diff --git a/packages/ack_generator/test/integration/ack_infer_graph_test.dart b/packages/ack_generator/test/integration/ack_infer_graph_test.dart
index 5e62973d..1b4f85a9 100644
--- a/packages/ack_generator/test/integration/ack_infer_graph_test.dart
+++ b/packages/ack_generator/test/integration/ack_infer_graph_test.dart
@@ -5,9 +5,10 @@ import 'package:build_test/build_test.dart';
 import 'package:logging/logging.dart';
 import 'package:test/test.dart';
 
+import '../test_utils/isolate_sources.dart';
+
 Future<void> _expectOutput(String source, Matcher matcher) async {
-  final readerWriter = TestReaderWriter(rootPackage: 'test_pkg');
-  await readerWriter.testing.loadIsolateSources();
+  final readerWriter = await createTestReaderWriter();
   await testBuilder(
     ackModelBuilder(BuilderOptions.empty),
     {'test_pkg|lib/schema.dart': source},
@@ -18,8 +19,7 @@ Future<void> _expectOutput(String source, Matcher matcher) async {
 }
 
 Future<void> _expectFailure(String source, List<String> messages) async {
-  final readerWriter = TestReaderWriter(rootPackage: 'test_pkg');
-  await readerWriter.testing.loadIsolateSources();
+  final readerWriter = await createTestReaderWriter();
   final seen = <String>{};
   await testBuilder(
     ackModelBuilder(BuilderOptions.empty),
@@ -47,8 +47,7 @@ part 'schema.ack.g.dart';
 
 void main() {
   test('rejects a codec type the library cannot name', () async {
-    final readerWriter = TestReaderWriter(rootPackage: 'test_pkg');
-    await readerWriter.testing.loadIsolateSources();
+    final readerWriter = await createTestReaderWriter();
     final errors = <String>[];
     await testBuilder(
       ackModelBuilder(BuilderOptions.empty),
diff --git a/packages/ack_generator/test/integration/class_model_graph_test.dart b/packages/ack_generator/test/integration/class_model_graph_test.dart
index 93f65eaa..67972287 100644
--- a/packages/ack_generator/test/integration/class_model_graph_test.dart
+++ b/packages/ack_generator/test/integration/class_model_graph_test.dart
@@ -4,6 +4,8 @@ import 'package:build_test/build_test.dart';
 import 'package:logging/logging.dart';
 import 'package:test/test.dart';
 
+import '../test_utils/isolate_sources.dart';
+
 Future<void> _expectFailure(
   String body,
   List<String> messages, {
@@ -11,8 +13,7 @@ Future<void> _expectFailure(
   Map<String, String> extraSources = const {},
   Map<String, Object> allowedOutputs = const {},
 }) async {
-  final readerWriter = TestReaderWriter(rootPackage: 'test_pkg');
-  await readerWriter.testing.loadIsolateSources();
+  final readerWriter = await createTestReaderWriter();
   final seen = <String>{};
   await testBuilder(
     ackModelBuilder(BuilderOptions.empty),
@@ -35,8 +36,7 @@ Future<void> _expectFailure(
 }
 
 Future<void> _expectWarning(String body, List<String> messages) async {
-  final readerWriter = TestReaderWriter(rootPackage: 'test_pkg');
-  await readerWriter.testing.loadIsolateSources();
+  final readerWriter = await createTestReaderWriter();
   final seen = <String>{};
   await testBuilder(
     ackModelBuilder(BuilderOptions.empty),
@@ -1198,8 +1198,7 @@ final class Address with _\$AddressAck {
   );
 
