diff --git a/lib/CleantalkSP/SpbctWP/RemoteCalls.php b/lib/CleantalkSP/SpbctWP/RemoteCalls.php
index 674ad7990..409dac1e5 100644
--- a/lib/CleantalkSP/SpbctWP/RemoteCalls.php
+++ b/lib/CleantalkSP/SpbctWP/RemoteCalls.php
@@ -33,11 +33,61 @@ class RemoteCalls extends \CleantalkSP\Common\RemoteCalls
         'spbc_key',
     ];
 
+    /** Default Content-Type for responses */
+    const DEFAULT_CONTENT_TYPE = 'text/plain; charset=UTF-8';
+
+    /**
+     * Action Content-Type list
+     *
+     * @var array<string, string>
+     */
+    private static $actionContentTypes = [
+        // HTML page
+        'debug'                     => 'text/html; charset=UTF-8',
+        // JSON payloads
+        'update_settings'           => 'application/json; charset=UTF-8',
+        'post_api_key'              => 'application/json; charset=UTF-8',
+        'license_update'            => 'application/json; charset=UTF-8',
+        'run_service_template_get'  => 'application/json; charset=UTF-8',
+        'scanner__check_dir'        => 'application/json; charset=UTF-8',
+        'update_pscan_statuses'     => 'application/json; charset=UTF-8',
+    ];
+
     public function __construct(&$state)
     {
         $this->without_token = self::checkWithoutToken();
         $this->state = $state;
         $this->class_name = __CLASS__;
+
+        // Declare the response type before any handler outputs (runs only for RC).
+        self::sendContentTypeHeader(Request::getString('spbc_remote_call_action'));
+    }
+
+    /**
+     * Resolve the full Content-Type header value for action.
+     *
+     * @param mixed $action
+     * @return string
+     */
+    public static function resolveContentType($action)
+    {
+        $action = strtolower((string)$action);
+        return isset(self::$actionContentTypes[$action])
+            ? self::$actionContentTypes[$action]
+            : self::DEFAULT_CONTENT_TYPE;
+    }
+
+    /**
+     * Send response Content-Type header unless headers have already been sent.
+     *
+     * @param mixed $action Remote call action name ('' when absent).
+     * @return void
+     */
+    private static function sendContentTypeHeader($action)
+    {
+        if (! headers_sent()) {
+            header('Content-Type: ' . self::resolveContentType($action));
+        }
     }
 
     /**
diff --git a/tests/lib/CleantalkSP/SpbctWP/RemoteCalls/RemoteCallsContentTypeTest.php b/tests/lib/CleantalkSP/SpbctWP/RemoteCalls/RemoteCallsContentTypeTest.php
new file mode 100644
index 000000000..8ee964e8f
--- /dev/null
+++ b/tests/lib/CleantalkSP/SpbctWP/RemoteCalls/RemoteCallsContentTypeTest.php
@@ -0,0 +1,64 @@
+<?php
+
+namespace CleantalkSP\SpbctWP;
+
+use PHPUnit\Framework\TestCase;
+
+/**
+ * Covers the Content-Type declaration added to Remote Call responses.
+ *
+ * @see \CleantalkSP\SpbctWP\RemoteCalls::resolveContentType()
+ */
+class RemoteCallsContentTypeTest extends TestCase
+{
+    public function testDefaultContentTypeIsPlainText()
+    {
+        $this->assertSame('text/plain; charset=UTF-8', RemoteCalls::DEFAULT_CONTENT_TYPE);
+    }
+
+    public function testPlainTextActionsFallBackToDefault()
+    {
+        $this->assertSame('text/plain; charset=UTF-8', RemoteCalls::resolveContentType('scanner__controller'));
+        $this->assertSame('text/plain; charset=UTF-8', RemoteCalls::resolveContentType('launch_background_scan'));
+        $this->assertSame('text/plain; charset=UTF-8', RemoteCalls::resolveContentType('check_website'));
+        $this->assertSame('text/plain; charset=UTF-8', RemoteCalls::resolveContentType('update_security_firewall__worker'));
+    }
+
+    public function testUnknownActionFallsBackToDefault()
+    {
+        $this->assertSame('text/plain; charset=UTF-8', RemoteCalls::resolveContentType('no_such_action_at_all'));
+        $this->assertSame('text/plain; charset=UTF-8', RemoteCalls::resolveContentType(''));
+    }
+
+    public function testDebugActionRendersHtml()
+    {
+        $this->assertSame('text/html; charset=UTF-8', RemoteCalls::resolveContentType('debug'));
+    }
+
+    public function testJsonActionsUseApplicationJson()
+    {
+        foreach (['update_settings', 'post_api_key', 'license_update', 'run_service_template_get', 'scanner__check_dir', 'update_pscan_statuses'] as $action) {
+            $this->assertSame(
+                'application/json; charset=UTF-8',
+                RemoteCalls::resolveContentType($action),
+                $action . ' must respond as JSON'
+            );
+        }
+    }
+
+    public function testFileDownloadFallsBackToDefaultBeforeHandlerOverrides()
+    {
+        $this->assertSame('text/plain; charset=UTF-8', RemoteCalls::resolveContentType('download__quarantine_file'));
+    }
+
+    public function testActionNameIsCaseInsensitive()
+    {
+        $this->assertSame('text/html; charset=UTF-8', RemoteCalls::resolveContentType('DEBUG'));
+        $this->assertSame('application/json; charset=UTF-8', RemoteCalls::resolveContentType('Update_Settings'));
+    }
+
+    public function testNonScalarSafeResolution()
+    {
+        $this->assertSame('text/plain; charset=UTF-8', RemoteCalls::resolveContentType(null));
+    }
+}
\ No newline at end of file