diff --git a/admin/mobile-app-settings.php b/admin/mobile-app-settings.php
index 1c9891a..cc92f58 100644
--- a/admin/mobile-app-settings.php
+++ b/admin/mobile-app-settings.php
@@ -36,7 +36,19 @@ if (isset($_POST['twp_test_notification']) && check_admin_referer('twp_mobile_se
 
 // Save settings
 if (isset($_POST['twp_save_mobile_settings']) && check_admin_referer('twp_mobile_settings')) {
-    update_option('twp_fcm_server_key', sanitize_text_field($_POST['twp_fcm_server_key']));
+    update_option('twp_fcm_project_id', sanitize_text_field($_POST['twp_fcm_project_id']));
+    // Service account JSON — validate it parses as JSON before saving
+    $sa_json_raw = isset($_POST['twp_fcm_service_account_json']) ? wp_unslash($_POST['twp_fcm_service_account_json']) : '';
+    if (!empty($sa_json_raw)) {
+        $sa_parsed = json_decode($sa_json_raw, true);
+        if ($sa_parsed && isset($sa_parsed['client_email'], $sa_parsed['private_key'])) {
+            update_option('twp_fcm_service_account_json', $sa_json_raw);
+        } else {
+            $sa_json_error = 'Invalid service account JSON — must contain client_email and private_key fields.';
+        }
+    } else {
+        update_option('twp_fcm_service_account_json', '');
+    }
     update_option('twp_auto_update_enabled', isset($_POST['twp_auto_update_enabled']) ? '1' : '0');
     update_option('twp_gitea_repo', sanitize_text_field($_POST['twp_gitea_repo']));
     update_option('twp_gitea_token', sanitize_text_field($_POST['twp_gitea_token']));
@@ -48,7 +60,9 @@ if (isset($_POST['twp_save_mobile_settings']) && check_admin_referer('twp_mobile
 }
 
 // Get current settings
-$fcm_server_key = get_option('twp_fcm_server_key', '');
+$fcm_project_id = get_option('twp_fcm_project_id', '');
+$fcm_service_account_json = get_option('twp_fcm_service_account_json', '');
+$fcm_sa_configured = !empty($fcm_service_account_json) && !empty($fcm_project_id);
 $auto_update_enabled = get_option('twp_auto_update_enabled', '1') === '1';
 $gitea_repo = get_option('twp_gitea_repo', 'wp-plugins/twilio-wp-plugin');
 $gitea_token = get_option('twp_gitea_token', '');
@@ -90,6 +104,12 @@ $total_sessions = $wpdb->get_var("SELECT COUNT(*) FROM $sessions_table");
         </div>
     <?php endif; ?>
 
+    <?php if (isset($sa_json_error)): ?>
+        <div class="notice notice-error is-dismissible">
+            <p><strong><?php echo esc_html($sa_json_error); ?></strong></p>
+        </div>
+    <?php endif; ?>
+
     <div class="twp-mobile-settings">
         <!-- Mobile App Overview -->
         <div class="card" style="max-width: 100%; margin-bottom: 20px;">
@@ -118,26 +138,45 @@ $total_sessions = $wpdb->get_var("SELECT COUNT(*) FROM $sessions_table");
 
             <!-- FCM Configuration -->
             <div class="card" style="max-width: 100%; margin-bottom: 20px;">
-                <h2>Firebase Cloud Messaging (FCM)</h2>
-                <p>Configure FCM to enable push notifications for the mobile app.</p>
+                <h2>Firebase Cloud Messaging (FCM) — HTTP v2 API</h2>
+                <p>Configure FCM using a service account for push notifications. The legacy server key API has been retired by Google.</p>
 
                 <table class="form-table">
                     <tr>
                         <th scope="row">
-                            <label for="twp_fcm_server_key">FCM Server Key</label>
+                            <label for="twp_fcm_project_id">Firebase Project ID</label>
                         </th>
                         <td>
                             <input type="text"
-                                   id="twp_fcm_server_key"
-                                   name="twp_fcm_server_key"
-                                   value="<?php echo esc_attr($fcm_server_key); ?>"
+                                   id="twp_fcm_project_id"
+                                   name="twp_fcm_project_id"
+                                   value="<?php echo esc_attr($fcm_project_id); ?>"
                                    class="regular-text"
-                                   placeholder="AAAA...">
+                                   placeholder="my-project-12345">
                             <p class="description">
-                                Get your server key from Firebase Console > Project Settings > Cloud Messaging > Server Key
+                                Found in Firebase Console &gt; Project Settings &gt; General &gt; Project ID
                             </p>
                         </td>
                     </tr>
+                    <tr>
+                        <th scope="row">
+                            <label for="twp_fcm_service_account_json">Service Account JSON</label>
+                        </th>
+                        <td>
+                            <textarea id="twp_fcm_service_account_json"
+                                      name="twp_fcm_service_account_json"
+                                      rows="6"
+                                      class="large-text code"
+                                      placeholder='Paste the entire contents of your service account JSON file...'><?php echo esc_textarea($fcm_service_account_json); ?></textarea>
+                            <p class="description">
+                                Generate in Firebase Console &gt; Project Settings &gt; Service Accounts &gt; Generate New Private Key.
+                                Paste the entire JSON file contents here. Must contain <code>client_email</code> and <code>private_key</code> fields.
+                            </p>
+                            <?php if ($fcm_sa_configured): ?>
+                                <p style="color: #00a32a; margin-top: 5px;">&#10003; Service account configured</p>
+                            <?php endif; ?>
+                        </td>
+                    </tr>
                     <tr>
                         <th scope="row">
                             <label for="twp_twilio_api_key_sid">Twilio API Key SID</label>
@@ -181,13 +220,13 @@ $total_sessions = $wpdb->get_var("SELECT COUNT(*) FROM $sessions_table");
                                    class="regular-text"
                                    placeholder="CR...">
                             <p class="description">
-                                Twilio Push Credential SID. Create in Twilio Console &gt; Messaging &gt; Push Credentials using your FCM server key. Required for incoming call push notifications.
+                                Twilio Push Credential SID. Create in Twilio Console &gt; Messaging &gt; Push Credentials using your FCM service account JSON. Required for incoming call push notifications.
                             </p>
                         </td>
                     </tr>
                 </table>
 
