diff --git a/AUTHORS b/AUTHORS
index 673c3f2fb551..c5b332610c80 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -80,3 +80,4 @@ LinXunFeng <linxunfeng@yeah.net>
 Hashir Shoaib <hashirshoaeb@gmail.com>
 Ricardo Dalarme <ricardodalarme@outlook.com>
 Andrei Kabylin <sys.int64@gmail.com>
+Ernesto Ramirez <neartox@outlook.com>
diff --git a/packages/go_router_builder/CHANGELOG.md b/packages/go_router_builder/CHANGELOG.md
index c09cc5874309..9ace42f1ed79 100644
--- a/packages/go_router_builder/CHANGELOG.md
+++ b/packages/go_router_builder/CHANGELOG.md
@@ -1,6 +1,11 @@
+## 4.1.0
+
+- Adds support for classes that support fromJson/toJson. [#117261](https://github.com/flutter/flutter/issues/117261)
+- Adds annotation that enable custom string encoder/decoder [#110781](https://github.com/flutter/flutter/issues/110781)
+
 ## 4.0.1
 
-- Fixes unnecessary whitespace in generated `RelativeGoRouteData`. 
+- Fixes unnecessary whitespace in generated `RelativeGoRouteData`.
 
 ## 4.0.0
 
diff --git a/packages/go_router_builder/example/lib/custom_encoder_example.dart b/packages/go_router_builder/example/lib/custom_encoder_example.dart
new file mode 100644
index 000000000000..a2d488c63bee
--- /dev/null
+++ b/packages/go_router_builder/example/lib/custom_encoder_example.dart
@@ -0,0 +1,94 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// ignore_for_file: public_member_api_docs, unreachable_from_main
+
+import 'dart:convert';
+
+import 'package:flutter/material.dart';
+import 'package:go_router/go_router.dart';
+
+part 'custom_encoder_example.g.dart';
+
+void main() => runApp(App());
+
+class App extends StatelessWidget {
+  App({super.key});
+
+  @override
+  Widget build(BuildContext context) =>
+      MaterialApp.router(routerConfig: _router, title: _appTitle);
+
+  final GoRouter _router = GoRouter(routes: $appRoutes);
+}
+
+@TypedGoRoute<HomeRoute>(
+  path: '/',
+  name: 'Home',
+  routes: <TypedGoRoute<GoRouteData>>[
+    TypedGoRoute<EncodedRoute>(path: 'encoded'),
+  ],
+)
+class HomeRoute extends GoRouteData with $HomeRoute {
+  const HomeRoute();
+
+  @override
+  Widget build(BuildContext context, GoRouterState state) => const HomeScreen();
+}
+
+class EncodedRoute extends GoRouteData with $EncodedRoute {
+  const EncodedRoute(this.token);
+
+  @CustomParameterCodec(encode: toBase64, decode: fromBase64)
+  final String token;
+
+  @override
+  Widget build(BuildContext context, GoRouterState state) =>
+      EncodedScreen(token: token);
+}
+
+class HomeScreen extends StatelessWidget {
+  const HomeScreen({super.key});
+
+  @override
+  Widget build(BuildContext context) => Scaffold(
+    appBar: AppBar(title: const Text(_appTitle)),
+    body: ListView(
+      children: <Widget>[
+        ListTile(
+          title: const Text('Base64Token'),
+          onTap: () => const EncodedRoute('Base64Token').go(context),
+        ),
+        ListTile(
+          title: const Text('from url only'),
+          // like in deep links
+          onTap: () => context.go('/encoded?token=ZW5jb2RlZCBpbmZvIQ'),
+        ),
+      ],
+    ),
+  );
+}
+
+class EncodedScreen extends StatelessWidget {
+  const EncodedScreen({super.key, required this.token});
+  final String token;
+
+  @override
+  Widget build(BuildContext context) => Scaffold(
+    appBar: AppBar(title: const Text('Base64Token')),
+    body: Center(child: Text(token)),
+  );
+}
+
+String fromBase64(String value) {
+  return const Utf8Decoder().convert(
+    base64Url.decode(base64Url.normalize(value)),
+  );
+}
+
+String toBase64(String value) {
+  return base64Url.encode(const Utf8Encoder().convert(value));
+}
+
+const String _appTitle = 'GoRouter Example: custom encoder';
diff --git a/packages/go_router_builder/example/lib/custom_encoder_example.g.dart b/packages/go_router_builder/example/lib/custom_encoder_example.g.dart
new file mode 100644
index 000000000000..ab002c500782
--- /dev/null
+++ b/packages/go_router_builder/example/lib/custom_encoder_example.g.dart
@@ -0,0 +1,66 @@
+// GENERATED CODE - DO NOT MODIFY BY HAND
+
+// ignore_for_file: always_specify_types, public_member_api_docs
+
+part of 'custom_encoder_example.dart';
+
+// **************************************************************************
+// GoRouterGenerator
+// **************************************************************************
+
+List<RouteBase> get $appRoutes => [$homeRoute];
+
+RouteBase get $homeRoute => GoRouteData.$route(
+  path: '/',
+  name: 'Home',
+  factory: $HomeRoute._fromState,
+  routes: [
+    GoRouteData.$route(path: 'encoded', factory: $EncodedRoute._fromState),
+  ],
+);
+
+mixin $HomeRoute on GoRouteData {
+  static HomeRoute _fromState(GoRouterState state) => const HomeRoute();
+
+  @override
+  String get location => GoRouteData.$location('/');
+
+  @override
+  void go(BuildContext context) => context.go(location);
+
+  @override
+  Future<T?> push<T>(BuildContext context) => context.push<T>(location);
+
+  @override
+  void pushReplacement(BuildContext context) =>
+      context.pushReplacement(location);
+
+  @override
+  void replace(BuildContext context) => context.replace(location);
+}
+
+mixin $EncodedRoute on GoRouteData {
+  static EncodedRoute _fromState(GoRouterState state) =>
+      EncodedRoute(fromBase64(state.uri.queryParameters['token']!));
+
+  EncodedRoute get _self => this as EncodedRoute;
+
+  @override
+  String get location => GoRouteData.$location(
+    '/encoded',
+    queryParams: {'token': toBase64(_self.token)},
+  );
+
+  @override
+  void go(BuildContext context) => context.go(location);
+
+  @override
+  Future<T?> push<T>(BuildContext context) => context.push<T>(location);
+
+  @override
+  void pushReplacement(BuildContext context) =>
+      context.pushReplacement(location);
+
+  @override
+  void replace(BuildContext context) => context.replace(location);
+}
diff --git a/packages/go_router_builder/example/lib/json_example.dart b/packages/go_router_builder/example/lib/json_example.dart
new file mode 100644
index 000000000000..4033b9a08468
--- /dev/null
+++ b/packages/go_router_builder/example/lib/json_example.dart
@@ -0,0 +1,82 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// ignore_for_file: public_member_api_docs, unreachable_from_main
+
+import 'dart:convert';
+
+import 'package:flutter/material.dart';
+import 'package:go_router/go_router.dart';
+
+import 'shared/json_example.dart';
+
+part 'json_example.g.dart';
+
+void main() => runApp(App());
+
+class App extends StatelessWidget {
+  App({super.key});
+
+  @override
+  Widget build(BuildContext context) =>
+      MaterialApp.router(routerConfig: _router, title: _appTitle);
+
+  final GoRouter _router = GoRouter(routes: $appRoutes);
+}
+
+@TypedGoRoute<HomeRoute>(
+  path: '/',
+  name: 'Home',
+  routes: <TypedGoRoute<GoRouteData>>[TypedGoRoute<JsonRoute>(path: 'json')],
+)
+class HomeRoute extends GoRouteData with $HomeRoute {
+  const HomeRoute();
+
+  @override
+  Widget build(BuildContext context, GoRouterState state) => const HomeScreen();
+}
+
+class JsonRoute extends GoRouteData with $JsonRoute {
+  const JsonRoute(this.json);
+
+  final JsonExample json;
+
+  @override
+  Widget build(BuildContext context, GoRouterState state) =>
+      JsonScreen(json: json);
+}
+
+class HomeScreen extends StatelessWidget {
+  const HomeScreen({super.key});
+
+  @override
+  Widget build(BuildContext context) => Scaffold(
+    appBar: AppBar(title: const Text(_appTitle)),
+    body: ListView(
+      children: <Widget>[
+        for (final JsonExample json in jsonData)
+          ListTile(
+            title: Text(json.name),
+            onTap: () => JsonRoute(json).go(context),
+          ),
+      ],
+    ),
+  );
+}
+
+class JsonScreen extends StatelessWidget {
+  const JsonScreen({required this.json, super.key});
+  final JsonExample json;
+
+  @override
+  Widget build(BuildContext context) => Scaffold(
+    appBar: AppBar(title: Text(json.name)),
+    body: ListView(
+      key: ValueKey<String>(json.id),
+      children: <Widget>[Text(json.id), Text(json.name)],
+    ),
+  );
+}
+
+const String _appTitle = 'GoRouter Example: builder';
diff --git a/packages/go_router_builder/example/lib/json_example.g.dart b/packages/go_router_builder/example/lib/json_example.g.dart
new file mode 100644
index 000000000000..99600db150ef
--- /dev/null
+++ b/packages/go_router_builder/example/lib/json_example.g.dart
@@ -0,0 +1,65 @@
+// GENERATED CODE - DO NOT MODIFY BY HAND
+
+// ignore_for_file: always_specify_types, public_member_api_docs
+
+part of 'json_example.dart';
+
+// **************************************************************************
