MCP Apps vs A2UI: UI as a Resource vs UI as a Protocol
With the introduction of MCP Apps, how does Anthropic's Model Context Protocol compare to Google's A2UI? A definitive guide on when to use which generative UI paradigm.
The world of Agentic UI is evolving at a breakneck pace. Following the massive, industry-wide success of the Model Context Protocol (MCP) originally spearheaded by Anthropic for data retrieval and tool-calling, the recent introduction of MCP Apps expanded the protocol’s ambitions to handle actual user interfaces.
At the exact same time, Google’s open-source A2UI is gaining immense traction as the definitive standard for “Agent-Driven Interfaces.”
Both protocols ultimately allow an AI agent to “show” a visual UI to the end user instead of just spitting out plain text. However, they take fundamentally different—and often opposing—architectural approaches to achieve this goal. In this deep dive, our engineering team compares MCP Apps (UI as a Resource) against A2UI (UI as a Protocol) across four critical dimensions to help you choose the right paradigm for your Next.js, Flutter, or React Native stack.
1. The Core Architecture: Sandboxes vs Blueprints
MCP Apps: The Iframe Sandbox
The MCP Apps mental model treats the UI as an opaque, external resource. When an agent determines it needs to show a custom interface, it requests a UI resource via a specialized ui:// URI. The responding MCP server generates and returns a complete bundle of pre-rendered HTML, CSS, and potentially executable JavaScript.
The host application (your chat interface) then takes this bundle and renders it blindly, typically by isolating it within a highly restricted <iframe>. The host app does not know—or care—what the HTML consists of; it simply acts as a dumb display container.
A2UI: The Native Blueprint
A2UI takes entirely the opposite approach. When an AI agent utilizes A2UI, it sends a streaming, strongly-typed JSON payload across the wire. This JSON contains absolutely no executable code. It is simply a declarative, mathematical blueprint (e.g., {"type": "Card", "children": [{"type": "Button", "label": "Click Me"}]}).
The host application receives this raw data structure and uses its own native, local components (like your custom React components or Flutter widgets) to securely draw the interface pixel-by-pixel.
2. Platform Portability
Winner: A2UI
If you are strictly building a web browser application, MCP Apps work brilliantly. Browsers possess decades of optimized engineering built specifically to render HTML via isolated iframes.
However, the moment you step outside the browser, the facade crumbles. If you are building a cross-platform product—say, a mobile app in Flutter or React Native, or a native desktop app utilizing Tauri or Electron—forcing those native engines to spin up off-screen webviews just to render arbitrary HTML iframes feels clunky, incredibly slow, and distinctly non-native.
Because A2UI relies on a dumb data protocol, the exact same JSON payload streamed from the agent will elegantly render as a native iOS UIButton on an iPhone, a Material Button on an Android device, and a React <Button> on the web.
3. Styling and Design Continuity
Winner: A2UI
Have you ever embedded a third-party weather widget or a stock ticker on your beautiful, carefully crafted website, only to find that its font, border radius, and clashing colors completely ruined your brand’s aesthetic? That is the exact risk you assume with MCP Apps.
Because the MCP App developer dictates the HTML and CSS bundle on their server, forcing that remote UI to perfectly match your host application’s highly specific design system (like your custom Tailwind config) is exceedingly difficult and often requires messy CSS overrides.
With A2UI’s upcoming advanced theming engine in v0.9, the AI agent never decides what the UI visually looks like; the agent only dictates what information is semantically shown. The host application’s local renderer retains absolute, dictatorial control over the styling cascade. A “primary button” generated by an agent via A2UI will look algorithmically identical to every other primary button in your app, automatically inheriting your dark mode status and exact design tokens.
4. Security, Isolation, and Vulnerability Boundaries
Winner: A2UI (for embedded workflows), Tie (for isolated third-party tools)
Following Anthropic’s release of the official MCP Apps Sandboxing Specification in January 2026, the security model relies heavily on iframe containment. The host application runs the generated HTML bundle inside a container with restricted permissions:
sandbox="allow-scripts allow-forms" (but crucially blocking allow-top-navigation and allow-same-origin).
While iframe isolation blocks access to host localStorage or cookie stores, it leaves a significant vulnerability surface open: Generative Phishing. If an LLM is compromised by a prompt injection attack, it can easily generate a pixel-perfect clone of a bank login portal or a Sign-in-with-Google form inside the iframe. Since the iframe has allow-forms and allow-scripts enabled, the user cannot easily visually distinguish this nested, AI-generated phishing field from a trusted host component.
A2UI bypasses this vulnerability class entirely via its Component Registry Firewall. Since the LLM is physically incapable of transmitting HTML or executable scripts, it can only instruct the client’s local renderer to mount pre-audited native widgets. The agent cannot craft a raw <form> or <input> to exfiltrate passwords. To understand this in depth, see our guide on Preventing Generative UI Phishing with the A2UI Firewall.
5. Accessibility (A11y) and Mobile Overhead
Winner: A2UI
Beyond security, production systems in 2026 must evaluate two critical dimensions:
- Accessibility (A11y): Iframes create isolated document fragments. Screen readers and assistive tools cannot easily traverse the host DOM into an opaque, third-party iframe bundle, especially when components inside are dynamically generated without semantic HTML tags. Because A2UI renders everything using local native components, it inherits the host app’s accessibility trees (like React ARIA, iOS Accessibility API, or Android Semantics), maintaining strict compliance.
- Performance Overhead: On web apps, rendering a few iframes is relatively cheap. However, on mobile apps built with Flutter, React Native, or SwiftUI, hosting MCP Apps requires spinning up heavy webviews (e.g.,
WKWebVieworAndroid WebView). This causes massive memory spikes, slows down transitions, and kills battery life. A2UI uses lightweight JSON payload parsing, drawing UI elements on the native canvas at a buttery-smooth 120Hz.
The Verdict: When to Choose Which?
- Use MCP Apps when: You are hosting a highly complex, self-contained, and isolated monolithic app (such as an interactive 3D WebGL renderer, an offline SQL console, or a complete drawing canvas) that has a bespoke interface that would be impractical to rebuild as reusable JSON components.
- Use A2UI when: You are embedding interactive widgets (forms, charts, tables, checkouts) inline within chat streams, copilots, or smart sidebars. A2UI ensures complete visual consistency, zero performance overhead on mobile/native platforms, and bulletproof security against UI injection attacks.