Belgie: Build MCP App UIs for Claude and ChatGPT from Python

Documentation: mplemay.github.io/belgie

Belgie lets a Python MCP server ship interactive React UIs that render inside AI hosts. Write the tool in Python and the view in React. Belgie runs Vite with hot reload during development, bundles each view into a single self-contained HTML resource for production, generates typed tool callers from your tool schemas, and gives components the host's theme, layout, and display mode.

Views follow the open MCP Apps standard, so one widget renders in Claude, in ChatGPT apps and plugins, and in other MCP Apps hosts such as VS Code and Goose.

• Python tools, React views: Attach a widget.tsx to an MCP tool with one decorator argument.
• One UI across hosts: Target MCP Apps once; use useHostInfo() and isWidget() when a view needs to adapt to Claude, ChatGPT, or a normal web page.
• Vite without a Node toolchain: Declare JavaScript dependencies in pyproject.toml. Belgie's embedded Deno runtime resolves them and runs Vite, so the Python project does not need Node.js.
• Typed tool calls: belgie generate turns MCP tool schemas into typed TypeScript callers, and useToolResult wires them to the opening result and later refreshes.
• Host-native behavior: Read theme, locale, safe areas, and device type; request fullscreen, open modals, send follow-up messages, and update model context from the view.
• Agent-generated UI: Let Pydantic AI and LangChain agents author React views at runtime with render_widget, alongside a sandboxed JavaScript and TypeScript tool.

Installation

uv add "belgie[mcp,cli]"
uvx library-skills install  # optional: install the use-belgie skill for Cursor, Codex, Claude, etc.

Install the base package with uv add belgie when you only need the runtime or agent integrations.

Build an MCP App

MCP Apps fit tasks where text alone is a poor interface: charts and dashboards over query results, review and approval forms before a write action, pickers for products, files, or records, and viewers or editors for structured data. The model calls the tool, the host renders your view with the result, and the user keeps working in the UI.

Attach a React widget to a Python MCP tool. BelgieExtension starts Vite in the background for development and runs a one-time production build:

from datetime import UTC, datetime
from pathlib import Path

from mcp.server import MCPServer

from belgie.mcp import BelgieExtension

belgie = BelgieExtension(project=".")


@belgie.tool(
    widget=Path("src/widgets/get-time/widget.tsx"),
    name="get-time",
    title="Get Time",
    description="Get the current server time in ISO 8601 format.",
)
def get_time() -> dict[str, str]:
    return {"time": datetime.now(tz=UTC).isoformat()}


mcp = MCPServer(name="Get Time Server", extensions=[belgie])

The widget is a normal React entry. @belgie/mcp connects to the MCP Apps host and surfaces the opening tool result:

import { Widget, useToolResult } from "@belgie/mcp";
import { getTime } from "@widgets/tools";

function AppView() {
  const { data, isLoading, execute } = useToolResult(getTime);
  return (
    <main>
      <p>{data?.time ?? (isLoading ? "Waiting..." : "No time returned.")}</p>
      <button onClick={() => void execute()}>Refresh</button>
    </main>
  );
}

export default function GetTime() {
  return (
    <Widget metadata={{ name: "Get Time", version: "1.0.0" }}>
      <AppView />
    </Widget>
  );
}

Declare JS deps under [tool.belgie.dependencies], then:

uv run belgie lock
uv run belgie install
# start your MCP server; Belgie starts Vite with widget HMR

Pass build=False to BelgieExtension when Vite is managed separately or production assets are already built.

Connect the server to Claude as a custom connector, or to ChatGPT through developer mode or a plugin, and the view renders when the model calls the tool.

Runnable projects:

• mcp: Minimal MCP Apps widget.
• shadcn: Same pattern with Tailwind CSS and shadcn/ui.
• tanstack: One React codebase served as a TanStack Start SPA and as an MCP widget through FastAPI.

Agent-generated UI

Pydantic AI and LangChain agents can return a complete inline React widget through the render_widget tool (alongside run_typescript / run_code). Enable rendering on the sandbox or middleware. The model passes a default-export TSX module and does not call render():

export default function Widget() {
  return <main>Hello from Belgie</main>;
}

from belgie.pydantic_ai import BelgieSandbox

capability = BelgieSandbox(enable_rendering=True, plugins=[])

render_widget builds HTML with @belgie/vite on a Belgie-owned renderer side-channel (not in the model-visible Deno worker). The agent Script stays workspace-restricted (no host /etc//proc, allow_sys, or allow_ffi), while Vite runs only in that host-mediated worker (workspace read/write, FFI under node_modules, localhost network, limited allow_sys; host env denied) and returns one self-contained HTML string with inline JavaScript, CSS, and assets. Host-configured Vite plugins run only during the server build; treat them as reviewed application code and use plugins=() for untrusted agents. This API is independent from @belgie/mcp and its path-based widget.tsx development and production flow.