+// GoRouterGenerator
+// **************************************************************************
+
+List<RouteBase> get $appRoutes => [$homeRoute];
+
+RouteBase get $homeRoute => GoRouteData.$route(
+  path: '/',
+  name: 'Home',
+  factory: $HomeRoute._fromState,
+  routes: [GoRouteData.$route(path: 'json', factory: $JsonRoute._fromState)],
+);
+
+mixin $HomeRoute on GoRouteData {
+  static HomeRoute _fromState(GoRouterState state) => const HomeRoute();
+
+  @override
+  String get location => GoRouteData.$location('/');
+
+  @override
+  void go(BuildContext context) => context.go(location);
+
+  @override
+  Future<T?> push<T>(BuildContext context) => context.push<T>(location);
+
+  @override
+  void pushReplacement(BuildContext context) =>
+      context.pushReplacement(location);
+
+  @override
+  void replace(BuildContext context) => context.replace(location);
+}
+
+mixin $JsonRoute on GoRouteData {
+  static JsonRoute _fromState(GoRouterState state) => JsonRoute((String json0) {
+    return JsonExample.fromJson(jsonDecode(json0) as Map<String, dynamic>);
+  }(state.uri.queryParameters['json']!));
+
+  JsonRoute get _self => this as JsonRoute;
+
+  @override
+  String get location => GoRouteData.$location(
+    '/json',
+    queryParams: {'json': jsonEncode(_self.json.toJson())},
+  );
+
+  @override
+  void go(BuildContext context) => context.go(location);
+
+  @override
+  Future<T?> push<T>(BuildContext context) => context.push<T>(location);
+
+  @override
+  void pushReplacement(BuildContext context) =>
+      context.pushReplacement(location);
+
+  @override
+  void replace(BuildContext context) => context.replace(location);
+}
diff --git a/packages/go_router_builder/example/lib/json_nested_example.dart b/packages/go_router_builder/example/lib/json_nested_example.dart
new file mode 100644
index 000000000000..16d0b5a45fce
--- /dev/null
+++ b/packages/go_router_builder/example/lib/json_nested_example.dart
@@ -0,0 +1,85 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// ignore_for_file: public_member_api_docs, unreachable_from_main
+
+import 'dart:convert';
+
+import 'package:flutter/material.dart';
+import 'package:go_router/go_router.dart';
+
+import 'shared/json_example.dart';
+
+part 'json_nested_example.g.dart';
+
+void main() => runApp(App());
+
+class App extends StatelessWidget {
+  App({super.key});
+
+  @override
+  Widget build(BuildContext context) =>
+      MaterialApp.router(routerConfig: _router, title: _appTitle);
+
+  final GoRouter _router = GoRouter(routes: $appRoutes);
+}
+
+@TypedGoRoute<HomeRoute>(
+  path: '/',
+  name: 'Home',
+  routes: <TypedGoRoute<GoRouteData>>[TypedGoRoute<JsonRoute>(path: 'json')],
+)
+class HomeRoute extends GoRouteData with $HomeRoute {
+  const HomeRoute();
+
+  @override
+  Widget build(BuildContext context, GoRouterState state) => const HomeScreen();
+}
+
+class JsonRoute extends GoRouteData with $JsonRoute {
+  const JsonRoute(this.json);
+
+  final JsonExampleNested<JsonExample> json;
+
+  @override
+  Widget build(BuildContext context, GoRouterState state) =>
+      JsonScreen(json: json.child);
+}
+
+class HomeScreen extends StatelessWidget {
+  const HomeScreen({super.key});
+
+  @override
+  Widget build(BuildContext context) => Scaffold(
+    appBar: AppBar(title: const Text(_appTitle)),
+    body: ListView(
+      children: <Widget>[
+        for (final JsonExample json in jsonData)
+          ListTile(
+            title: Text(json.name),
+            onTap:
+                () => JsonRoute(
+                  JsonExampleNested<JsonExample>(child: json),
+                ).go(context),
+          ),
+      ],
+    ),
+  );
+}
+
+class JsonScreen extends StatelessWidget {
+  const JsonScreen({required this.json, super.key});
+  final JsonExample json;
+
+  @override
+  Widget build(BuildContext context) => Scaffold(
+    appBar: AppBar(title: Text(json.name)),
+    body: ListView(
+      key: ValueKey<String>(json.id),
+      children: <Widget>[Text(json.id), Text(json.name)],
+    ),
+  );
+}
+
+const String _appTitle = 'GoRouter Example: builder';
diff --git a/packages/go_router_builder/example/lib/json_nested_example.g.dart b/packages/go_router_builder/example/lib/json_nested_example.g.dart
new file mode 100644
index 000000000000..18411ebc9a7c
--- /dev/null
+++ b/packages/go_router_builder/example/lib/json_nested_example.g.dart
@@ -0,0 +1,70 @@
+// GENERATED CODE - DO NOT MODIFY BY HAND
+
+// ignore_for_file: always_specify_types, public_member_api_docs
+
+part of 'json_nested_example.dart';
+
+// **************************************************************************
+// GoRouterGenerator
+// **************************************************************************
+
+List<RouteBase> get $appRoutes => [$homeRoute];
+
+RouteBase get $homeRoute => GoRouteData.$route(
+  path: '/',
+  name: 'Home',
+  factory: $HomeRoute._fromState,
+  routes: [GoRouteData.$route(path: 'json', factory: $JsonRoute._fromState)],
+);
+
+mixin $HomeRoute on GoRouteData {
+  static HomeRoute _fromState(GoRouterState state) => const HomeRoute();
+
+  @override
+  String get location => GoRouteData.$location('/');
+
+  @override
+  void go(BuildContext context) => context.go(location);
+
+  @override
+  Future<T?> push<T>(BuildContext context) => context.push<T>(location);
+
+  @override
+  void pushReplacement(BuildContext context) =>
+      context.pushReplacement(location);
+
+  @override
+  void replace(BuildContext context) => context.replace(location);
+}
+
+mixin $JsonRoute on GoRouteData {
+  static JsonRoute _fromState(GoRouterState state) => JsonRoute((String json0) {
+    return JsonExampleNested.fromJson(
+      jsonDecode(json0) as Map<String, dynamic>,
+      (Object? json1) {
+        return JsonExample.fromJson(json1 as Map<String, dynamic>);
+      },
+    );
+  }(state.uri.queryParameters['json']!));
+
+  JsonRoute get _self => this as JsonRoute;
+
+  @override
+  String get location => GoRouteData.$location(
+    '/json',
+    queryParams: {'json': jsonEncode(_self.json.toJson())},
+  );
+
+  @override
+  void go(BuildContext context) => context.go(location);
+
+  @override
+  Future<T?> push<T>(BuildContext context) => context.push<T>(location);
+
+  @override
+  void pushReplacement(BuildContext context) =>
+      context.pushReplacement(location);
+
+  @override
+  void replace(BuildContext context) => context.replace(location);
+}
diff --git a/packages/go_router_builder/example/lib/shared/json_example.dart b/packages/go_router_builder/example/lib/shared/json_example.dart
new file mode 100644
index 000000000000..7d20e88e4c50
--- /dev/null
+++ b/packages/go_router_builder/example/lib/shared/json_example.dart
@@ -0,0 +1,53 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+/// json example
+class JsonExample {
+  /// json example
+  const JsonExample({required this.id, required this.name});
+
+  /// fromJson decoder
+  factory JsonExample.fromJson(Map<String, dynamic> json) {
+    return JsonExample(id: json['id'] as String, name: json['name'] as String);
+  }
+
+  /// toJson encoder
+  Map<String, dynamic> toJson() {
+    return <String, dynamic>{'id': id, 'name': name};
+  }
+
+  /// id
+  final String id;
+
+  /// name
+  final String name;
+}
+
+/// example info
+const List<JsonExample> jsonData = <JsonExample>[
+  JsonExample(id: '1', name: 'people'),
+  JsonExample(id: '2', name: 'animals'),
+];
+
+/// Json Nested Example
+class JsonExampleNested<T> {
+  /// Json Nested Example
+  const JsonExampleNested({required this.child});
+
+  /// toJson decoder
+  factory JsonExampleNested.fromJson(
+    Map<String, dynamic> json,
+    T Function(Object? json) fromJsonT,
+  ) {
+    return JsonExampleNested<T>(child: fromJsonT(json['child']));
+  }
+
+  /// toJson encoder
+  Map<String, dynamic> toJson() {
+    return <String, dynamic>{'child': child};
+  }
+
+  /// child
+  final T child;
+}
diff --git a/packages/go_router_builder/lib/src/route_config.dart b/packages/go_router_builder/lib/src/route_config.dart
index b1eab450219b..8cf6e72de0e9 100644
--- a/packages/go_router_builder/lib/src/route_config.dart
+++ b/packages/go_router_builder/lib/src/route_config.dart
@@ -273,7 +273,14 @@ mixin _GoRouteMixin on RouteBaseConfig {
         );
       }
     }
-    final String fromStateExpression = decodeParameter(element, _pathParams);
+    final List<ElementAnnotation>? metadata = _fieldMetadata(
+      element.displayName,
+    );
+    final String fromStateExpression = decodeParameter(
+      element,
+      _pathParams,
+      metadata,
+    );
 
