Overview
Open-ended generative UI is at the agent-created end of the generative UI spectrum. The interface is authored outside your application, for example by an MCP server, and your frontend renders it inside a sandbox. Neither you nor the agent writes the components: a third party ships them, and your app hosts them. This approach gives the widest expressive range: the agent owns the canvas. A capability can arrive with its own interface already built, so you can surface interactive tools you never implemented, with no frontend code on your side. It suits one-off visualizations and bespoke answers where a result that is surprising and good enough beats one that is predictable. It is also the most experimental approach, the least deterministic, slower, and more expensive to run, and untrusted UI is the hardest to make consistent, accessible, and safe, so it must be isolated from the rest of your application.When to use this approach
Reach for open-ended generative UI when you want to surface capabilities and interfaces that live outside your application and evolve independently of it, such as tools published by an ecosystem of MCP servers. When you need to guarantee branding, accessibility, or layout, move back along the spectrum toward declarative or controlled generative UI, where your application owns the components.MCP Apps
The Model Context Protocol lets an agent connect to external servers that provide tools and resources. MCP Apps extend that idea to interface: an MCP server ships interactive UI, and the frontend renders it, usually in an iframe, directly in the conversation. The server owns the components, the data, and the interactions, while your application provides the frame and the connection to the agent. CopilotKit documents this pattern as MCP Apps.Sandboxing and safety
Because the interface comes from a third party, treat it as untrusted. Render it in an isolated context, such as a sandboxed iframe, and constrain what it can access so a misbehaving or malicious app cannot reach the rest of your page or your users’ data. Sandboxing is what makes the open end of the spectrum usable in production: it contains the expressive range rather than limiting it.See also
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