A2UI vs MCP-UI: Understanding the Protocol Landscape

TL;DR: MCP handles context (what the AI knows). A2UI handles presentation (what the user sees). They’re complementary, not competing.

The Confusion

With multiple “AI protocols” emerging in 2024-2025, developers are understandably confused. This article clarifies the landscape.

Protocol Overview

Feature	A2UI	MCP (Model Context Protocol)
Primary Purpose	UI Rendering	Context Injection
Data Flow	Agent → User	Data Sources → Agent
Output	Visual Components	Text/Structured Data
Security Focus	Prevent UI injection	Authorize data access

When to Use A2UI

A2UI shines when your agent needs to show something to the user beyond plain text:

1. Rich Data Display: Flight cards, restaurant listings, product catalogs.
2. Interactive Forms: Dynamic surveys, booking flows, multi-step wizards.
3. Data Visualization: Charts, maps, dashboards.

{
  "type": "flight-card",
  "props": {
    "airline": "TechAir",
    "departure": "SFO",
    "arrival": "NRT",
    "price": 1250
  }
}

When to Use MCP

MCP excels when your agent needs to know something:

1. Database Access: Query your PostgreSQL, Supabase, or Firebase.
2. File System: Read local documents, CSVs, PDFs.
3. API Integration: Connect to Salesforce, Jira, Notion.

The Synergy: Using Both Together

The most powerful agentic applications use both protocols:

[User Query] → [MCP: Fetch Data] → [LLM: Reason] → [A2UI: Render UI]

Example Flow:

1. User asks: “Show me my upcoming flights.”
2. MCP fetches flight data from the user’s calendar and airline API.
3. LLM processes and prioritizes the results.
4. A2UI renders a beautiful <FlightList /> component.

Security Model Comparison

A2UI: “Vending Machine” Approach

• The client pre-registers allowed components.
• The agent can only request from the approved menu.
• Result: Zero risk of arbitrary code execution.

MCP: “Gated Access” Approach

• Each data source has explicit permissions.
• The user authorizes which sources the agent can access.
• Result: Controlled data exposure.

Conclusion

Don’t choose between A2UI and MCP. Choose both for a complete agentic stack:

Layer	Protocol
Context (Input)	MCP
Presentation (Output)	A2UI

---