diff --git a/packages/ack_generator/CHANGELOG.md b/packages/ack_generator/CHANGELOG.md
index c336cf58..48148e21 100644
--- a/packages/ack_generator/CHANGELOG.md
+++ b/packages/ack_generator/CHANGELOG.md
@@ -7,6 +7,8 @@
   strict/passthrough policy to object models and aliases.
 * Reject object shape-changing methods that cannot be modeled statically,
   instead of generating fields that disagree with the authored schema.
+* Resolve same-build scalar, list, and codec model references using their
+  actual runtime types, and generate valid codecs for nullable nested Sets.
 
 * Allow built-in and contextual Dart keywords such as `required` as generated
   field, discriminator, and capture-member names without changing their JSON
diff --git a/packages/ack_generator/lib/src/analyzer/custom_schema_resolver.dart b/packages/ack_generator/lib/src/analyzer/custom_schema_resolver.dart
index 4f7a270e..0b3c308d 100644
--- a/packages/ack_generator/lib/src/analyzer/custom_schema_resolver.dart
+++ b/packages/ack_generator/lib/src/analyzer/custom_schema_resolver.dart
@@ -826,6 +826,22 @@ final class CustomSchemaResolver {
         field: field,
       );
       if (target == null) return null;
+      final schemaType = switch (target) {
+        TopLevelVariableElement(:final type) => type,
+        GetterElement(:final returnType) => returnType,
+        _ => null,
+      };
+      final runtime = schemaType == null
+          ? null
+          : _base.ackSchemaRuntimeType(schemaType);
+      if (runtime == null || _base.containsInvalidType(runtime)) {
+        throw InvalidGenerationSource(
+          '${_fieldPath(field)} resolves to generated model "$name", but '
+          '${target.name} has no resolvable AckSchema runtime type. Declare '
+          'the schema type explicitly.',
+          element: field,
+        );
+      }
       final visibleName = qualifySymbol(prefix, name);
       result = (
         schemaExpression: '$visibleName.\$ack.schema',
@@ -835,7 +851,11 @@ final class CustomSchemaResolver {
             declarationName: target.name!,
           ),
           className: name,
-          runtimeRef: ModelGraphBuilderBase.jsonMapRef,
+          runtimeRef: _base.inferTypeRef(
+            runtime,
+            path: _fieldPath(field),
+            context: field,
+          ),
           importPrefix: prefix,
         ),
         setListSchema: null,
diff --git a/packages/ack_generator/lib/src/inference/schema_inference.dart b/packages/ack_generator/lib/src/inference/schema_inference.dart
index fa254bda..6c831136 100644
--- a/packages/ack_generator/lib/src/inference/schema_inference.dart
+++ b/packages/ack_generator/lib/src/inference/schema_inference.dart
@@ -70,7 +70,11 @@ final class AckSchemaInference {
         rejectNullableCollectionElement: rejectNullableCollectionElement,
         validateMapKey: validateMapKey,
       );
-      return '${_ack('Ack')}.list($itemSchema).codec<${renderType(type)}>(decode: (list) => list.toSet(), encode: (set) => set.toList(growable: false),)';
+      final runtimeType = type.element.instantiate(
+        typeArguments: type.typeArguments,
+        nullabilitySuffix: NullabilitySuffix.none,
+      );
+      return '${_ack('Ack')}.list($itemSchema).codec<${renderType(runtimeType)}>(decode: (list) => list.toSet(), encode: (set) => set.toList(growable: false),)';
     }
     if (type.isDartCoreMap) {
       validateMapKey(type);
diff --git a/packages/ack_generator/test/integration/class_first_nested_runtime_build_test.dart b/packages/ack_generator/test/integration/class_first_nested_runtime_build_test.dart