   test('allows split imports for a class-first model and its facade', () async {
-    final readerWriter = TestReaderWriter(rootPackage: 'test_pkg');
-    await readerWriter.testing.loadIsolateSources();
+    final readerWriter = await createTestReaderWriter();
     await testBuilder(
       ackModelBuilder(BuilderOptions.empty),
       {
diff --git a/packages/ack_generator/test/test_utils/isolate_sources.dart b/packages/ack_generator/test/test_utils/isolate_sources.dart
new file mode 100644
index 00000000..8a0c1ee1
--- /dev/null
+++ b/packages/ack_generator/test/test_utils/isolate_sources.dart
@@ -0,0 +1,30 @@
+import 'dart:typed_data';
+
+import 'package:build/build.dart';
+import 'package:build_test/build_test.dart';
+
+final _isolateSources = _loadIsolateSources();
+
+/// Loads immutable dependency bytes into a fresh filesystem for each case.
+///
+/// Visible package files must stay unchanged during the test isolate. A new
+/// test invocation reloads them. Fixtures and generated output are never cached.
+Future<TestReaderWriter> createTestReaderWriter() async {
+  final sources = await _isolateSources;
+  final readerWriter = TestReaderWriter(rootPackage: 'test_pkg');
+  for (final entry in sources.entries) {
+    // Seed inputs without recording them as builder outputs. writeBytes copies
+    // the bytes, so a case can also mutate its own read buffers independently.
+    readerWriter.testing.writeBytes(entry.key, entry.value);
+  }
+  return readerWriter;
+}
+
+Future<Map<AssetId, Uint8List>> _loadIsolateSources() async {
+  final readerWriter = TestReaderWriter();
+  await readerWriter.testing.loadIsolateSources();
+  return Map.unmodifiable({
+    for (final id in readerWriter.testing.assets)
+      id: readerWriter.testing.readBytes(id).asUnmodifiableView(),
+  });
+}
diff --git a/packages/ack_generator/test/test_utils/isolate_sources_test.dart b/packages/ack_generator/test/test_utils/isolate_sources_test.dart
new file mode 100644
index 00000000..255ca259
--- /dev/null
+++ b/packages/ack_generator/test/test_utils/isolate_sources_test.dart
@@ -0,0 +1,127 @@
+import 'package:ack_generator/src/builder.dart';
+import 'package:build/build.dart';
+import 'package:build_test/build_test.dart';
+import 'package:test/test.dart';
+
+import 'isolate_sources.dart';
+
+void main() {
+  test('preserves every visible dependency asset and its bytes', () async {
+    final original = TestReaderWriter(rootPackage: 'test_pkg');
+    await original.testing.loadIsolateSources();
+    final readerWriter = await createTestReaderWriter();
+
+    expect(original.testing.assets, isNotEmpty);
+    expect(
+      readerWriter.testing.assets,
+      unorderedEquals(original.testing.assets),
+    );
+    for (final id in original.testing.assets) {
+      expect(
+        readerWriter.testing.readBytes(id),
+        original.testing.readBytes(id),
+        reason: 'Dependency bytes changed for $id',
+      );
+    }
+    expect(readerWriter.testing.assetsRead, isEmpty);
+    expect(readerWriter.testing.assetsWritten, isEmpty);
+  });
+
+  test('reader mutations, outputs and tracking stay in their case', () async {
+    final first = await createTestReaderWriter();
+    final dependency = AssetId('ack', 'lib/ack.dart');
+    final removed = AssetId('ack', 'lib/annotations.dart');
+    final fixture = AssetId('test_pkg', 'lib/schema.dart');
+    final output = AssetId('test_pkg', 'lib/schema.ack.dart');
+    final originalBytes = List<int>.of(first.testing.readBytes(dependency));
+
+    await first.readAsBytes(dependency);
+    first.testing.readBytes(dependency)[0] ^= 0xff;
+    first.testing.delete(removed);
+    first.testing.writeString(fixture, 'final fixture = 1;');
+    await first.writeAsString(output, 'generated in the first case');
+    expect(first.testing.assetsRead, contains(dependency));
+    expect(first.testing.assetsWritten, contains(output));
+
+    final second = await createTestReaderWriter();
+    expect(second, isNot(same(first)));
+    expect(second.testing.readBytes(dependency), originalBytes);
+    expect(second.testing.exists(removed), isTrue);
+    expect(second.testing.exists(fixture), isFalse);
+    expect(second.testing.exists(output), isFalse);
+    expect(second.testing.assetsRead, isEmpty);
+    expect(second.testing.assetsWritten, isEmpty);
+  });
+
+  test(
+    'concurrent requests get independent readers and byte buffers',
+    () async {
+      final readers = await Future.wait([
+        createTestReaderWriter(),
+        createTestReaderWriter(),
+      ]);
+      final dependency = AssetId('ack', 'lib/ack.dart');
+      final originalBytes = List<int>.of(
+        readers[1].testing.readBytes(dependency),
+      );
+      readers[0].testing.readBytes(dependency)[0] ^= 0xff;
+
+      expect(readers[0], isNot(same(readers[1])));
+      expect(readers[1].testing.readBytes(dependency), originalBytes);
+      final later = await createTestReaderWriter();
+      expect(later.testing.readBytes(dependency), originalBytes);
+    },
+  );
+
+  for (final validFirst in [true, false]) {
+    test('resolves fresh fixtures and outputs with valid case '
+        '${validFirst ? 'before' : 'after'} a missing JSON part', () async {
+      for (final valid in [validFirst, !validFirst]) {
+        final readerWriter = await createTestReaderWriter();
+        final errors = <String>[];
+        expect(
+          readerWriter.testing.assets.where((id) => id.package == 'test_pkg'),
+          isEmpty,
+        );
+        expect(readerWriter.testing.assetsRead, isEmpty);
+        expect(readerWriter.testing.assetsWritten, isEmpty);
+
+        await testBuilder(
+          ackModelBuilder(BuilderOptions.empty),
+          {
+            'test_pkg|lib/schema.dart': valid
+                ? _source
+                : _source.replaceFirst("part 'schema.ack.g.dart';", ''),
+          },
+          generateFor: const {'test_pkg|lib/schema.dart'},
+          readerWriter: readerWriter,
+          outputs: valid
+              ? {
+                  'test_pkg|lib/schema.ack.dart': decodedMatches(
+                    contains('final class User'),
+                  ),
+                }
+              : const {},
+          onLog: (log) {
+            if (log.level.name == 'SEVERE') errors.add(log.message);
+          },
+        );
+        expect(
+          errors,
+          valid ? isEmpty : contains(contains("part 'schema.ack.g.dart';")),
+        );
+      }
+    });
+  }
+}
+
+const _source = '''
+import 'package:ack/ack.dart';
+import 'package:ack/annotations.dart';
+
+part 'schema.ack.dart';
+part 'schema.ack.g.dart';
+
+@Schemable()
+final userSchema = Ack.object({'name': Ack.string()});
+''';