     if (element.isPositional) {
       return '$fromStateExpression,';
@@ -298,7 +305,8 @@ mixin _GoRouteMixin on RouteBaseConfig {
       );
     }
 
-    return encodeField(field);
+    final List<ElementAnnotation>? metadata = _fieldMetadata(fieldName);
+    return encodeField(field, metadata);
   }
 
   String get _locationQueryParams {
@@ -473,19 +481,19 @@ mixin $_mixinName on $routeDataClassName {
   $_castedSelf
   @override
   String get location => GoRouteData.\$location($_locationArgs,$_locationQueryParams);
-  
+
   @override
   void go(BuildContext context) =>
       context.go(location${_extraParam != null ? ', extra: $selfFieldName.$extraFieldName' : ''});
-  
+
   @override
   Future<T?> push<T>(BuildContext context) =>
       context.push<T>(location${_extraParam != null ? ', extra: $selfFieldName.$extraFieldName' : ''});
-  
+
   @override
   void pushReplacement(BuildContext context) =>
       context.pushReplacement(location${_extraParam != null ? ', extra: $selfFieldName.$extraFieldName' : ''});
-  
+
   @override
   void replace(BuildContext context) =>
       context.replace(location${_extraParam != null ? ', extra: $selfFieldName.$extraFieldName' : ''});
@@ -548,7 +556,7 @@ mixin $_mixinName on $routeDataClassName {
   $_castedSelf
   @override
   String get subLocation => RelativeGoRouteData.\$location($_locationArgs,$_locationQueryParams);
-  
+
   @override
   String get relativeLocation => './\$subLocation';
 
@@ -559,11 +567,11 @@ mixin $_mixinName on $routeDataClassName {
   @override
   Future<T?> pushRelative<T>(BuildContext context) =>
       context.push<T>(relativeLocation${_extraParam != null ? ', extra: $selfFieldName.$extraFieldName' : ''});
-  
+
   @override
   void pushReplacementRelative(BuildContext context) =>
       context.pushReplacement(relativeLocation${_extraParam != null ? ', extra: $selfFieldName.$extraFieldName' : ''});
-  
+
   @override
   void replaceRelative(BuildContext context) =>
       context.replace(relativeLocation${_extraParam != null ? ', extra: $selfFieldName.$extraFieldName' : ''});
@@ -901,6 +909,14 @@ $routeDataClassName.$dataConvertionFunctionName(
   PropertyAccessorElement2? _field(String name) =>
       routeDataClass.getGetter2(name);
 
+  List<ElementAnnotation>? _fieldMetadata(String name) =>
+      routeDataClass.fields2
+          .firstWhereOrNull(
+            (FieldElement2 element) => element.displayName == name,
+          )
+          ?.metadata2
+          .annotations;
+
   /// The name of `RouteData` subclass this configuration represents.
   @protected
   String get routeDataClassName;
diff --git a/packages/go_router_builder/lib/src/type_helpers.dart b/packages/go_router_builder/lib/src/type_helpers.dart
index 294e1d325dda..0bced8c2d4fb 100644
--- a/packages/go_router_builder/lib/src/type_helpers.dart
+++ b/packages/go_router_builder/lib/src/type_helpers.dart
@@ -5,8 +5,10 @@
 import 'package:analyzer/dart/analysis/results.dart';
 import 'package:analyzer/dart/analysis/session.dart';
 import 'package:analyzer/dart/ast/ast.dart';
+import 'package:analyzer/dart/constant/value.dart';
 import 'package:analyzer/dart/element/element2.dart';
 import 'package:analyzer/dart/element/type.dart';
+import 'package:collection/collection.dart';
 import 'package:source_gen/source_gen.dart';
 import 'package:source_helper/source_helper.dart';
 
