Text-based dialogues, often reminiscent of cumbersome question-answer games, are increasingly being replaced by dynamic, AI-generated user interfaces. Imagine, instead of asking a chatbot multiple times for date, time, and number of guests for a table reservation, the AI agent instantly generates a native form directly in your application. This vision is exactly what drives the development of so-called “Agentic UIs.”
At the forefront of this movement, two competing technological approaches are crystallizing: Google’s open standard A2UI (Agent-to-User Interface) and the MCP-UI (Model Context Protocol UI) initiated by Anthropic and further developed with OpenAI, now part of MCP Apps. This article provides an in-depth technical comparison of both protocols, analyzes their fundamental philosophies, and offers strategic guidance for developers, product managers, and architects.
What is Google A2UI? The “Native-First” Approach
A2UI is an open-source standard developed by Google, published under the Apache 2.0 license, designed to enable standardized communication between AI agents and graphical user interfaces (GUIs). A2UI’s approach can be described as “native-first” and is based on a clear separation of structure and presentation.
The Core Idea: A Declarative, Native Approach
Instead of sending executable code like HTML, CSS, or JavaScript, the AI agent in A2UI transmits a purely declarative JSON structure. This structure abstractly describes which UI elements should be displayed. The receiving client application (e.g., a web app, a mobile Flutter/SwiftUI app, or a desktop application) is responsible for interpreting this abstract description and rendering it with its own native UI components.
- Security by Design: Since no executable code is transmitted, the risk of code injection attacks is systemically minimized. The agent can only request components from a catalog predefined and approved by the client (e.g., “Button”, “TextField”, “Card”). Control remains entirely with the host application.
- Seamless User Experience and Brand Conformity: Because the client application uses its own, already styled and compliant UI libraries, the generated interface fits perfectly into the existing design, accessibility, and performance of the app. Visual breaks, common with iframes, are avoided.
- Platform Independence: A single A2UI response from the agent can be rendered on vastly different platforms. The same JSON payload works for an Angular web application as well as a native iOS app.
A2UI is already actively used in Google products such as the GenUI SDK for Flutter and Gemini Enterprise. Key ecosystem partners, such as the developers of CopilotKit and AG UI, support the standard and drive its adoption forward.
What is MCP-UI? The “Resource-First” Approach
The approach initiated by Anthropic and standardized in cooperation with OpenAI as “MCP Apps” follows a fundamentally different philosophy. Here, the user interface is considered as a self-contained “resource” provided by an AI tool.
The Core Idea: UI as Isolated Resource
An AI model or a connected tool can return a special ui:// URI. The host application retrieves this resource, which typically consists of pre-built HTML, CSS, and JavaScript. To ensure security, this resource is not directly integrated into the page but executed in an isolated sandbox, usually an iframe.
- Security through Sandboxing: Strict isolation within a sandbox prevents the UI code from accessing the host application or local data. This creates a robust security boundary between the trusted host and potentially untrusted UI code.
- Easy Deployment: Developers of AI tools can create and deploy UI components as standalone web applications (HTML/JS). The complexity of client-side integration is low, as only a standardized container (the iframe) is needed.
- Potential Drawbacks: The biggest compromise is user experience. A UI rendered in an iframe rarely adapts seamlessly to the host application’s design. Styling, fonts, animations, and the overall “look-and-feel” can vary significantly, leading to a fragmented and less professional impression.
Direct Comparison: A2UI vs. MCP-UI
The choice between A2UI and MCP-UI depends on a project’s priorities: Is maximum integration and native feel the goal, or maximum isolation and easy deployment?
| Criterion | A2UI (Google) | MCP-UI (Anthropic/OpenAI) |
|---|---|---|
| Philosophy | Native-First: Sends an abstract UI blueprint (JSON) that is rendered natively. | Resource-First: Sends a URI to a pre-built UI (HTML/JS) that is loaded. |
| Rendering Technique | Client-side by the host application with its own UI components (Angular, React, Flutter, etc.). | In an isolated sandbox, typically an iframe. |
| User Experience & Styling | Seamless integration into the host app’s design. Feels 100% native. | Often visually inconsistent and recognizable as an “embedded foreign element.” |
| Security Model | Declarative data transmission. No executable code. Protection against UI injection. | Code execution in a strict sandbox to isolate from the host application. |
| Portability | Very high. One agent payload works on web, mobile, and desktop, as long as a renderer exists. | High, as long as the client platform can provide a web renderer for the sandbox. |
The Role of OpenAI’s ChatKit: The Ecosystem Solution
Besides the open protocols A2UI and MCP-UI, ChatKit from OpenAI exists as another relevant solution. It’s important to correctly classify ChatKit: It’s less an open protocol and more a highly integrated framework for creating AI applications within the OpenAI ecosystem. You could compare it to an “end-to-end” solution perfectly tuned to OpenAI’s models and services, but in return, less focused on interoperability with external agents or platforms.
Strategic Decision: Which Approach is Right for Your Project?
Choosing the right technology is a strategic decision with direct impacts on product quality, security, and development effort.
Choose A2UI if…
- …a seamless, brand-compliant user experience is the top priority.
- …you want full control over the design and behavior of every UI element.
- …you develop for multiple platforms (web, iOS, Android) and want to use the same agent backend.
Choose MCP-UI if…
- …maximum isolation and simple integration is a priority.
- …you want to integrate third-party UIs whose code you don’t fully trust.
- …development speed is crucial and you want to leverage existing web components.
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