> ## Documentation Index
> Fetch the complete documentation index at: https://docs.fastapps.org/llms.txt
> Use this file to discover all available pages before exploring further.

# How ChatGPT apps work

> Understand how ChatGPT apps work, and how to build one with FastApps

<img src="https://mintcdn.com/dooilabs/1i2xYz35eFX4C0kU/images/howappssdk.png?fit=max&auto=format&n=1i2xYz35eFX4C0kU&q=85&s=07879acea655d84824d84ead725ef61d" alt="How Apps SDK works" width="100%" data-path="images/howappssdk.png" />

## 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.

## Building ChatGPT apps

With FastApps, you could simply use the prebuilt library to resgister tools and widgets.

**Tools** (`server/tools/hello_tool.py`)

```python theme={null}
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`)

```jsx theme={null}
import React from 'react';

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

## 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.

## Next Steps

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