MCP UI: Extending the frontier — Liad Yosef and Ido Salomon, MCP Apps

Liad Yosef and Ido Salomon, co-creators of MCPUI and the MCP Apps specification, present the evolution of interactive UI within AI chat hosts at AI Engineer Europe 2026. The core thesis is that text-based tool responses are a fundamental limitation of current MCP implementations: companies like Shopify, Booking, and Expedia are reduced to unbranded walls of text when their tools respond, stripping them of identity and making outputs hard to parse.

Salomon originally released MCPUI in May 2025 as a mechanism for passing rich HTML UI over the MCP protocol. The concept was straightforward: instead of a tool returning plain text, it returns a resource — an actual HTML component — which the host renders inside a secure sandbox. The host then handles all bidirectional message-passing between the UI and the model, ensuring every user interaction (clicks, form submissions) stays in context rather than routing directly to the service's backend. This architectural decision is critical: if a user favorites a Spotify track via an embedded UI that talks directly to Spotify's backend, the model loses context of that action. MCP Apps solves this by standardizing a messaging layer so all interactions flow back through the host.

Within a few months of the original release, Anthropic and OpenAI formally partnered to adopt MCPUI as the first official extension to the MCP standard, renamed MCP Apps. Host adoption is already broad: VS Code, Cursor, Claude, ChatGPT, Microsoft Copilot, GitHub, Postman, Goose, LibreChat, and the terminal tool Spy all support MCP Apps. ChatGPT now recommends MCP Apps as the canonical way to build ChatGPT-integrated applications. Even prior to standardization, major companies had adopted MCPUI: all Shopify storefronts were already sending MCPUI chunks, and all Hugging Face Spaces were MCPUI widgets.

The architecture is technically straightforward for both server and host developers. Server-side, developers register a resource that returns HTML and wire it to a tool call. Host-side, implementation is a React component that accepts the resource and a message-passing callback, then renders inside a sandbox. The PostHog demonstration shown live in Claude illustrates the contrast: a funnel analysis request that previously returned a long text summary instead renders PostHog's own branded funnel chart component, which the user can click to drill into specific funnel steps.

The speakers introduce an interaction message spectrum ranging from notifications (UI retains most control, e.g., updating cart quantity without host involvement) through tool calls (UI instructs the host to call a specific tool) to prompts (UI surrenders full control and asks the host model to run a prompt). This spectrum standardizes how embedded UI hands off agency to the model.

On the generative UI front, the speakers clarify that MCP Apps is agnostic to how UI is generated. The spectrum runs from predefined (black-box server-generated HTML, as with Airbnb building its own component) through declarative UI (JSON Render-style schemas where the host supplies components) to fully generative UI (host model generates UI from scratch). Anthropic's recently shipped generative UI feature in Claude uses MCP Apps under the hood, streaming model-generated UI into an MCP App wrapper. The team is also pursuing interoperability with Google's A2UI protocol and WebMCP.

Upcoming roadmap items include reusable views (addressing Autodesk's performance complaint about re-rendering heavy apps on every turn), and model-driven UI interaction — a proposed standard by which Claude itself can click buttons or fill forms inside an embedded UI, exposing tools from the view to the model rather than only from user to model. An official SDK (xapps) keeps implementations automatically spec-compliant as the standard evolves.

Sam Altman's cited figure of 800 million weekly ChatGPT users (now reportedly 1 billion) is used to quantify the distribution opportunity: MCP Apps' potential audience is described as at least 160 times the iPhone App Store's user base at launch. The speakers frame 2026 as the year MCP Apps becomes the global standard for UI inside chat applications, and present the shift as a once-in-20-years opportunity to rethink application distribution — moving from monolithic destination websites to atomic UI chunks embedded within personal assistant flows.