diff --git a/lib/core/editor/querya_highlight_controller.dart b/lib/core/editor/querya_highlight_controller.dart
index 5b463a50..793d78a5 100644
--- a/lib/core/editor/querya_highlight_controller.dart
+++ b/lib/core/editor/querya_highlight_controller.dart
@@ -74,7 +74,7 @@ class QueryaHighlightController extends TextEditingController {
       style: style,
     );
 
-    if (_cachedSpan != null) {
+    if (_cachedSpan != null && _cachedText == text) {
       return TextSpan(style: style, children: [_cachedSpan!]);
     }
 
diff --git a/test/core/editor/querya_highlight_controller_test.dart b/test/core/editor/querya_highlight_controller_test.dart
new file mode 100644
index 00000000..2fe0bcb6
--- /dev/null
+++ b/test/core/editor/querya_highlight_controller_test.dart
@@ -0,0 +1,105 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:querya_desktop/core/editor/querya_code_language.dart';
+import 'package:querya_desktop/core/editor/querya_highlight_controller.dart';
+import 'package:querya_desktop/core/editor/syntax_highlight_isolate.dart';
+import 'package:querya_desktop/core/editor/syntax_highlight_service.dart';
+import 'package:querya_desktop/core/theme/parser/token_colors_highlighter_config.dart';
+import 'package:querya_desktop/core/theme/querya_theme.dart';
+
+void main() {
+  setUpAll(() async {
+    TestWidgetsFlutterBinding.ensureInitialized();
+    await SyntaxHighlightService.ensureInitialized();
+  });
+
+  testWidgets('buildTextSpan ensures span.toPlainText() == text when text changes for > 8KB script',
+      (tester) async {
+    final pair = SyntaxHighlightService.createPair(
+      language: QueryaCodeLanguage.sql,
+      queryaTheme: QueryaTheme.darkDefault,
+    );
+    final lightHighlighter = pair.light;
+    final darkHighlighter = pair.dark;
+    final lightConfig = buildDefaultEditorHighlighterConfig(
+      QueryaTheme.lightDefault.editor,
+    );
+    final darkConfig = buildDefaultEditorHighlighterConfig(
+      QueryaTheme.darkDefault.editor,
+    );
+    final grammarJson = SyntaxHighlightService.grammarJsonFor(
+      QueryaCodeLanguage.sql,
+    );
+
+    // 1. Initialize controller with small text (< threshold) to populate _cachedSpan synchronously
+    const smallText = 'SELECT id FROM users;';
+    final controller = QueryaHighlightController(
+      text: smallText,
+      language: QueryaCodeLanguage.sql,
+      lightHighlighter: lightHighlighter,
+      darkHighlighter: darkHighlighter,
+      lightThemeConfig: lightConfig,
+      darkThemeConfig: darkConfig,
+      grammarJson: grammarJson,
+      wrapperColor: Colors.black,
+    );
+
+    late BuildContext buildContext;
+    await tester.pumpWidget(
+      MaterialApp(
+        theme: ThemeData.light(),
+        home: Builder(
+          builder: (ctx) {
+            buildContext = ctx;
+            return const SizedBox();
+          },
+        ),
+      ),
+    );
+
+    // Initial highlight for small text populates _cachedSpan
+    var span = controller.buildTextSpan(
+      context: buildContext,
+      withComposing: false,
+    );
+    expect(span.toPlainText(), smallText);
+    expect(span.children, isNotNull);
+
+    // 2. Switch to large text (> 8KB). Stale _cachedSpan from smallText must NOT be returned.
+    final largeText = List.filled(500, 'SELECT id FROM users; -- row').join('\n');
+    expect(largeText.length, greaterThan(kSyntaxHighlightIsolateThreshold));
+    controller.text = largeText;
+
+    span = controller.buildTextSpan(
+      context: buildContext,
+      withComposing: false,
+    );
+    // span.toPlainText() MUST match largeText, never stale smallText
+    expect(span.toPlainText(), largeText);
+    expect(span.toPlainText() == controller.text, isTrue);
+
+    // 3. User edits text while isolate highlight is pending (typing characters)
+    final modifiedLargeText = '$largeText\n-- extra query';
+    controller.text = modifiedLargeText;
+
+    span = controller.buildTextSpan(
+      context: buildContext,
+      withComposing: false,
+    );
+    expect(span.toPlainText(), modifiedLargeText);
+    expect(span.toPlainText() == controller.text, isTrue);
+
+    // 4. User deletes characters
+    final deletedText = modifiedLargeText.substring(0, modifiedLargeText.length - 25);
+    controller.text = deletedText;
+
+    span = controller.buildTextSpan(
+      context: buildContext,
+      withComposing: false,
+    );
+    expect(span.toPlainText(), deletedText);
+    expect(span.toPlainText() == controller.text, isTrue);
+
+    controller.dispose();
+  });
+}