index f8c81d35..4df7eb74 100644
--- a/packages/ack_generator/test/integration/class_first_nested_runtime_build_test.dart
+++ b/packages/ack_generator/test/integration/class_first_nested_runtime_build_test.dart
@@ -252,6 +252,76 @@ final class Holder with _$HolderAck {
 }
 ''',
         );
+        File(
+          p.join(temporary.path, 'lib', 'value_models.dart'),
+        ).writeAsStringSync(r'''
+import 'package:ack/ack.dart';
+
+part 'value_models.g.dart';
+
+final class Token {
+  const Token(this.value);
+  final String value;
+  static int decodes = 0;
+  static int encodes = 0;
+
+  static Token decode(String value) {
+    decodes++;
+    return Token(value);
+  }
+
+  static String encode(Token token) {
+    encodes++;
+    return token.value;
+  }
+
+  @override
+  bool operator ==(Object other) => other is Token && other.value == value;
+  @override
+  int get hashCode => value.hashCode;
+}
+
+@Schemable()
+final scalarSchema = Ack.integer();
+@Schemable()
+final stringsSchema = Ack.list(Ack.string());
+@Schemable()
+final encodedSchema = Ack.string().codec<Token>(
+  decode: Token.decode,
+  encode: Token.encode,
+);
+''');
+        File(
+          p.join(temporary.path, 'lib', 'value_holder.dart'),
+        ).writeAsStringSync(r'''
+import 'package:ack/ack.dart';
+import 'value_models.dart' as values;
+
+part 'value_holder.g.dart';
+
+@Schemable()
+final class ValueHolder with _$ValueHolderAck {
+  const ValueHolder({
+    required this.scalar,
+    required this.strings,
+    required this.encoded,
+    required this.history,
+    required this.unique,
+    this.patch = const JsonMaybe.absent(),
+    required this.sets,
+    required this.nestedSets,
+  });
+
+  final values.Scalar scalar;
+  final values.Strings strings;
+  final values.Encoded encoded;
+  final List<values.Scalar> history;
+  final Set<values.Scalar> unique;
+  final JsonMaybe<values.Encoded> patch;
+  final Map<String, Set<int>?> sets;
+  final Map<String, Map<String, Set<String>?>?> nestedSets;
+}
+''');
         File(
           p.join(temporary.path, 'lib', 'address_envelope.dart'),
         ).writeAsStringSync(r'''
@@ -284,9 +354,55 @@ import 'package:ack_class_first_nested/itinerary.dart';
 import 'package:ack_class_first_nested/order.dart';
 import 'package:ack_class_first_nested/parcel.dart';
 import 'package:ack_class_first_nested/pet.dart';
+import 'package:ack_class_first_nested/value_holder.dart';
+import 'package:ack_class_first_nested/value_models.dart' as values;
+import 'package:ack/ack.dart';
 import 'package:test/test.dart';
 
 void main() {
+  test('same-build value models and nullable nested Sets round-trip', () {
+    values.Token.decodes = 0;
+    values.Token.encodes = 0;
+    final json = <String, Object?>{
+      'scalar': 7,
+      'strings': ['one', 'two'],
+      'encoded': 'token',
+      'history': [1, 2],
+      'unique': [3, 4],
+      'sets': {'present': [1, 2], 'none': null},
+      'nestedSets': {
+        'present': {'items': ['a', 'b'], 'none': null},
+        'none': null,
+      },
+    };
+    final holder = ValueHolderSchema.parse(json);
+    expect(holder.scalar.value, 7);
+    expect(holder.strings.value, ['one', 'two']);
+    expect(holder.encoded.value.value, 'token');
+    expect(holder.history.map((model) => model.value), [1, 2]);
+    expect(holder.unique.map((model) => model.value), [3, 4]);
+    expect(holder.patch, const JsonMaybe<values.Encoded>.absent());
+    expect(holder.sets['present'], {1, 2});
+    expect(holder.sets['none'], isNull);
+    expect(holder.nestedSets['present']!['items'], {'a', 'b'});
