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How does ChatGPT apps work?

Your MCP server is the foundation of every ChatGPT App. It exposes tools that the model can call, enforces authentication, and packages the structured data plus component that the ChatGPT client renders inline. This guide walks through the core building blocks with examples in FastApps.

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Tool Architecture

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BaseWidget Class

Every FastApps tool must inherit from BaseWidget: 
class MyWidget(BaseWidget):
    # Required attributes
    identifier = "my_widget"           # Unique identifier
    title = "My Widget Title"          # Display name
    input_schema = MyInputModel        # Pydantic model
    invoking = "Setting up widget…"    # Progress message
    invoked = "Widget ready!"          # Completion message
    
    # Optional attributes
    description = "Widget description" # Help text
    widget_accessible = True           # Allow component calls
    
    def execute(self, inputs, ctx):
        # Your business logic here
        return {"data": "processed"}

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Next Steps

Ready to start building?

Tool Basics

Learn the fundamentals

API Integration

Connect to external services

Advanced Patterns

Complex business logic
Or jump straight to the Quick Start Guide for a complete walkthrough.