fix: flutterfire configure command when existing firebase_options.dart has a wrong format

packages/flutterfire_cli/lib/src/firebase/firebase_dart_configuration_write.dart

@@ -59,11 +59,8 @@ class FirebaseDartConfigurationWrite {
       );
       outputFile.writeAsStringSync(updatedFileString);
     } else {
-      _writeHeader();
-      _writeClass();
-
       outputFile.createSync(recursive: true);
-      outputFile.writeAsStringSync(_stringBuffer.toString());
+      outputFile.writeAsStringSync(_buildConfigurationFile());
     }
 
     return _firebaseJsonWrites();
@@ -130,14 +127,28 @@ class FirebaseDartConfigurationWrite {
             platform == kWeb ? 'if (kIsWeb)' : 'case TargetPlatform.$platform:',
           ),
         );
-        final unsupportedErrorLineIndex = startIndex + 1;
 
-        if (fileConfigurationLines[unsupportedErrorLineIndex]
-            .contains('UnsupportedError')) {
+        // Existing file does not match generated FlutterFire structure.
+        // Regenerate so configure can recover from template placeholders.
+        if (startIndex == -1) {
+          return _buildConfigurationFile();
+        }
+
+        final unsupportedErrorLineIndex = fileConfigurationLines.indexWhere(
+          (line) =>
+              line.contains('throw UnsupportedError(') ||
+              line.contains('throw UnimplementedError('),
+          startIndex,
+        );
+
+        if (unsupportedErrorLineIndex != -1) {
           final endIndex = fileConfigurationLines.indexWhere(
             (line) => line.contains(');'),
             unsupportedErrorLineIndex,
           );
+          if (endIndex == -1) {
+            return _buildConfigurationFile();
+          }
           fileConfigurationLines.removeRange(
             unsupportedErrorLineIndex,
             endIndex + 1,
@@ -150,7 +161,7 @@ class FirebaseDartConfigurationWrite {
                 : '        return ${platform.toLowerCase()};',
           );
         } else {
-          throw Exception('`UnsupportedError` not found in $platform');
+          return _buildConfigurationFile();
         }
 
         final insertIndex = fileConfigurationLines.lastIndexOf('}');
@@ -169,6 +180,13 @@ class FirebaseDartConfigurationWrite {
     return formatList(fileConfigurationLines).join('\n');
   }
 
+  String _buildConfigurationFile() {
+    _stringBuffer.clear();
+    _writeHeader();
+    _writeClass();
+    return formatList(_stringBuffer.toString().split('\n')).join('\n');
+  }
+
   // ensure only one empty line between each static property
   List<String> formatList(List<String> items) {
     return items.fold<List<String>>([], (acc, item) {

packages/flutterfire_cli/test/firebase_dart_configuration_write_test.dart

@@ -0,0 +1,62 @@
+import 'dart:io';
+
+import 'package:flutterfire_cli/src/firebase/firebase_dart_configuration_write.dart';
+import 'package:flutterfire_cli/src/firebase/firebase_options.dart';
+import 'package:path/path.dart' as p;
+import 'package:test/test.dart';
+
+void main() {
+  group('FirebaseDartConfigurationWrite', () {
+    // Regression test for
+    // https://github.com/invertase/flutterfire_cli/issues/422
+    test(
+      'replaces placeholder firebase_options.dart with generated output',
+      () {
+        final tempDir = Directory.systemTemp.createTempSync();
+        addTearDown(() => tempDir.deleteSync(recursive: true));
+
+        final filePath = p.join(tempDir.path, 'lib', 'firebase_options.dart');
+        File(filePath)
+          ..createSync(recursive: true)
+          ..writeAsStringSync('''
+// File normally generated by FlutterFire CLI. This is a stand-in.
+// See README.md for details.
+import 'package:firebase_core/firebase_core.dart' show FirebaseOptions;
+
+class DefaultFirebaseOptions {
+  // TODO: Remove this file and run `flutterfire config` for Firebase options
+  static FirebaseOptions get currentPlatform {
+    throw UnimplementedError(
+      'Generate this file by running `flutterfire configure`. '
+      'See README.md for details.',
+    );
+  }
+}
+''');
+
+        final writer = FirebaseDartConfigurationWrite(
+          configurationFilePath: filePath,
+          flutterAppPath: tempDir.path,
+          firebaseProjectId: 'test-project-id',
+          webOptions: const FirebaseOptions(
+            optionsSourceContent: '{}',
+            optionsSourceFileName: 'firebase-config.json',
+            apiKey: 'api-key',
+            appId: 'app-id',
+            messagingSenderId: 'sender-id',
+            projectId: 'test-project-id',
+            measurementId: 'measurement-id',
+          ),
+        );
+
+        writer.write();
+
+        final updatedContent = File(filePath).readAsStringSync();
+        expect(updatedContent, contains('if (kIsWeb) {'));
+        expect(updatedContent, contains('return web;'));
+        expect(updatedContent, contains('static const FirebaseOptions web'));
+        expect(updatedContent, isNot(contains('UnimplementedError')));
+      },
+    );
+  });
+}