missing methods to override in Freezed Flutter

I am trying to use freeze with json_serializer . Response from Api is

{
    "CustomData": {
        "message": "OTP sent successfully",
        "msisdn": "1273"
    }
}

Now as this response comes from sendOTP api . so I do it like

import 'package:freezed_annotation/freezed_annotation.dart';

part 'send_otp_response.freezed.dart';
part 'send_otp_response.g.dart';

@freezed
class SendOtpResponse with _$SendOtpResponse {
  const factory SendOtpResponse({
    @JsonKey(name: 'CustomData') required CustomData customData,
  }) = _SendOtpResponse;

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

@freezed
class CustomData with _$CustomData {
  const factory CustomData({
    @Default('') String message,
    @Default('') String msisdn,
  }) = _CustomData;

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

and I also run the command

flutter packages pub run build_runner watch --delete-conflicting-outputs

But when I hover over SendOtpResponse . It shows an error

Missing concrete implementations of '_$SendOtpResponse. toJson' and 'getter _$SendOtpResponse. customData'. (Documentation) Try implementing the missing methods, or make the class abstract.

and when I hover customData it says

Missing concrete implementations of '_$CustomData. toJson', 'getter _$CustomData. message', and 'getter _$CustomData. msisdn'. (Documentation) Try implementing the missing methods, or make the class abstract.

for customData . It says two missing methods and for sendOTPResponse it says 3 missing methods.

I also make sure the name of the class is send_otp_response.dart

if you using freezed 3.0.0 or newer make you class to abstract class can fix this issue

you can read in this patch note in other change section
ref. https://pub.dev/packages/freezed/changelog#300---2025-02-25

import 'package:freezed_annotation/freezed_annotation.dart';

part 'send_otp_response.freezed.dart';
part 'send_otp_response.g.dart';

@freezed
abstract class SendOtpResponse with _$SendOtpResponse {
  const factory SendOtpResponse({
    @JsonKey(name: 'CustomData') required CustomData customData,
  }) = _SendOtpResponse;

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

@freezed
abstract class CustomData with _$CustomData {
  const factory CustomData({
    @Default('') String message,
    @Default('') String msisdn,
  }) = _CustomData;

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