diff --git a/composer.json b/composer.json
index cf236a5..dd7f9d0 100644
--- a/composer.json
+++ b/composer.json
@@ -10,6 +10,7 @@
         "psr-4": {"MBBParser\\": "src/"}
     },
     "require": {
+    	"php": ">=7.4",
     	"wpmetabox/support": "dev-master",
         "riimu/kit-phpencoder": "^2.4"
     }
diff --git a/src/SettingsTrait.php b/src/SettingsTrait.php
index 7270b18..61d36f9 100644
--- a/src/SettingsTrait.php
+++ b/src/SettingsTrait.php
@@ -3,23 +3,41 @@
 
 /**
  * Use overloading magic methods for short syntax.
+ * Property accesses are recorded so unparse_custom_settings() can detect
+ * which keys were touched (= natively handled) vs untouched (= potentially custom).
  */
 trait SettingsTrait {
 	protected $settings;
 
+	/**
+	 * Keys accessed via __get / __set / __isset during this object's lifetime.
+	 * Used by unparse_custom_settings() to detect natively-handled keys.
+	 *
+	 * @var string[]
+	 */
+	private array $accessed_keys = [];
+
 	public function get_settings(): array {
 		return $this->settings;
 	}
 
+	/** Return all keys that were accessed (touched) so far. */
+	public function get_accessed_keys(): array {
+		return $this->accessed_keys;
+	}
+
 	public function __get( string $key ) {
+		$this->accessed_keys[] = $key;
 		return $this->settings[ $key ] ?? null;
 	}
 
 	public function __set( string $key, $value ): void {
+		$this->accessed_keys[] = $key;
 		$this->settings[ $key ] = $value;
 	}
 
 	public function __isset( string $key ): bool {
+		$this->accessed_keys[] = $key;
 		return isset( $this->settings[ $key ] );
 	}
 
diff --git a/src/Unparsers/Field.php b/src/Unparsers/Field.php
index 124b68f..a3cfb18 100644
--- a/src/Unparsers/Field.php
+++ b/src/Unparsers/Field.php
@@ -10,6 +10,23 @@ class Field extends Base {
 
 	private $choice_types = [ 'select', 'radio', 'checkbox_list', 'select_advanced', 'button_group', 'image_select', 'autocomplete' ];
 
+	/**
+	 * Native keys that no unparse_* method accesses via $this->key (magic method),
+	 * so they are never auto-tracked by SettingsTrait. List them explicitly here to
+	 * prevent unparse_custom_settings() from mis-classifying them as custom attributes.
+	 *
+	 * Every other native key is auto-tracked the moment any method reads or writes
+	 * $this->key — no need to list those here.
+	 */
+	private const STRUCTURAL_KEYS = [
+		'type', 'id', '_id', 'name', 'std', 'placeholder',
+		'columns', 'field_name', '_state', 'input_attributes',
+		'multiple', 'fields', 'tab', 'validation',
+		'custom_settings', '_callback', 'options',
+		'limit', 'limit_type',
+	];
+
+
 	/**
 	 * This is revert of parse method. While parse method converts to the minimal format,
 	 * this method converts back to the original format.
@@ -33,13 +50,17 @@ public function unparse() {
 			->unparse_text_limiter()
 			->unparse_conditional_logic()
 			->unparse_tooltip()
-			->unparse_admin_columns()
-			->ensure_boolean( 'save_field' );
+			->unparse_admin_columns();
 
+		// Field-type handler must run BEFORE unparse_custom_settings() so its
+		// property accesses are tracked and not mis-classified as custom attributes.
 		$func = "unparse_field_{$this->type}";
 		if ( method_exists( $this, $func ) ) {
 			$this->$func();
 		}
+
+		$this->unparse_custom_settings();
+		$this->ensure_boolean( 'save_field' );
 	}
 
 	private function unparse_datalist() {
@@ -206,6 +227,40 @@ private function unparse_admin_columns() {
 		return $this;
 	}
 
+	/**
+	 * Detect unknown top-level keys and move them into the custom_settings
+	 * format `{ uid: { id, key, value } }` that the Builder UI renders in
+	 * the "Custom Settings" tab.
+	 *
+	 * "Known" = auto-tracked by SettingsTrait + STRUCTURAL_KEYS pass-through list.
+	 */
+	private function unparse_custom_settings(): self {
+		$known  = array_merge( $this->get_accessed_keys(), self::STRUCTURAL_KEYS );
+		$custom = $this->custom_settings ?? [];
+
+		foreach ( $this->settings as $key => $value ) {
+			if ( in_array( $key, $known, true ) ) {
+				continue;
+			}
+
+			$uid            = uniqid();
+			$custom[ $uid ] = [
+				'id'    => $uid,
+				'key'   => $key,
+				'value' => ( is_array( $value ) || is_object( $value ) )
+					? wp_json_encode( $value )
+					: (string) $value,
+			];
+			unset( $this->settings[ $key ] );
+		}
+
+		if ( ! empty( $custom ) ) {
+			$this->custom_settings = $custom;
+		}
+
+		return $this;
+	}
+
 	public function unparse_default_values() {
 		$this->id  = $this->id ?? uniqid();
 		$this->_id = $this->_id ?? $this->id;