Instead of sending HTML or JavaScript, agents send JSON "blueprints" that describe what UI components should be rendered. The client application then renders these using its own native component library.

How It Works

1. Agent generates JSON describing the desired UI
2. Client validates the request against its component catalog
3. Client renders using native components (React, SwiftUI, Flutter, etc.)
4. User interactions are captured and sent back to the agent

Strengths

• Native look and feel: UI matches the host application perfectly
• Security: No code execution, only declarative data
• Performance: Native rendering is faster than WebViews
• Accessibility: Native components inherit platform accessibility features
• LLM-optimized: Flat JSON structure is easy for models to generate

Weaknesses

• Limited flexibility: Can only use pre-approved components
• Renderer dependency: Need a renderer for each platform
• Early stage: Still in preview, API may change
• Smaller ecosystem: Fewer components and tools available

Deep Dive: MCP Apps

MCP Apps extends the Model Context Protocol with UI capabilities, taking a web-centric approach. Agents can generate HTML, CSS, and JavaScript that runs in a sandboxed iframe.

How It Works

1. Agent generates web content (HTML/CSS/JS)
2. Client creates a sandboxed iframe
3. Content is loaded into the iframe with strict CSP
4. Communication happens via postMessage API

Strengths

• Maximum flexibility: Can create any web UI
• Familiar technology: Uses standard web technologies
• Rich ecosystem: Access to web libraries and frameworks
• Production ready: Mature and battle-tested
• Quick prototyping: Easy to experiment and iterate

Weaknesses

• Non-native feel: May look different from host app
• Security complexity: Requires careful sandboxing
• Performance overhead: WebView has more overhead than native
• Mobile limitations: Some web features don't work well in mobile WebViews

When to Use A2UI

A2UI is the better choice when:

• Building native applications: If you're developing for iOS, Android, or desktop with native technologies, A2UI ensures UI consistency.
• Security is paramount: For applications handling sensitive data where code execution risks are unacceptable.
• Cross-platform consistency: When the same agent needs to work across web, mobile, and desktop with a unified experience.
• Performance matters: For real-time or resource-constrained applications where WebView overhead is problematic.

When to Use MCP Apps

MCP Apps is the better choice when:

• Web-first applications: If your agent primarily runs in web browsers, MCP Apps provides a more direct path.
• Maximum UI flexibility: When you need to create complex visualizations, charts, or highly custom interfaces.
• Rapid prototyping: For quickly testing ideas without building custom components.
• Existing MCP integration: If you're already using MCP for tools and data, adding Apps is a natural extension.

Can You Use Both?

Yes! A2UI and MCP Apps are not mutually exclusive. In fact, they complement each other well:

• MCP handles tool calls, data fetching, and external integrations
• A2UI handles the UI presentation layer

A typical architecture might use MCP to connect your agent to databases, APIs, and external services, while using A2UI to present the results in a native-feeling interface.

Additionally, AG-UI (Agent-User Interface protocol from CopilotKit) has achieved "Day-Zero" compatibility with A2UI, providing another bridge between these ecosystems.

Decision Framework

Ask yourself these questions:

1. Is native platform integration important?
   Yes → Lean toward A2UI
2. Do I need maximum UI flexibility?
   Yes → Lean toward MCP Apps
3. Am I building for mobile platforms?
   Yes → A2UI typically provides better UX
4. Do I need production stability today?
   Yes → MCP Apps is more mature
5. Is my team more comfortable with native or web development?
   Native → A2UI; Web → MCP Apps

Conclusion

Both A2UI and MCP Apps are powerful solutions for agent-generated UI, each with distinct strengths:

• Choose A2UI for native applications, security-critical use cases, and cross-platform consistency
• Choose MCP Apps for web-first applications, maximum flexibility, and rapid prototyping

The good news is that you don't have to make an all-or-nothing choice. Many applications will benefit from using both protocols together, leveraging each for its strengths.

As the agent ecosystem continues to evolve, expect these protocols to mature and potentially converge in some areas. Stay updated by following the A2UI GitHub repository and the MCP documentation.

Further Reading

• Introduction to A2UI
• Getting Started with A2UI
• Official A2UI Documentation
• Google's A2UI Announcement