Access parent component value from the Repeater in Filament

If you want to interact with fields outside of Repeater input, then you can use use $get('../../parent_field_name').

Yes, $get is very powerful function.

Let's say you have few Fields in the form - Order fields.

<?php

use App\Models\Product;
use Filament\Forms;
use Filament\Forms\Form;

public static function form(Form $form): Form
{
    return $form
        ->schema([
            Forms\Components\TextInput::make('order_number')
                ->label('Order Number')
                ->placeholder('Order Number')
                ->maxLength(20)
                ->required(),

            Forms\Components\Repeater::make('products')
                ->relationship()
                ->schema([
                    Forms\Components\TextInput::make('product_code')
                        ->label('Product Code')
                        ->required(),

                    Forms\Components\Select::make('product_id')
                        ->label('Product')
                        ->options(Product::query()->pluck('name', 'id'))
                        ->required()
                        ->reactive()
                        ->afterStateUpdated(fn ($state, Forms\Set $set) =>
                            $set('price', Product::find($state)?->retail_price ?? 0)
                        )
                        ->distinct()
                        ->disableOptionsWhenSelectedInSiblingRepeaterItems()
                        ->searchable(),

                    Forms\Components\TextInput::make('quantity')
                        ->label('Quantity')
                        ->numeric()
                        ->default(1)
                        ->required(),

                    Forms\Components\TextInput::make('price')
                        ->label('Unit Price')
                        ->disabled()
                        ->numeric()
                        ->required(),
                ]),
        ]);
}

Now, you want to access the order number in the product repeater, you can do with

Forms\Components\TextInput::make('product_code')
    ->label('Product Code')
    ->required()
    ->afterStateUpdated(function(Forms\Get $get) {
        return $get('../../order_number'); // using .. you can access parent component value. same as directory structure.
    }),

For more details you can refer the Filament doc for the same.

and there you go!..