diff --git a/backend/app/Services/GeminiService.php b/backend/app/Services/GeminiService.php
index a644cb4..2f0517c 100644
--- a/backend/app/Services/GeminiService.php
+++ b/backend/app/Services/GeminiService.php
@@ -4,12 +4,14 @@ namespace App\Services;
 
 class GeminiService
 {
+    public const DEFAULT_MODEL = 'gemini-3.1-flash-lite';
+
     /**
      * Call Gemini API to generate a response
      */
     public static function generateResponse(string $apiKey, string $systemPrompt, string $userMessage): ?string
     {
-        $url = 'https://generativelanguage.googleapis.com/v1beta/models/gemini-flash-lite-latest:generateContent?key=' . $apiKey;
+        $url = 'https://generativelanguage.googleapis.com/v1beta/models/' . self::DEFAULT_MODEL . ':generateContent?key=' . $apiKey;
 
         $payload = json_encode([
             'contents' => [
@@ -54,7 +56,7 @@ class GeminiService
      */
     public static function generatePromptFromAudio(string $apiKey, string $audioBase64, string $mimeType): ?string
     {
-        $url = 'https://generativelanguage.googleapis.com/v1beta/models/gemini-flash-lite-latest:generateContent?key=' . $apiKey;
+        $url = 'https://generativelanguage.googleapis.com/v1beta/models/' . self::DEFAULT_MODEL . ':generateContent?key=' . $apiKey;
 
         $payload = json_encode([
             'contents' => [
@@ -102,7 +104,7 @@ class GeminiService
      */
     public static function generateResponseFromAudio(string $apiKey, string $systemPrompt, string $audioBase64, string $mimeType): ?string
     {
-        $url = 'https://generativelanguage.googleapis.com/v1beta/models/gemini-flash-lite-latest:generateContent?key=' . $apiKey;
+        $url = 'https://generativelanguage.googleapis.com/v1beta/models/' . self::DEFAULT_MODEL . ':generateContent?key=' . $apiKey;
 
         // Clean mimeType if it contains codec info
         if (strpos($mimeType, ';') !== false) {
@@ -156,11 +158,11 @@ class GeminiService
     }
 
     /**
-     * Transcribe incoming audio voice note to text using gemini-2.0-flash-lite
+     * Transcribe incoming audio voice note to text using gemini-3.1-flash-lite
      */
     public static function transcribeAudio(string $apiKey, string $audioBase64, string $mimeType): ?string
     {
-        $url = 'https://generativelanguage.googleapis.com/v1beta/models/gemini-2.0-flash-lite:generateContent?key=' . $apiKey;
+        $url = 'https://generativelanguage.googleapis.com/v1beta/models/' . self::DEFAULT_MODEL . ':generateContent?key=' . $apiKey;
 
         // Clean mimeType if it contains codec info
         if (strpos($mimeType, ';') !== false) {
@@ -213,7 +215,7 @@ class GeminiService
      */
     public static function generateResponseFromImage(string $apiKey, string $systemPrompt, string $imageBase64, string $mimeType): ?string
     {
-        $url = 'https://generativelanguage.googleapis.com/v1beta/models/gemini-flash-lite-latest:generateContent?key=' . $apiKey;
+        $url = 'https://generativelanguage.googleapis.com/v1beta/models/' . self::DEFAULT_MODEL . ':generateContent?key=' . $apiKey;
 
         // Clean mimeType if it contains codec info
         if (strpos($mimeType, ';') !== false) {
@@ -271,7 +273,7 @@ class GeminiService
      */
     public static function generateOcrFromImage(string $apiKey, string $prompt, string $imageBase64, string $mimeType): ?string
     {
-        $url = 'https://generativelanguage.googleapis.com/v1beta/models/gemini-2.0-flash-lite:generateContent?key=' . $apiKey;
+        $url = 'https://generativelanguage.googleapis.com/v1beta/models/' . self::DEFAULT_MODEL . ':generateContent?key=' . $apiKey;
 
         // Clean mimeType if it contains codec info
         if (strpos($mimeType, ';') !== false) {