CONFIGURATION

Root class name Defensive number casts Detect DateTime Force all nullable

{ }input.json

{
  "order_id": "ord_8F42",
  "customer": {
    "first_name": "Alex",
    "email": "[email protected]",
    "is_verified": true
  },
  "total": 149.95,
  "created_at": "2026-11-03T10:20:30Z",
  "items": [
    {"sku": "KB-101", "quantity": 1, "price": 129.95},
    {"sku": "ST-204", "quantity": 2, "price": 10.0, "note": "Gift"}
  ]
}

                    {
                      "order_id": "ord_8F42",
                      "customer": {
                        "first_name": "Alex",
                        "email": "alex@example.com",
                        "is_verified": true
                      },
                      "total": 149.95,
                      "created_at": "2026-11-03T10:20:30Z",
                      "items": [
                        {"sku": "KB-101", "quantity": 1, "price": 129.95},
                        {"sku": "ST-204", "quantity": 2, "price": 10.0, "note": "Gift"}
                      ]
                    }

Dorder.dart

import 'package:freezed_annotation/freezed_annotation.dart';
part 'order.freezed.dart';
part 'order.g.dart';

@Freezed(makeCollectionsUnmodifiable: false)
abstract class Order with _$Order {
  const factory Order({
    @JsonKey(name: "order_id")
    required String orderId,
    required Customer customer,
    required double total,
    @JsonKey(name: "created_at")
    required DateTime createdAt,
    required List<Item> items,
  }) = _Order;

  factory Order.fromJson(Map<String, dynamic> json) => _$OrderFromJson(json);
}

@Freezed(makeCollectionsUnmodifiable: false)
abstract class Customer with _$Customer {
  const factory Customer({
    @JsonKey(name: "first_name")
    required String firstName,
    required String email,
    @JsonKey(name: "is_verified")
    required bool isVerified,
  }) = _Customer;

  factory Customer.fromJson(Map<String, dynamic> json) => _$CustomerFromJson(json);
}

@Freezed(makeCollectionsUnmodifiable: false)
abstract class Item with _$Item {
  const factory Item({
    required String sku,
    required int quantity,
    required double price,
    String? note,
  }) = _Item;

  factory Item.fromJson(Map<String, dynamic> json) => _$ItemFromJson(json);
}

Convert JSON to Freezed 3.x

Paste a JSON object to generate Freezed model declarations. Copy the output into a Dart file whose name matches the generated part directives.

Output behavior

Each inferred object becomes an @Freezed class with a fromJson factory. The file declares matching .freezed.dart and .g.dart parts. @JsonKey(name: ...) keeps original JSON keys when Dart fields use camelCase. The output sets makeCollectionsUnmodifiable to false.

Project setup

flutter pub add freezed_annotation json_annotation
flutter pub add --dev freezed:^3.2.5 build_runner json_serializable
dart run build_runner build

Before using it

Run build_runner from the project root after saving the file. Keep the generated part files with the source file; changing the root class name changes their expected filenames.