See examples/ui/pydantic-ai for a web app that renders agent-authored UI from a prompt.

AI agents

When an agent needs a browser-style API or a JavaScript transformation, give it the Belgie sandbox tool. Pydantic AI uses run_typescript; LangChain uses run_code. Belgie executes JavaScript, TypeScript, or TSX in the embedded Deno sandbox. The Python runtime does not require a separate Node install.

Pydantic AI

Install with uv add "belgie[pydantic-ai]", set OPENAI_API_KEY, then:

from pydantic_ai import Agent

from belgie.pydantic_ai import BelgieSandbox

agent = Agent("openai:gpt-5", capabilities=[BelgieSandbox()])

result = agent.run_sync(
    "Convert 'foo-bar' to camelCase using TypeScript.",
)
print(result.output)

See examples/ai/pydantic-ai.

LangChain

Install with uv add "belgie[langchain]", set OPENAI_API_KEY, then:

from langchain.agents import create_agent

from belgie.langchain import BelgieMiddleware

agent = create_agent(
    model="openai:gpt-5",
    tools=[],
    middleware=[BelgieMiddleware()],
    system_prompt="You can execute JavaScript or TypeScript in a Deno sandbox with run_code.",
)

result = agent.invoke(
    {
        "messages": [
            (
                "user",
                "Convert 'foo-bar' to camelCase using TypeScript.",
            ),
        ],
    },
)
print(result["messages"][-1].content)

See examples/ai/langchain.

Under the hood: Deno in Python

MCP App builds, agent-generated UI, and both agent sandbox integrations use Belgie's embedded Deno runtime. Call it directly when you need JavaScript or TypeScript from Python without MCP or an agent framework:

• Scripts: Inline or file-based JS/TS with Runtime and Script, sync or async.
• Inline dependencies: Import npm, JSR, and URL modules from source.
• Environments: Lockfiles, custom cache/options, local packages, and Command for npm binaries (Vite, esbuild, etc.).
• Data bridge: Pass JSON-safe dicts, lists, and primitives across the boundary.

Runtime permissions

RuntimePermissions gates Deno APIs and every host-backed module read, including static and dynamic imports, JSON modules, and Node require(). File entrypoints created with Script.from_file and command entrypoints must be covered by allow_read; inline and in-memory sources do not need a host read grant. Belgie-managed npm packages are available to the module loader without adding their node_modules or cache roots to the runtime's general read grants. Package imports therefore work in restricted runtimes, while Deno.readFile, arbitrary absolute file: URLs, and other direct host reads remain subject to the caller's allow_read and deny_read settings.

import asyncio

from belgie import Runtime, Script

script = Script[[str], str](
    """
import camelcase from "npm:camelcase@8.0.0";

export default function run(input: string): string {
  return camelcase(input);
}
"""
)


async def main() -> None:
    async with Runtime() as run:
        print(await run(script)("foo-bar"))  # prints: fooBar


asyncio.run(main())

Examples

Small, runnable projects. Each focuses on one capability.

UI

• mcp: MCP Apps extension with a React widget built through Belgie.
• shadcn: MCP Apps widget styled with Tailwind CSS and shadcn/ui.
• tanstack: TanStack Start SPA and MCP widget served together through FastAPI.
• pydantic-ai: FastAPI SPA that renders agent-authored UI from a prompt.

AI

• pydantic-ai: Pydantic AI agent with BelgieSandbox() for sandboxed JS/TS/TSX execution.
• langchain: LangChain agent with BelgieMiddleware() for sandboxed JS/TS/TSX execution.

Basic

• simple: Async Runtime with a TypeScript file on disk.
• inline-deps: Direct npm:, jsr:, and URL imports in a script.
• jsr-deps: JSR packages declared through an explicit Environment.
• pyproject: Manage project package dependencies with belgie[cli] and [tool.belgie.dependencies].
• environment: Sync and async Environment setup with path.
• commands: npm package binaries via Runtime and Command.

For deeper integration guidance, optionally install the use-belgie skill with uvx library-skills install.