@@ -49,14 +51,41 @@ const List<_TypeHelper> _helpers = <_TypeHelper>[
   _TypeHelperString(),
   _TypeHelperUri(),
   _TypeHelperIterable(),
+  _TypeHelperJson(),
 ];
 
+/// Checks if has a function that converts string to string, such as encode and decode.
+bool _isStringToStringFunction(
+  ExecutableElement2? executableElement,
+  String name,
+) {
+  if (executableElement == null) {
+    return false;
+  }
+  final List<FormalParameterElement> parameters =
+      executableElement.formalParameters;
+  return parameters.length == 1 &&
+      parameters.first.type.isDartCoreString &&
+      executableElement.returnType.isDartCoreString;
+}
+
+/// Returns the custom codec for the annotation.
+String? _getCustomCodec(ElementAnnotation annotation, String name) {
+  final ExecutableElement2? executableElement =
+      annotation.computeConstantValue()?.getField(name)?.toFunctionValue2();
+  if (_isStringToStringFunction(executableElement, name)) {
+    return executableElement!.displayName;
+  }
+  return null;
+}
+
 /// Returns the decoded [String] value for [element], if its type is supported.
 ///
 /// Otherwise, throws an [InvalidGenerationSourceError].
 String decodeParameter(
   FormalParameterElement element,
   Set<String> pathParameters,
+  List<ElementAnnotation>? metadata,
 ) {
   if (element.isExtraField) {
     return 'state.${_stateValueAccess(element, pathParameters)}';
@@ -65,13 +94,37 @@ String decodeParameter(
   final DartType paramType = element.type;
   for (final _TypeHelper helper in _helpers) {
     if (helper._matchesType(paramType)) {
-      String decoded = helper._decode(element, pathParameters);
+      String? decoder;
+
+      final ElementAnnotation? annotation = metadata?.firstWhereOrNull((
+        ElementAnnotation annotation,
+      ) {
+        return annotation.computeConstantValue()?.type?.getDisplayString() ==
+            'CustomParameterCodec';
+      });
+      if (annotation != null) {
+        final String? decode = _getCustomCodec(annotation, 'decode');
+        final String? encode = _getCustomCodec(annotation, 'encode');
+        if (decode != null && encode != null) {
+          decoder = decode;
+        } else {
+          throw InvalidGenerationSourceError(
+            'The parameter type '
+            '`${paramType.getDisplayString(withNullability: false)}` not have a well defined CustomParameterCodec decorator.',
+            element: element,
+          );
+        }
+      }
+      String decoded = helper._decode(element, pathParameters, decoder);
       if (element.isOptional && element.hasDefaultValue) {
         if (element.type.isNullableType) {
           throw NullableDefaultValueError(element);
         }
         decoded += ' ?? ${element.defaultValueCode!}';
       }
+      if (helper is _TypeHelperString && decoder != null) {
+        return _fieldWithEncoder(decoded, decoder);
+      }
       return decoded;
     }
   }
@@ -86,13 +139,38 @@ String decodeParameter(
 /// Returns the encoded [String] value for [element], if its type is supported.
 ///
 /// Otherwise, throws an [InvalidGenerationSourceError].
-String encodeField(PropertyAccessorElement2 element) {
+String encodeField(
+  PropertyAccessorElement2 element,
+  List<ElementAnnotation>? metadata,
+) {
   for (final _TypeHelper helper in _helpers) {
     if (helper._matchesType(element.returnType)) {
-      return helper._encode(
+      String? encoder;
+      final ElementAnnotation? annotation = metadata?.firstWhereOrNull((
+        ElementAnnotation annotation,
+      ) {
+        final DartObject? constant = annotation.computeConstantValue();
+        return constant?.type?.getDisplayString() == 'CustomParameterCodec';
+      });
+      if (annotation != null) {
+        final String? decode = _getCustomCodec(annotation, 'decode');
+        final String? encode = _getCustomCodec(annotation, 'encode');
+        if (decode != null && encode != null) {
+          encoder = encode;
+        } else {
+          throw InvalidGenerationSourceError(
+            'The parameter type '
+            '`${element.type.getDisplayString(withNullability: false)}` not have a well defined CustomParameterCodec decorator.',
+            element: element,
+          );
+        }
+      }
+      final String encoded = helper._encode(
         '$selfFieldName.${element.displayName}',
         element.returnType,
+        encoder,
       );
+      return encoded;
     }
   }
 
@@ -102,7 +180,7 @@ String encodeField(PropertyAccessorElement2 element) {
   );
 }
 
-/// Returns an AstNode type from a InterfaceElement.
+/// Returns an AstNode type from a InterfaceElement2.
 T? getNodeDeclaration<T extends AstNode>(InterfaceElement2 element) {
   final AnalysisSession? session = element.session;
   if (session == null) {
@@ -145,6 +223,9 @@ String compareField(
 /// Gets the name of the `const` map generated to help encode [Enum] types.
 String enumMapName(InterfaceType type) => '_\$${type.element.name}EnumMap';
 
+/// Gets the name of the `const` map generated to help encode [Json] types.
+String jsonMapName(InterfaceType type) => type.element.name;
+
 String _stateValueAccess(
   FormalParameterElement element,
   Set<String> pathParameters,
@@ -177,6 +258,10 @@ String withoutNullability(String type) {
   return _isNullableType(type) ? type.substring(0, type.length - 1) : type;
 }
 
+String _fieldWithEncoder(String field, String? customEncoder) {
+  return customEncoder != null ? '$customEncoder($field)' : field;
+}
+
 abstract class _TypeHelper {
   const _TypeHelper();
 
@@ -184,10 +269,11 @@ abstract class _TypeHelper {
   String _decode(
     FormalParameterElement parameterElement,
     Set<String> pathParameters,
+    String? customDecoder,
   );
 
   /// Encodes the value from its string representation in the URL.
-  String _encode(String fieldName, DartType type);
+  String _encode(String fieldName, DartType type, String? customEncoder);
 
   bool _matchesType(DartType type);
 
@@ -206,8 +292,11 @@ class _TypeHelperBigInt extends _TypeHelperWithHelper {
   }
 
   @override
-  String _encode(String fieldName, DartType type) =>
-      '$fieldName${type.ensureNotNull}.toString()';
+  String _encode(String fieldName, DartType type, String? customEncoder) =>
+      _fieldWithEncoder(
+        '$fieldName${type.ensureNotNull}.toString()',
+        customEncoder,
+      );
 
   @override
   bool _matchesType(DartType type) =>
@@ -221,8 +310,11 @@ class _TypeHelperBool extends _TypeHelperWithHelper {
   String helperName(DartType paramType) => boolConverterHelperName;
 
   @override
-  String _encode(String fieldName, DartType type) =>
-      '$fieldName${type.ensureNotNull}.toString()';
+  String _encode(String fieldName, DartType type, String? customEncoder) =>
+      _fieldWithEncoder(
+        '$fieldName${type.ensureNotNull}.toString()',
+        customEncoder,
+      );
 