+    expect(holder.nestedSets['present']!['none'], isNull);
+    expect(holder.nestedSets['none'], isNull);
+    expect(values.Token.decodes, 1);
+    expect(holder.toJson(), json);
+    expect(values.Token.encodes, 1);
+    expect(ValueHolderSchema.parse(holder.toJson()), holder);
+    expect(() => holder.strings.value.add('three'), throwsUnsupportedError);
+    expect(() => holder.sets['present']!.add(3), throwsUnsupportedError);
+    expect(
+      () => holder.nestedSets['present']!['items']!.add('c'),
+      throwsUnsupportedError,
+    );
+
+    final patched = ValueHolderSchema.parse({...json, 'patch': 'changed'});
+    expect(patched.patch.valueOrNull!.value.value, 'changed');
+    expect(patched.toJson()['patch'], 'changed');
+  });
+
   test('nested models parse, encode, and export through facades', () {
     final order = OrderSchema.parse({
       'shipping': {'city': 'Amsterdam'},
diff --git a/packages/ack_generator/test/integration/class_first_value_runtime_refs_test.dart b/packages/ack_generator/test/integration/class_first_value_runtime_refs_test.dart
new file mode 100644
index 00000000..f76ffb7d
--- /dev/null
+++ b/packages/ack_generator/test/integration/class_first_value_runtime_refs_test.dart
@@ -0,0 +1,82 @@
+import 'package:ack_generator/src/builder.dart';
+import 'package:build/build.dart';
+import 'package:build_test/build_test.dart';
+import 'package:test/test.dart';
+
+Future<void> _expectOutput(String source, Matcher matcher) async {
+  final readerWriter = TestReaderWriter(rootPackage: 'test_pkg');
+  await readerWriter.testing.loadIsolateSources();
+  await testBuilder(
+    ackModelBuilder(BuilderOptions.empty),
+    {'test_pkg|lib/models.dart': source},
+    generateFor: const {'test_pkg|lib/models.dart'},
+    readerWriter: readerWriter,
+    outputs: {'test_pkg|lib/models.g.dart': decodedMatches(matcher)},
+  );
+}
+
+void main() {
+  test(
+    'same-build value models use their actual schema runtime types',
+    () async {
+      await _expectOutput(
+        r'''
+import 'package:ack/ack.dart';
+part 'models.g.dart';
+
+final class Token {
+  const Token(this.value);
+  final String value;
+}
+
+@Schemable()
+final scalarSchema = Ack.integer();
+@Schemable()
+final stringsSchema = Ack.list(Ack.string());
+@Schemable()
+final encodedSchema = Ack.string().codec<Token>(
+  decode: Token.new,
+  encode: (token) => token.value,
+);
+
+@Schemable()
+final class Holder with _$HolderAck {
+  const Holder({required this.scalar, required this.strings, required this.encoded});
+  final Scalar scalar;
+  final Strings strings;
+  final Encoded encoded;
+}
+''',
+        allOf([
+          contains(r"Scalar.$ack.fromRuntime(value['scalar'] as int)"),
+          contains(
+            r"Strings.$ack.fromRuntime(value['strings'] as List<String>)",
+          ),
+          contains(r"Encoded.$ack.fromRuntime(value['encoded'] as Token)"),
+        ]),
+      );
+    },
+  );
+
+  test('nullable nested Sets use non-nullable codec type arguments', () async {
+    await _expectOutput(
+      r'''
+import 'package:ack/ack.dart';
+part 'models.g.dart';
+
+@Schemable()
+final class Holder with _$HolderAck {
+  const Holder({required this.values, required this.nested});
+  final Map<String, Set<int>?> values;
+  final Map<String, Map<String, Set<String>?>?> nested;
+}
+''',
+      allOf([
+        contains('.codec<Set<int>>('),
+        contains('.codec<Set<String>>('),
+        isNot(contains('.codec<Set<int>?>(')),
+        isNot(contains('.codec<Set<String>?>(')),
+      ]),
+    );
+  });
+}