-                <?php if (!empty($fcm_server_key)): ?>
+                <?php if ($fcm_sa_configured): ?>
                     <p>
                         <button type="submit" name="twp_test_notification" class="button">
                             Send Test Notification
diff --git a/includes/class-twp-fcm.php b/includes/class-twp-fcm.php
index 88b8059..579f4ec 100644
--- a/includes/class-twp-fcm.php
+++ b/includes/class-twp-fcm.php
@@ -1,27 +1,33 @@
 <?php
 /**
- * Firebase Cloud Messaging (FCM) Integration
+ * Firebase Cloud Messaging (FCM) Integration — HTTP v2 API
  *
- * Handles push notifications to mobile devices via FCM
+ * Handles push notifications to mobile devices via FCM using
+ * service account credentials and OAuth2 access tokens.
  */
 class TWP_FCM {
 
-    private $server_key;
-    private $fcm_url = 'https://fcm.googleapis.com/fcm/send';
+    private $project_id;
+    private $service_account;
+    private $fcm_url_template = 'https://fcm.googleapis.com/v1/projects/%s/messages:send';
 
     /**
      * Constructor
      */
     public function __construct() {
-        $this->server_key = get_option('twp_fcm_server_key', '');
+        $this->project_id = get_option('twp_fcm_project_id', '');
+        $sa_json = get_option('twp_fcm_service_account_json', '');
+        if (!empty($sa_json)) {
+            $this->service_account = json_decode($sa_json, true);
+        }
     }
 
     /**
      * Send push notification to user's devices
      */
     public function send_notification($user_id, $title, $body, $data = array(), $data_only = false) {
-        if (empty($this->server_key)) {
-            error_log('TWP FCM: Server key not configured');
+        if (empty($this->project_id) || empty($this->service_account)) {
+            error_log('TWP FCM: Project ID or service account not configured');
             return false;
         }
 
@@ -57,47 +63,54 @@ class TWP_FCM {
     }
 
     /**
-     * Send notification to specific token
+     * Send notification to specific token via FCM HTTP v2 API
      */
     private function send_to_token($token, $title, $body, $data = array(), $data_only = false) {
-        if ($data_only) {
-            $payload = array(
-                'to' => $token,
-                'data' => array_merge($data, array(
-                    'title' => $title,
-                    'body' => $body,
-                    'timestamp' => time()
-                )),
-                'priority' => 'high'
-            );
-        } else {
-            $notification = array(
+        $access_token = $this->get_access_token();
+        if (!$access_token) {
+            return array('success' => false, 'error' => 'auth_failed');
+        }
+
+        // FCM v2 requires all data values to be strings
+        $string_data = array();
+        foreach ($data as $key => $value) {
+            $string_data[$key] = is_string($value) ? $value : (string)$value;
+        }
+        $string_data['title'] = $title;
+        $string_data['body'] = $body;
+        $string_data['timestamp'] = (string)time();
+
+        // Build the v2 message payload
+        $message = array(
+            'token' => $token,
+            'data' => $string_data,
+            'android' => array(
+                'priority' => 'high',
+            ),
+        );
+
+        if (!$data_only) {
+            $message['notification'] = array(
                 'title' => $title,
                 'body' => $body,
-                'sound' => 'default',
-                'priority' => 'high',
-                'click_action' => 'FLUTTER_NOTIFICATION_CLICK'
             );
-
-            $payload = array(
-                'to' => $token,
-                'notification' => $notification,
-                'data' => array_merge($data, array(
-                    'title' => $title,
-                    'body' => $body,
-                    'timestamp' => time()
-                )),
-                'priority' => 'high'
+            $message['android']['notification'] = array(
+                'sound' => 'default',
+                'click_action' => 'FLUTTER_NOTIFICATION_CLICK',
             );
         }
 