   @override
   bool _matchesType(DartType type) => type.isDartCoreBool;
@@ -240,8 +332,11 @@ class _TypeHelperDateTime extends _TypeHelperWithHelper {
   }
 
   @override
-  String _encode(String fieldName, DartType type) =>
-      '$fieldName${type.ensureNotNull}.toString()';
+  String _encode(String fieldName, DartType type, String? customEncoder) =>
+      _fieldWithEncoder(
+        '$fieldName${type.ensureNotNull}.toString()',
+        customEncoder,
+      );
 
   @override
   bool _matchesType(DartType type) =>
@@ -260,8 +355,11 @@ class _TypeHelperDouble extends _TypeHelperWithHelper {
   }
 
   @override
-  String _encode(String fieldName, DartType type) =>
-      '$fieldName${type.ensureNotNull}.toString()';
+  String _encode(String fieldName, DartType type, String? customEncoder) =>
+      _fieldWithEncoder(
+        '$fieldName${type.ensureNotNull}.toString()',
+        customEncoder,
+      );
 
   @override
   bool _matchesType(DartType type) => type.isDartCoreDouble;
@@ -275,8 +373,14 @@ class _TypeHelperEnum extends _TypeHelperWithHelper {
       '${enumMapName(paramType as InterfaceType)}.$enumExtensionHelperName';
 
   @override
-  String _encode(String fieldName, DartType type) =>
-      '${enumMapName(type as InterfaceType)}[$fieldName${type.ensureNotNull}]';
+  String _encode(
+    String fieldName,
+    DartType type,
+    String? customEncoder,
+  ) => _fieldWithEncoder(
+    '${enumMapName(type as InterfaceType)}[$fieldName${type.ensureNotNull}]',
+    customEncoder,
+  );
 
   @override
   bool _matchesType(DartType type) => type.isEnum;
@@ -300,6 +404,7 @@ class _TypeHelperExtensionType extends _TypeHelper {
   String _decode(
     FormalParameterElement parameterElement,
     Set<String> pathParameters,
+    String? customDecoder,
   ) {
     final DartType paramType = parameterElement.type;
     if (paramType.isNullableType && parameterElement.hasDefaultValue) {
@@ -336,7 +441,7 @@ class _TypeHelperExtensionType extends _TypeHelper {
   }
 
   @override
-  String _encode(String fieldName, DartType type) {
+  String _encode(String fieldName, DartType type, String? customDecoder) {
     final DartType representationType = type.extensionTypeErasure;
     if (representationType.isDartCoreString) {
       return '$fieldName${type.ensureNotNull} as String';
@@ -388,8 +493,11 @@ class _TypeHelperInt extends _TypeHelperWithHelper {
   }
 
   @override
-  String _encode(String fieldName, DartType type) =>
-      '$fieldName${type.ensureNotNull}.toString()';
+  String _encode(String fieldName, DartType type, String? customEncoder) =>
+      _fieldWithEncoder(
+        '$fieldName${type.ensureNotNull}.toString()',
+        customEncoder,
+      );
 
   @override
   bool _matchesType(DartType type) => type.isDartCoreInt;
@@ -407,8 +515,11 @@ class _TypeHelperNum extends _TypeHelperWithHelper {
   }
 
   @override
-  String _encode(String fieldName, DartType type) =>
-      '$fieldName${type.ensureNotNull}.toString()';
+  String _encode(String fieldName, DartType type, String? customEncoder) =>
+      _fieldWithEncoder(
+        '$fieldName${type.ensureNotNull}.toString()',
+        customEncoder,
+      );
 
   @override
   bool _matchesType(DartType type) => type.isDartCoreNum;
@@ -421,10 +532,12 @@ class _TypeHelperString extends _TypeHelper {
   String _decode(
     FormalParameterElement parameterElement,
     Set<String> pathParameters,
+    String? customDecoder,
   ) => 'state.${_stateValueAccess(parameterElement, pathParameters)}';
 
   @override
-  String _encode(String fieldName, DartType type) => fieldName;
+  String _encode(String fieldName, DartType type, String? customEncoder) =>
+      _fieldWithEncoder(fieldName, customEncoder);
 
   @override
   bool _matchesType(DartType type) => type.isDartCoreString;
@@ -442,8 +555,11 @@ class _TypeHelperUri extends _TypeHelperWithHelper {
   }
 
   @override
-  String _encode(String fieldName, DartType type) =>
-      '$fieldName${type.ensureNotNull}.toString()';
+  String _encode(String fieldName, DartType type, String? customEncoder) =>
+      _fieldWithEncoder(
+        '$fieldName${type.ensureNotNull}.toString()',
+        customEncoder,
+      );
 
   @override
   bool _matchesType(DartType type) =>
@@ -460,6 +576,7 @@ class _TypeHelperIterable extends _TypeHelperWithHelper {
   String _decode(
     FormalParameterElement parameterElement,
     Set<String> pathParameters,
+    String? customDecoder,
   ) {
     if (parameterElement.type is ParameterizedType) {
       final DartType iterableType =
@@ -476,6 +593,10 @@ class _TypeHelperIterable extends _TypeHelperWithHelper {
             convertToNotNull = '.cast<$iterableType>()';
           }
           entriesTypeDecoder = helper.helperName(iterableType);
+          if (customDecoder != null) {
+            entriesTypeDecoder =
+                '(e) => $entriesTypeDecoder($customDecoder(e))';
+          }
         }
       }
 
@@ -510,7 +631,7 @@ state.uri.queryParametersAll[${escapeDartString(parameterElement.displayName.keb
   }
 
   @override
-  String _encode(String fieldName, DartType type) {
+  String _encode(String fieldName, DartType type, String? customEncoder) {
     final String nullAwareAccess = type.isNullableType ? '?' : '';
     if (type is ParameterizedType) {
       final DartType iterableType = type.typeArguments.first;
@@ -520,7 +641,7 @@ state.uri.queryParametersAll[${escapeDartString(parameterElement.displayName.keb
       for (final _TypeHelper helper in _helpers) {
         if (helper._matchesType(iterableType)) {
           entriesTypeEncoder = '''
-$nullAwareAccess.map((e) => ${helper._encode('e', iterableType)}).toList()''';
+$nullAwareAccess.map((e) => ${helper._encode('e', iterableType, customEncoder)}).toList()''';
         }
       }
       return '''
@@ -540,6 +661,135 @@ $fieldName$nullAwareAccess.map((e) => e.toString()).toList()''';
       '!$iterablesEqualHelperName($value1, $value2)';
 }
 
