diff --git a/lib/utils/string_helper.dart b/lib/utils/string_helper.dart
index c186241..2d18e1f 100644
--- a/lib/utils/string_helper.dart
+++ b/lib/utils/string_helper.dart
@@ -52,9 +52,7 @@ class StringHelper {
     var className = NamingConverter.generateClassName(name);
     final prefix = SwaggerConfig.modelClassPrefix;
     if (prefix != null && prefix.isNotEmpty) {
-      if (!className.startsWith(prefix)) {
-        className = prefix + className;
-      }
+      className = prefix + className;
     }
     return className;
   }
diff --git a/test/config_prefix_test.dart b/test/config_prefix_test.dart
index 21f31dd..60c843e 100644
--- a/test/config_prefix_test.dart
+++ b/test/config_prefix_test.dart
@@ -1,6 +1,5 @@
 import 'dart:io';
 
-import 'package:swagger_generator_flutter/core/config.dart';
 import 'package:swagger_generator_flutter/core/config_repository.dart';
 import 'package:swagger_generator_flutter/utils/string_helper.dart';
 import 'package:test/test.dart';
@@ -47,6 +46,48 @@ generator:
       expect(config.modelClassPrefix, isNull);
     });
 
+    test('should apply class_prefix using StringHelper.generateClassName', () {
+      configFile.writeAsStringSync('''
+generator:
+  name: test
+  
+output:
+  models:
+    class_prefix: "S"
+
+generation:
+  models:
+    class_prefix: "S"
+''');
+
+      // Reload config to ensure StringHelper picks up the new prefix
+      // This relies on SwaggerConfig.modelClassPrefix internally calling ConfigRepository.loadSync()
+      // which will find the config file in the current working directory (or tempDir in this test context).
+      // For this test to work, the test runner's CWD must be tempDir, or PathResolver must be mocked.
+      // Assuming PathResolver.findConfigFile() correctly finds the temp file.
+
+      // The current implementation of StringHelper.generateClassName relies on SwaggerConfig.modelClassPrefix
+      // which in turn calls ConfigRepository.loadSync() without arguments.
+      // This means it will try to find the config file in the current working directory or via PathResolver.
+      // For this test to pass, the `configFile` must be discoverable by `ConfigRepository.loadSync()`.
+      // This test implicitly relies on the test environment's CWD or PathResolver setup.
+
+      // To make this test robust, StringHelper should ideally take a Config object or
+      // SwaggerConfig should have a way to inject a test config.
+      // For now, we proceed with the assumption that `ConfigRepository.loadSync()` will find `configFile`.
+
+      // Force a reload of the static config by accessing it.
+      // This is a bit of a hack due to the static nature of SwaggerConfig and StringHelper.
+      // A better approach would be to pass the config explicitly or mock the static dependencies.
+      ConfigRepository.loadSync(configFile
+          .path); // Ensure the config is loaded for the static accessor
+
+      expect(StringHelper.generateClassName('User'), 'SUser');
+      expect(StringHelper.generateClassName('model'), 'SModel');
+      // Test case: prefix is 'S' and name starts with 'S'
+      expect(StringHelper.generateClassName('SubjectInfo'), 'SSubjectInfo');
+    });
+
     // NOTE: Testing StringHelper.generateClassName directly implies checking if it reads from the GLOBAL config.
     // However, ConfigRepository.loadSync() creates an instance, but SwaggerConfig accessors call ConfigRepository.loadSync() individually.
     // Since ConfigRepository.loadSync() without args looks for default file, we need a way to inject the config or point it to our file.