Open Generative UI

What is this?

Open Generative UI lets the agent generate complete, sandboxed UI on the fly (HTML, CSS, and JavaScript) and stream it live into the chat. The user sees the interface build in real time: styles apply first, then HTML streams in progressively, and finally JavaScript expressions execute one by one.

Key benefits:

No predefined components — the agent creates any UI it needs, on demand
Live streaming — HTML streams into a preview as it's generated
CDN libraries — the generated UI can load Chart.js, D3, Three.js, etc. via <script> tags
Secure sandboxing — content runs in an isolated iframe without same-origin access
Sandbox functions — optionally expose host functions to the generated UI for two-way communication

Minimal setup

Turning on Open Generative UI takes one flag in the runtime plus a plain <CopilotChat /> on the frontend; the built-in activity renderer is auto-registered by CopilotKitProvider, so no extra wiring is needed.

Enable it in the runtime

Add OpenGenerativeUIMiddleware to your runtime configuration:

The OpenGenerativeUIMiddleware then converts the agent's streamed generateSandboxedUi tool call into open-generative-ui activity events, which the built-in OpenGenerativeUIActivityRenderer mounts inside a sandboxed iframe.

Drop `<CopilotChat />` into the page

Wrap your app in CopilotKitProvider and render <CopilotChat> — no extra props needed:

That's it. Ask the agent "build me a simple greeting card" to see HTML stream into a sandboxed preview live.

Advanced: With app tool calling

Sandbox functions let the generated UI call back into your host application — a generated settings panel can toggle your app's theme, a product card can push items into your cart, or a data view can ask the host to fetch data the iframe can't reach directly.

Runtime is unchanged

The server-side flag is identical to the minimal cell; the advanced behaviour is a pure frontend addition.

Register sandbox functions on the provider

Each sandbox function is a Zod-validated, host-side bridge the agent can invoke from inside the generated iframe via Websandbox.connection.remote.<name>(args). The handler runs in the host page and its description is appended to the agent's context, so the agent knows which bridges are available when generating HTML/JS.

<Snippet region="sandbox-function-registration" cell="open-gen-ui-advanced" /> <Callout type="info" title="How the sandbox calls you back"> Inside the generated UI, the agent writes JS that calls `await Websandbox.connection.remote.notifyHost({ message: "hi" })`. The call is proxied back to the host page, where your `handler` runs with the validated args. </Callout>

Common use cases

Theme toggling — generated UI controls your app's appearance
Cart / state management — product cards push items into host state
Navigation — generated UI triggers route changes in the host app
Data fetching — sandbox asks the host to fetch data the iframe can't reach directly

How streaming works

The agent generates the tool call's parameters in an order optimized for the user experience:

placeholderMessages — shown immediately while generating
css — all styles first; the preview starts once CSS is complete
html — streams live into the preview as it's generated
jsFunctions — reusable helpers injected before expressions
jsExpressions — executed one by one; the user sees each take effect

The middleware parses the tool-call arguments incrementally and emits activity events as each parameter completes, so the preview updates progressively.

Using CDN libraries

The sandboxed iframe can load external libraries from CDNs; just include <script> or <link> tags in the generated HTML <head>. Chart.js, D3, Three.js, and any other CDN-hosted library work out of the box.

html

<head>
  <script src="https://cdn.jsdelivr.net/npm/chart.js"></script>
</head>
<body>
  <canvas id="myChart"></canvas>
</body>