+class _TypeHelperJson extends _TypeHelperWithHelper {
+  const _TypeHelperJson();
+
+  @override
+  String helperName(DartType paramType) {
+    return _helperNameN(paramType, 0);
+  }
+
+  @override
+  String _encode(String fieldName, DartType type, String? customEncoder) =>
+      _fieldWithEncoder(
+        'jsonEncode($fieldName${type.ensureNotNull}.toJson())',
+        customEncoder,
+      );
+
+  @override
+  bool _matchesType(DartType type) {
+    if (type is! InterfaceType) {
+      return false;
+    }
+
+    final MethodElement2? toJsonMethod = type.lookUpMethod3(
+      'toJson',
+      type.element3.library2,
+    );
+    if (toJsonMethod == null ||
+        !toJsonMethod.isPublic ||
+        toJsonMethod.formalParameters.isNotEmpty) {
+      return false;
+    }
+
+    // test for template
+    if (_isNestedTemplate(type)) {
+      // check for deep compatibility
+      return _matchesType(type.typeArguments.first);
+    }
+
+    final ConstructorElement2? fromJsonMethod = type.element3
+        .getNamedConstructor2('fromJson');
+
+    if (fromJsonMethod == null ||
+        !fromJsonMethod.isPublic ||
+        fromJsonMethod.formalParameters.length != 1 ||
+        fromJsonMethod.formalParameters.first.type.getDisplayString(
+              withNullability: false,
+            ) !=
+            'Map<String, dynamic>') {
+      throw InvalidGenerationSourceError(
+        'The parameter type '
+        '`${type.getDisplayString(withNullability: false)}` not have a supported fromJson definition.',
+        element: type.element3,
+      );
+    }
+
+    return true;
+  }
+
+  String _helperNameN(DartType paramType, int index) {
+    final String mainType = index == 0 ? 'String' : 'Object?';
+    final String mainDecoder =
+        index == 0
+            ? 'jsonDecode(json$index) as Map<String, dynamic>'
+            : 'json$index as Map<String, dynamic>';
+    if (_isNestedTemplate(paramType as InterfaceType)) {
+      return '''
+($mainType json$index) {
+  return ${jsonMapName(paramType)}.fromJson(
+    $mainDecoder,
+    ${_helperNameN(paramType.typeArguments.first, index + 1)},
+  );
+}''';
+    }
+    return '''
+($mainType json$index) {
+  return ${jsonMapName(paramType)}.fromJson($mainDecoder);
+}''';
+  }
+
+  bool _isNestedTemplate(InterfaceType type) {
+    // check if has fromJson constructor
+    final ConstructorElement2? fromJsonMethod = type.element3
+        .getNamedConstructor2('fromJson');
+    if (fromJsonMethod == null || !fromJsonMethod.isPublic) {
+      return false;
+    }
+
+    if (type.typeArguments.length != 1) {
+      return false;
+    }
+
+    // check if fromJson method receive two parameters
+    final List<FormalParameterElement> parameters =
+        fromJsonMethod.formalParameters;
+    if (parameters.length != 2) {
+      return false;
+    }
+
+    final FormalParameterElement firstParam = parameters[0];
+    if (firstParam.type.getDisplayString(withNullability: false) !=
+        'Map<String, dynamic>') {
+      throw InvalidGenerationSourceError(
+        'The parameter type '
+        '`${type.getDisplayString(withNullability: false)}` not have a supported fromJson definition.',
+        element: type.element3,
+      );
+    }
+
+    // Test for (T Function(Object? json)).
+    final FormalParameterElement secondParam = parameters[1];
+    if (secondParam.type is! FunctionType) {
+      return false;
+    }
+
+    final FunctionType functionType = secondParam.type as FunctionType;
+    if (functionType.parameters.length != 1 ||
+        functionType.returnType.getDisplayString() !=
+            type.element.typeParameters.first.getDisplayString() ||
+        functionType.parameters[0].type.getDisplayString() != 'Object?') {
+      throw InvalidGenerationSourceError(
+        'The parameter type '
+        '`${type.getDisplayString(withNullability: false)}` not have a supported fromJson definition.',
+        element: type.element3,
+      );
+    }
+
+    return true;
+  }
+}
+
 abstract class _TypeHelperWithHelper extends _TypeHelper {
   const _TypeHelperWithHelper();
 
@@ -549,6 +799,7 @@ abstract class _TypeHelperWithHelper extends _TypeHelper {
   String _decode(
     FormalParameterElement parameterElement,
     Set<String> pathParameters,
+    String? customDecoder,
   ) {
     final DartType paramType = parameterElement.type;
     final String parameterName = parameterElement.displayName;
@@ -560,15 +811,17 @@ abstract class _TypeHelperWithHelper extends _TypeHelper {
           'state.uri.queryParameters, '
           '${helperName(paramType)})';
     }
-
     final String nullableSuffix =
         paramType.isNullableType ||
                 (paramType.isEnum && !paramType.isNullableType)
             ? '!'
             : '';
 
-    return '${helperName(paramType)}'
-        '(state.${_stateValueAccess(parameterElement, pathParameters)} ${!parameterElement.isRequired ? " ?? '' " : ''})$nullableSuffix';
+    final String decode = _fieldWithEncoder(
+      'state.${_stateValueAccess(parameterElement, pathParameters)} ${!parameterElement.isRequired ? " ?? '' " : ''}',
+      customDecoder,
+    );
+    return '${helperName(paramType)}($decode)$nullableSuffix';
   }
 }
 
diff --git a/packages/go_router_builder/pubspec.yaml b/packages/go_router_builder/pubspec.yaml
index 25e088a51c72..f25a19ef95f4 100644
--- a/packages/go_router_builder/pubspec.yaml
+++ b/packages/go_router_builder/pubspec.yaml
@@ -2,7 +2,7 @@ name: go_router_builder
 description: >-
   A builder that supports generated strongly-typed route helpers for
   package:go_router
-version: 4.0.1
+version: 4.1.0
 repository: https://github.com/flutter/packages/tree/main/packages/go_router_builder
 issue_tracker: https://github.com/flutter/flutter/issues?q=is%3Aissue+is%3Aopen+label%3A%22p%3A+go_router_builder%22
 