+        $payload = array('message' => $message);
+
+        $url = sprintf($this->fcm_url_template, $this->project_id);
+
         $headers = array(
-            'Authorization: key=' . $this->server_key,
+            'Authorization: Bearer ' . $access_token,
             'Content-Type: application/json'
         );
 
         $ch = curl_init();
-        curl_setopt($ch, CURLOPT_URL, $this->fcm_url);
+        curl_setopt($ch, CURLOPT_URL, $url);
         curl_setopt($ch, CURLOPT_POST, true);
         curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
         curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
@@ -111,10 +124,14 @@ class TWP_FCM {
         if ($http_code !== 200) {
             error_log("TWP FCM: Failed to send notification. HTTP $http_code: $response");
 
-            // Check if token is invalid
             $response_data = json_decode($response, true);
-            if (isset($response_data['results'][0]['error']) &&
-                in_array($response_data['results'][0]['error'], array('InvalidRegistration', 'NotRegistered'))) {
+            $error_code = isset($response_data['error']['details'][0]['errorCode'])
+                ? $response_data['error']['details'][0]['errorCode'] : '';
+            $error_status = isset($response_data['error']['status'])
+                ? $response_data['error']['status'] : '';
+
+            if (in_array($error_code, array('UNREGISTERED', 'INVALID_ARGUMENT')) ||
+                $error_status === 'NOT_FOUND') {
                 return array('success' => false, 'error' => 'invalid_token');
             }
 
@@ -124,6 +141,107 @@ class TWP_FCM {
         return array('success' => true);
     }
 
+    /**
+     * Get OAuth2 access token from service account credentials.
+     * Caches the token in a transient until near expiry.
+     */
+    private function get_access_token() {
+        $cached = get_transient('twp_fcm_access_token');
+        if ($cached) {
+            return $cached;
+        }
+
+        if (empty($this->service_account)) {
+            error_log('TWP FCM: Service account not configured');
+            return false;
+        }
+
+        $jwt = $this->create_jwt();
+        if (!$jwt) {
+            return false;
+        }
+
+        $ch = curl_init();
+        curl_setopt($ch, CURLOPT_URL, $this->service_account['token_uri']);
+        curl_setopt($ch, CURLOPT_POST, true);
+        curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
+        curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, true);
+        curl_setopt($ch, CURLOPT_POSTFIELDS, http_build_query(array(
+            'grant_type' => 'urn:ietf:params:oauth:grant-type:jwt-bearer',
+            'assertion' => $jwt,
+        )));
+
+        $response = curl_exec($ch);
+        $http_code = curl_getinfo($ch, CURLINFO_HTTP_CODE);
+        curl_close($ch);
+
+        if ($http_code !== 200) {
+            error_log("TWP FCM: Failed to get access token. HTTP $http_code: $response");
+            return false;
+        }
+
+        $token_data = json_decode($response, true);
+        $access_token = $token_data['access_token'];
+        $expires_in = isset($token_data['expires_in']) ? (int)$token_data['expires_in'] : 3600;
+
+        // Cache token for 5 minutes less than actual expiry
+        set_transient('twp_fcm_access_token', $access_token, max(60, $expires_in - 300));
+
+        return $access_token;
+    }
+
+    /**
+     * Create a signed JWT for the service account OAuth2 flow
+     */
+    private function create_jwt() {
+        $sa = $this->service_account;
+
+        if (empty($sa['client_email']) || empty($sa['private_key']) || empty($sa['token_uri'])) {
+            error_log('TWP FCM: Service account JSON missing required fields');
+            return false;
+        }
+
+        $now = time();
+        $header = array('alg' => 'RS256', 'typ' => 'JWT');
+        $claims = array(
+            'iss' => $sa['client_email'],
+            'scope' => 'https://www.googleapis.com/auth/firebase.messaging',
+            'aud' => $sa['token_uri'],
+            'iat' => $now,
+            'exp' => $now + 3600,
+        );
+
+        $segments = array(
+            $this->base64url_encode(json_encode($header)),
+            $this->base64url_encode(json_encode($claims)),
+        );
+
+        $signing_input = implode('.', $segments);
+
+        $private_key = openssl_pkey_get_private($sa['private_key']);
+        if (!$private_key) {
+            error_log('TWP FCM: Failed to parse service account private key');
+            return false;
+        }
+
+        $signature = '';
+        if (!openssl_sign($signing_input, $signature, $private_key, OPENSSL_ALGO_SHA256)) {
+            error_log('TWP FCM: Failed to sign JWT');
+            return false;
+        }
+
+        $segments[] = $this->base64url_encode($signature);
+
+        return implode('.', $segments);
+    }
+
+    /**
+     * Base64url encode (RFC 4648)
+     */
+    private function base64url_encode($data) {
+        return rtrim(strtr(base64_encode($data), '+/', '-_'), '=');
+    }
+
     /**
      * Get all active FCM tokens for a user
      */
@@ -218,7 +336,7 @@ class TWP_FCM {
 
         $data = array(
             'type' => 'test',
-            'test' => true
+            'test' => 'true'
         );
 
         return $this->send_notification($user_id, $title, $body, $data);