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GenerativeUI lets the LLM write Prefab Python code at runtime and render it as a streaming interactive UI. Instead of calling pre-built tools with fixed interfaces, the model creates tailored visualizations for whatever data it’s working with. 
from fastmcp import FastMCP
from fastmcp.apps.generative import GenerativeUI

mcp = FastMCP("My Server")
mcp.add_provider(GenerativeUI())
This registers:
ComponentTypePurpose
generate_prefab_uiToolAccepts Python code, executes in Pyodide sandbox, renders result
search_prefab_componentsToolLets the LLM discover available Prefab components
Generative rendererResourceui:// resource with browser-side Pyodide for streaming
The LLM writes real Python — loops, f-strings, computation — using Prefab’s component library (charts, tables, forms, cards, layout primitives). As the model generates tokens, the host streams partial code to the renderer via ontoolinputpartial, so the user watches the UI build up in real time.

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Configuration

GenerativeUI(
    tool_name="generate_prefab_ui",                # Rename the generation tool
    components_tool_name="search_prefab_components",  # Rename the search tool
    include_components_tool=True,                   # Set False to omit the search tool
)

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What the LLM Sees

The tool description includes code examples that teach the LLM the Prefab patterns. The LLM calls generate_prefab_ui with a code argument containing Prefab Python, and optionally a data argument to pass in real data from the conversation: 
# The LLM generates something like:
generate_prefab_ui(
    code="""
from prefab_ui.components import Column, Heading
from prefab_ui.components.charts import BarChart, ChartSeries
from prefab_ui.app import PrefabApp

with PrefabApp() as app:
    with Column(gap=4):
        Heading("Revenue")
        BarChart(data=data, series=[ChartSeries(data_key="revenue")], x_axis="quarter")
""",
    data={"data": [{"quarter": "Q1", "revenue": 42000}, ...]}
)
The component search tool lets the LLM discover what’s available before writing code — search_prefab_components("Chart") returns matching components with import paths.

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Requirements

Requires fastmcp[apps] (installs prefab-ui). The Pyodide sandbox for server-side validation requires Deno, which installs automatically on first use. The streaming renderer loads Pyodide from CDN in the browser — CSP is configured automatically. The sandbox includes the Python standard library and Prefab. External packages (NumPy, pandas, etc.) are not available.

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Learn More

The full Generative UI guide covers the streaming mechanics in detail, how to pass data, the component search tool, and sandbox limitations.