diff --git a/packages/go_router_builder/test_inputs/bad_json.dart b/packages/go_router_builder/test_inputs/bad_json.dart
new file mode 100644
index 000000000000..8d43c6bfd7b0
--- /dev/null
+++ b/packages/go_router_builder/test_inputs/bad_json.dart
@@ -0,0 +1,29 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:go_router/go_router.dart';
+
+mixin $BadJson {}
+
+@TypedGoRoute<BadJson>(path: '/')
+class BadJson extends GoRouteData with $BadJson {
+  const BadJson({required this.id});
+
+  final JsonExample id;
+}
+
+class JsonExample {
+  const JsonExample({required this.id});
+
+  // json parameter is not a Map<String, dynamic>
+  factory JsonExample.fromJson(/*Map<String, dynamic>*/ dynamic json) {
+    return JsonExample(id: (json as Map<String, dynamic>)['a'] as String);
+  }
+
+  Map<String, dynamic> toJson() {
+    return <String, dynamic>{'id': id};
+  }
+
+  final String id;
+}
diff --git a/packages/go_router_builder/test_inputs/bad_json.dart.expect b/packages/go_router_builder/test_inputs/bad_json.dart.expect
new file mode 100644
index 000000000000..fc70f3af316d
--- /dev/null
+++ b/packages/go_router_builder/test_inputs/bad_json.dart.expect
@@ -0,0 +1 @@
+The parameter type `JsonExample` not have a supported fromJson definition.
diff --git a/packages/go_router_builder/test_inputs/bad_json_template.dart b/packages/go_router_builder/test_inputs/bad_json_template.dart
new file mode 100644
index 000000000000..fd1656b61fd7
--- /dev/null
+++ b/packages/go_router_builder/test_inputs/bad_json_template.dart
@@ -0,0 +1,50 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:go_router/go_router.dart';
+
+mixin $BadJsonTemplate {}
+
+@TypedGoRoute<BadJsonTemplate>(path: '/')
+class BadJsonTemplate extends GoRouteData with $BadJsonTemplate {
+  const BadJsonTemplate({required this.nested, this.deepNested});
+
+  final JsonExampleNested<JsonExample> nested;
+  final JsonExampleNested<JsonExampleNested<JsonExample>>? deepNested;
+}
+
+class JsonExample {
+  const JsonExample({required this.id});
+
+  factory JsonExample.fromJson(Map<String, dynamic> json) {
+    return JsonExample(id: json['id'] as String);
+  }
+
+  Map<String, dynamic> toJson() {
+    return <String, dynamic>{'id': id};
+  }
+
+  final String id;
+}
+
+class JsonExampleNested<T> {
+  const JsonExampleNested({required this.child});
+
+  // from fromJson is not well formed
+  factory JsonExampleNested.fromJson(
+    // ignore: avoid_unused_constructor_parameters
+    Map<String, dynamic> json,
+    // ignore: avoid_unused_constructor_parameters
+    void Function(Object? json) fromJsonT,
+    /*T Function(Object? json) fromJsonT,*/
+  ) {
+    return JsonExampleNested<T>(child: /*fromJsonT(json['child'])*/ null);
+  }
+
+  Map<String, dynamic> toJson() {
+    return <String, dynamic>{'child': child};
+  }
+
+  final T? child;
+}
diff --git a/packages/go_router_builder/test_inputs/bad_json_template.dart.expect b/packages/go_router_builder/test_inputs/bad_json_template.dart.expect
new file mode 100644
index 000000000000..3302facea2c2
--- /dev/null
+++ b/packages/go_router_builder/test_inputs/bad_json_template.dart.expect
@@ -0,0 +1 @@
+The parameter type `JsonExampleNested<JsonExample>` not have a supported fromJson definition.
diff --git a/packages/go_router_builder/test_inputs/custom_encoder.dart b/packages/go_router_builder/test_inputs/custom_encoder.dart
new file mode 100644
index 000000000000..a0e0d7d04361
--- /dev/null
+++ b/packages/go_router_builder/test_inputs/custom_encoder.dart
@@ -0,0 +1,59 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:convert';
+
+import 'package:go_router/go_router.dart';
+
+String fromBase64(String value) {
+  return const Utf8Decoder().convert(
+    base64Url.decode(base64Url.normalize(value)),
+  );
+}
+
+String toBase64(String value) {
+  return base64Url.encode(const Utf8Encoder().convert(value));
+}
+
+mixin $CustomParameterRoute {}
+
+@TypedGoRoute<CustomParameterRoute>(path: '/default-value-route')
+class CustomParameterRoute extends GoRouteData with $CustomParameterRoute {
+  CustomParameterRoute({required this.param});
+
+  @CustomParameterCodec(encode: toBase64, decode: fromBase64)
+  final int param;
+}
+
+mixin $CustomParameterComplexRoute {}
+
+@TypedGoRoute<CustomParameterComplexRoute>(path: '/:id/')
+class CustomParameterComplexRoute extends GoRouteData
+    with $CustomParameterComplexRoute {
+  CustomParameterComplexRoute({
+    required this.id,
+    this.dir = '',
+    this.list = const <Uri>[],
+    required this.enumTest,
+  });
+
+  @CustomParameterCodec(encode: toBase64, decode: fromBase64)
+  final int id;
+
+  @CustomParameterCodec(encode: toBase64, decode: fromBase64)
+  final String dir;
+  @CustomParameterCodec(encode: toBase64, decode: fromBase64)
+  final List<Uri> list;
+  @CustomParameterCodec(encode: toBase64, decode: fromBase64)
+  final EnumTest enumTest;
+}
+
+enum EnumTest {
+  a(1),
+  b(3),
+  c(5);
+
+  const EnumTest(this.x);
+  final int x;
+}
diff --git a/packages/go_router_builder/test_inputs/custom_encoder.dart.expect b/packages/go_router_builder/test_inputs/custom_encoder.dart.expect
new file mode 100644
index 000000000000..d37b9cdc6a17
--- /dev/null
+++ b/packages/go_router_builder/test_inputs/custom_encoder.dart.expect
@@ -0,0 +1,102 @@
+RouteBase get $customParameterRoute => GoRouteData.$route(
+  path: '/default-value-route',
+  factory: $CustomParameterRoute._fromState,
+);
+
+mixin $CustomParameterRoute on GoRouteData {
+  static CustomParameterRoute _fromState(GoRouterState state) =>
+      CustomParameterRoute(
+        param: int.parse(fromBase64(state.uri.queryParameters['param']!)),
+      );
+
+  CustomParameterRoute get _self => this as CustomParameterRoute;
+
+  @override
+  String get location => GoRouteData.$location(
+    '/default-value-route',
+    queryParams: {'param': toBase64(_self.param.toString())},
+  );
+
+  @override
+  void go(BuildContext context) => context.go(location);
+
+  @override
+  Future<T?> push<T>(BuildContext context) => context.push<T>(location);
+
+  @override
+  void pushReplacement(BuildContext context) =>
+      context.pushReplacement(location);
+
+  @override
+  void replace(BuildContext context) => context.replace(location);
+}
+
+RouteBase get $customParameterComplexRoute => GoRouteData.$route(
+  path: '/:id/',
+  factory: $CustomParameterComplexRoute._fromState,
+);
+
+mixin $CustomParameterComplexRoute on GoRouteData {
+  static CustomParameterComplexRoute _fromState(GoRouterState state) =>
+      CustomParameterComplexRoute(
+        id: int.parse(fromBase64(state.pathParameters['id']!)),
+        dir: fromBase64(state.uri.queryParameters['dir'] ?? ''),
+        list:
+            state.uri.queryParametersAll['list']
+                ?.map((e) => Uri.parse(fromBase64(e)))
+                .cast<Uri>()
+                .toList() ??
+            const <Uri>[],
+        enumTest:
+            _$EnumTestEnumMap._$fromName(
+              fromBase64(state.uri.queryParameters['enum-test']!),
+            )!,
+      );
+
+  CustomParameterComplexRoute get _self => this as CustomParameterComplexRoute;
+
+  @override
+  String get location => GoRouteData.$location(
+    '/${Uri.encodeComponent(toBase64(_self.id.toString()))}/',
+    queryParams: {
+      if (_self.dir != '') 'dir': toBase64(_self.dir),
+      if (!_$iterablesEqual(_self.list, const <Uri>[]))
+        'list': _self.list.map((e) => toBase64(e.toString())).toList(),
