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How Apps SDK works

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How ChatGPT apps work

At its core, a ChatGPT app is an MCP server that exposes multiple tools. Each tools return structured contents including a widget. Specifically, each tools include
  • Metadata : Description of the tool (so that the model could call the right tool at the right time), input schema, the message to be displayed before/after the tool is executed, etc.
  • Widgets : This is the component that is being displayed on the screen.

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Building ChatGPT apps

With FastApps, you could simply use the prebuilt library to resgister tools and widgets. Tools (server/tools/hello_tool.py) 
from fastapps import BaseWidget, ConfigDict
from pydantic import BaseModel
from typing import Dict, Any


class MyWidgetInput(BaseModel):
    model_config = ConfigDict(populate_by_name=True)

class MyWidgetTool(BaseWidget):
    identifier = "hello"
    title = "My Widget"
    input_schema = MyWidgetInput
    invoking = "Loading widget..."
    invoked = "Widget ready!"

    widget_csp = {
        "connect_domains": [],
        "resource_domains": []
    }

    async def execute(self, input_data: MyWidgetInput, context=None, user=None) -> Dict[str, Any]:
        return {
            "message": "Welcome to FastApps"
        }
Widgets (widgets/hello/index.jsx) 
import React from 'react';

export default function HelloWidget() {
  return (
    <div>
      <h1>Hello world!</h1>
    </div>
  );
}

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What FastApps provide

FastApps provide everything you need to build sophisticated ChatGPT apps: Widget Registration: Use the BaseWidget class to simply register it as an MCP tool. We handle the rest - no complex configuration or manual registration needed. CLI commands: Just type fastapps init to set up the whole project. Use fastapps create mywidget to make an widget. Everything from component creation to MCP tool registeration will be set up automatically. Simple auth: Authentication and other advanced features are all handled with our simple decorators. Just add @auth_required and you’re done.

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Next Steps

Quick Start

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