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Beyond Components: Designing Generative UI for MCP Apps — Ruben Casas, Postman
Takeaway
Separate where an agent interface runs from how it is generated, and choose a UI approach that balances flexibility and predictability.
Summary
- MCP apps can embed third-party interfaces in an agent host, while individual products can also add their own chat interfaces.
- Static UI maps tool-call data onto predefined components; examples include AG-UI and Goose Auto Visualizer.
- Declarative UI lets models generate JSON or YAML layouts rendered through a component catalog, as in Vercel's JSON Render.
- Declarative generation currently balances personalization with design-system consistency, while runtime generation of React, JavaScript, and CSS offers a more flexible next step.
generative-uimcp-appsdesign-systems
Original description
Ruben Casas from Postman prompted a model to rewrite his blog. It built a search box with a blur animation and accessibility out of the box, without being asked. That was when he concluded the model writes better frontend code than he does. His question for the talk: if the models are this capable, why are most agent UIs still invoking static prebuilt components? The talk maps three points on the spectrum. Static components pass props to predefined React elements (AG UI, Goose auto visualizer). Declarative UI has the LLM generate JSON or YAML that a rendering engine maps to components at runtime. Ruben argues this is the right balance today. Fully generative UI skips components entirely: the model writes HTML, CSS, and JavaScript on demand. His weather agent does this in a single tool call. The catch is containment: LLM generated code needs a sandbox, which is why MCP apps and their double iframe default matter. He closes on the TV analogy. The first TV shows were radio shows with cameras because nobody could imagine what else to do with the new medium. We are in that era. Speaker info: https://x.com/Infoxicador / ruben-casas-17100383 Timestamps: 0:00 Introduction: The evolution from 'poor man's bit coding' to high-fidelity UI generation 2:56 Why are we still stuck with static UI? 3:33 The new computer: Searching for the interface of the future 4:47 The role of MCP apps and 'Super Apps' 5:52 Three levels of UI generation: Static, Declarative, and Generative 6:03 Understanding Static UI components (e.g., AG UI, Goose) 7:42 The benefits of Declarative UI (e.g., JSON/YAML renderers) 10:06 Moving to the next level: Generative UI components 11:25 The challenge of trust: The need for sandboxing and containment 12:22 Why MCP apps are the ideal delivery mechanism for Generative UI 13:21 The 'TV/Radio' analogy: Imagining the future of agent interaction 14:46 Beyond components: Towards true human-agent collaboration 16:16 Conclusion: Shaping the future of user interfaces