+      'enum-test': toBase64(_$EnumTestEnumMap[_self.enumTest]),
+    },
+  );
+
+  @override
+  void go(BuildContext context) => context.go(location);
+
+  @override
+  Future<T?> push<T>(BuildContext context) => context.push<T>(location);
+
+  @override
+  void pushReplacement(BuildContext context) =>
+      context.pushReplacement(location);
+
+  @override
+  void replace(BuildContext context) => context.replace(location);
+}
+
+const _$EnumTestEnumMap = {EnumTest.a: 'a', EnumTest.b: 'b', EnumTest.c: 'c'};
+
+extension<T extends Enum> on Map<T, String> {
+  T? _$fromName(String? value) =>
+      entries.where((element) => element.value == value).firstOrNull?.key;
+}
+
+bool _$iterablesEqual<T>(Iterable<T>? iterable1, Iterable<T>? iterable2) {
+  if (identical(iterable1, iterable2)) return true;
+  if (iterable1 == null || iterable2 == null) return false;
+  final iterator1 = iterable1.iterator;
+  final iterator2 = iterable2.iterator;
+  while (true) {
+    final hasNext1 = iterator1.moveNext();
+    final hasNext2 = iterator2.moveNext();
+    if (hasNext1 != hasNext2) return false;
+    if (!hasNext1) return true;
+    if (iterator1.current != iterator2.current) return false;
+  }
+}
diff --git a/packages/go_router_builder/test_inputs/json.dart b/packages/go_router_builder/test_inputs/json.dart
new file mode 100644
index 000000000000..90026d35049d
--- /dev/null
+++ b/packages/go_router_builder/test_inputs/json.dart
@@ -0,0 +1,29 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:go_router/go_router.dart';
+
+mixin $GoodJson {}
+
+@TypedGoRoute<GoodJson>(path: '/')
+class GoodJson extends GoRouteData with $GoodJson {
+  const GoodJson({required this.id, this.optionalField});
+
+  final JsonExample id;
+  final JsonExample? optionalField;
+}
+
+class JsonExample {
+  const JsonExample({required this.id});
+
+  factory JsonExample.fromJson(Map<String, dynamic> json) {
+    return JsonExample(id: json['id'] as String);
+  }
+
+  final String id;
+
+  Map<String, dynamic> toJson() {
+    return <String, dynamic>{'id': id};
+  }
+}
diff --git a/packages/go_router_builder/test_inputs/json.dart.expect b/packages/go_router_builder/test_inputs/json.dart.expect
new file mode 100644
index 000000000000..56e0bf4814a4
--- /dev/null
+++ b/packages/go_router_builder/test_inputs/json.dart.expect
@@ -0,0 +1,51 @@
+RouteBase get $goodJson =>
+    GoRouteData.$route(path: '/', factory: $GoodJson._fromState);
+
+mixin $GoodJson on GoRouteData {
+  static GoodJson _fromState(GoRouterState state) => GoodJson(
+    id: (String json0) {
+      return JsonExample.fromJson(jsonDecode(json0) as Map<String, dynamic>);
+    }(state.uri.queryParameters['id']!),
+    optionalField: _$convertMapValue(
+      'optional-field',
+      state.uri.queryParameters,
+      (String json0) {
+        return JsonExample.fromJson(jsonDecode(json0) as Map<String, dynamic>);
+      },
+    ),
+  );
+
+  GoodJson get _self => this as GoodJson;
+
+  @override
+  String get location => GoRouteData.$location(
+    '/',
+    queryParams: {
+      'id': jsonEncode(_self.id.toJson()),
+      if (_self.optionalField != null)
+        'optional-field': jsonEncode(_self.optionalField!.toJson()),
+    },
+  );
+
+  @override
+  void go(BuildContext context) => context.go(location);
+
+  @override
+  Future<T?> push<T>(BuildContext context) => context.push<T>(location);
+
+  @override
+  void pushReplacement(BuildContext context) =>
+      context.pushReplacement(location);
+
+  @override
+  void replace(BuildContext context) => context.replace(location);
+}
+
+T? _$convertMapValue<T>(
+  String key,
+  Map<String, String> map,
+  T? Function(String) converter,
+) {
+  final value = map[key];
+  return value == null ? null : converter(value);
+}
diff --git a/packages/go_router_builder/test_inputs/json_template.dart b/packages/go_router_builder/test_inputs/json_template.dart
new file mode 100644
index 000000000000..c4cc509a4c82
--- /dev/null
+++ b/packages/go_router_builder/test_inputs/json_template.dart
@@ -0,0 +1,46 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:go_router/go_router.dart';
+
+mixin $JsonTemplateRoute {}
+
+@TypedGoRoute<JsonTemplateRoute>(path: '/')
+class JsonTemplateRoute extends GoRouteData with $JsonTemplateRoute {
+  const JsonTemplateRoute({required this.nested, this.deepNested});
+
+  final JsonExampleNested<JsonExample> nested;
+  final JsonExampleNested<JsonExampleNested<JsonExample>>? deepNested;
+}
+
+class JsonExample {
+  const JsonExample({required this.id});
+
+  factory JsonExample.fromJson(Map<String, dynamic> json) {
+    return JsonExample(id: json['id'] as String);
+  }
+
+  Map<String, dynamic> toJson() {
+    return <String, dynamic>{'id': id};
+  }
+
+  final String id;
+}
+
+class JsonExampleNested<T> {
+  const JsonExampleNested({required this.child});
+
+  factory JsonExampleNested.fromJson(
+    Map<String, dynamic> json,
+    T Function(Object? json) fromJsonT,
+  ) {
+    return JsonExampleNested<T>(child: fromJsonT(json['child']));
+  }
+
+  Map<String, dynamic> toJson() {
+    return <String, dynamic>{'child': child};
+  }
+
+  final T child;
+}
diff --git a/packages/go_router_builder/test_inputs/json_template.dart.expect b/packages/go_router_builder/test_inputs/json_template.dart.expect
new file mode 100644
index 000000000000..a2c04c4392ba
--- /dev/null
+++ b/packages/go_router_builder/test_inputs/json_template.dart.expect
@@ -0,0 +1,65 @@
+RouteBase get $jsonTemplateRoute => GoRouteData.$route(
+      path: '/',
+      factory: $JsonTemplateRoute._fromState,
+    );
+
+mixin $JsonTemplateRoute on GoRouteData {
+  static JsonTemplateRoute _fromState(GoRouterState state) => JsonTemplateRoute(
+        nested: (String json0) {
+          return JsonExampleNested.fromJson(
+            jsonDecode(json0) as Map<String, dynamic>,
+            (Object? json1) {
+              return JsonExample.fromJson(json1 as Map<String, dynamic>);
+            },
+          );
+        }(state.uri.queryParameters['nested']!),
+        deepNested: _$convertMapValue('deep-nested', state.uri.queryParameters,
+            (String json0) {
+          return JsonExampleNested.fromJson(
+            jsonDecode(json0) as Map<String, dynamic>,
+            (Object? json1) {
+              return JsonExampleNested.fromJson(
+                json1 as Map<String, dynamic>,
+                (Object? json2) {
+                  return JsonExample.fromJson(json2 as Map<String, dynamic>);
+                },
+              );
+            },
+          );
+        }),
+      );
+
+  JsonTemplateRoute get _self => this as JsonTemplateRoute;
+
+  @override
+  String get location => GoRouteData.$location(
+        '/',
+        queryParams: {
+          'nested': jsonEncode(_self.nested.toJson()),
+          if (_self.deepNested != null)
+            'deep-nested': jsonEncode(_self.deepNested!.toJson()),
+        },
+      );
+
+  @override
+  void go(BuildContext context) => context.go(location);
+
+  @override
+  Future<T?> push<T>(BuildContext context) => context.push<T>(location);
+
+  @override
+  void pushReplacement(BuildContext context) =>
+      context.pushReplacement(location);
+
+  @override
+  void replace(BuildContext context) => context.replace(location);
+}
+
+T? _$convertMapValue<T>(
+  String key,
+  Map<String, String> map,
+  T? Function(String) converter,
+) {
+  final value = map[key];
+  return value == null ? null : converter(value);
+}