diff --git a/includes/engine/class-tmdo-entity-registry.php b/includes/engine/class-tmdo-entity-registry.php
index 6e4cdfe..cc86021 100644
--- a/includes/engine/class-tmdo-entity-registry.php
+++ b/includes/engine/class-tmdo-entity-registry.php
@@ -159,9 +159,12 @@ final class TMDO_Entity_Registry {
 			self::$scopes[ $entity_type ][ $group_name ] = $post_types;
 		}
 		$keys = array_column( $normalized, 'key' );
+		// A sort index may end with the entity ID (flat-first tie-break, 2.4 PR-118).
+		$id_col = (string) self::$adapters[ $entity_type ]->get_entity_id_column();
 		foreach ( (array) ( $args['indexes'] ?? array() ) as $index_name => $columns ) {
 			$columns = array_values( array_map( 'strval', (array) $columns ) );
-			if ( $columns && ! array_diff( $columns, $keys ) ) {
+			$fields  = '' !== $id_col && end( $columns ) === $id_col ? array_slice( $columns, 0, -1 ) : $columns;
+			if ( $fields && ! array_diff( $fields, $keys ) ) {
 				self::$indexes[ $entity_type ][ $group_name ][ (string) $index_name ] = $columns;
 			}
 		}
diff --git a/includes/query/class-tmdo-post-query-router.php b/includes/query/class-tmdo-post-query-router.php
index ede1648..443505e 100644
--- a/includes/query/class-tmdo-post-query-router.php
+++ b/includes/query/class-tmdo-post-query-router.php
@@ -797,10 +797,17 @@ class TMDO_Post_Query_Router {
 		if ( 1 === preg_match_all( '/\bJOIN\b/i', (string) $flat_first ) ) {
 			$flat_first = preg_replace( '/\s+GROUP BY\s+`?' . preg_quote( $wpdb->posts, '/' ) . '`?\.`?ID`?(?=\s)/', ' ', (string) $flat_first, 1 );
 		}
-		// Tie-breaks on posts.ID (ours, and any a plugin appended) read the flat post_id.
+		// Tie-breaks on posts.ID (ours, and any a plugin appended) read the flat
+		// post_id, once, in the sort's direction: a (column, post_id) index then
+		// returns rows in order with no filesort. Ties are ordered by ID the same
+		// way as the column (natively ID DESC); pagination stays stable.
+		$tie        = "`{$alias}`.`post_id` " . ( 'ASC' === strtoupper( (string) $sort['order'] ) ? 'ASC' : 'DESC' );
 		$flat_first = preg_replace_callback(
 			'/\bORDER BY\b(?!.*\bORDER BY\b).*$/s',
-			static fn( $m ) => preg_replace( '/`?' . preg_quote( $wpdb->posts, '/' ) . '`?\.`?ID`?(?!\w)/', "`{$alias}`.`post_id`", $m[0] ),
+			static function ( $m ) use ( $wpdb, $tie ) {
+				$order = (string) preg_replace( '/`?' . preg_quote( $wpdb->posts, '/' ) . '`?\.`?ID`?(?!\w)(\s+(ASC|DESC)\b)?/i', $tie, $m[0] );
+				return (string) preg_replace( '/(,\s*' . preg_quote( $tie, '/' ) . ')(\s*,\s*' . preg_quote( $tie, '/' ) . ')+/', '$1', $order );
+			},
 			(string) $flat_first
 		);
 		return (string) preg_replace( '/^\s*SELECT\s/', 'SELECT STRAIGHT_JOIN ', (string) $flat_first, 1 );
diff --git a/tests/integration/PostQueryRouterFoundRowsTest.php b/tests/integration/PostQueryRouterFoundRowsTest.php
index a806a54..3dd49db 100644
--- a/tests/integration/PostQueryRouterFoundRowsTest.php
+++ b/tests/integration/PostQueryRouterFoundRowsTest.php
@@ -226,14 +226,14 @@ class PostQueryRouterFoundRowsTest extends TestCase {
 	 *
 	 * @return array{0:TMDO_Post_Query_Router, 1:WP_Query, 2:string}
 	 */
-	private function sorted( string $compare = '>=' ): array {
+	private function sorted( string $compare = '>=', string $order = 'DESC' ): array {
 		global $wpdb;
 		$query = new WP_Query();
 		$query->set( 'post_type', 'fr_item' );
 		$query->set( 'post_status', 'publish' );
 		$query->set( 'meta_query', array( 'price' => array( 'key' => 'fr_price', 'value' => 15, 'compare' => $compare, 'type' => 'NUMERIC' ) ) );
 		$query->set( 'orderby', 'price' );
-		$query->set( 'order', 'DESC' );
+		$query->set( 'order', $order );
 		$router = new TMDO_Post_Query_Router();
 		$router->pre_get_posts( $query );
 
@@ -252,11 +252,26 @@ class PostQueryRouterFoundRowsTest extends TestCase {
 		$this->assertStringStartsWith( 'SELECT STRAIGHT_JOIN ', $sql );
 		$this->assertStringContainsString( 'FROM `' . self::TABLE . '` AS `wpdoflat_fr_item_fr_core` INNER JOIN ' . $wpdb->posts . ' ON', $sql );
 		$this->assertStringNotContainsString( 'LEFT JOIN `' . self::TABLE . '`', $sql );
-		$this->assertStringEndsWith( 'ORDER BY `wpdoflat_fr_item_fr_core`.`fr_price` DESC, `wpdoflat_fr_item_fr_core`.`post_id` DESC, `wpdoflat_fr_item_fr_core`.`post_id` DESC LIMIT 0, 10', $sql, 'posts.ID tie-breaks read the flat post_id' );
+		$this->assertStringEndsWith( 'ORDER BY `wpdoflat_fr_item_fr_core`.`fr_price` DESC, `wpdoflat_fr_item_fr_core`.`post_id` DESC LIMIT 0, 10', $sql, 'posts.ID tie-breaks read the flat post_id, once' );
 		$this->assertSame( array( '3', '2' ), $wpdb->get_col( $sql ) );
 		$this->assertSame( $wpdb->get_col( str_replace( 'SQL_CALC_FOUND_ROWS ', '', $request ) ), $wpdb->get_col( $sql ) );
 	}
 
+	/**
+	 * The ID tie-break follows the sort direction, so a (column, post_id)
+	 * index returns rows in order with no filesort (2.4 PR-118); ties are
+	 * ordered by ID the same way as the column.
+	 */
+	public function test_flat_first_tie_break_follows_the_sort_direction(): void {
+		global $wpdb;
+		list( $router, $query, $request ) = $this->sorted( '>=', 'ASC' );
+
+		$sql = $router->posts_request( $request, $query );
+
+		$this->assertStringEndsWith( 'ORDER BY `wpdoflat_fr_item_fr_core`.`fr_price` ASC, `wpdoflat_fr_item_fr_core`.`post_id` ASC LIMIT 0, 10', $sql );
+		$this->assertSame( array( '2', '3' ), $wpdb->get_col( $sql ) );
+	}
+
 	public function test_date_ordered_filtered_request_keeps_wordpress_join_order(): void {
 		list( $router, $query, $request ) = $this->routed();
 
diff --git a/tests/unit/EntityRegistryIndexesTest.php b/tests/unit/EntityRegistryIndexesTest.php
index cb4a399..02d4103 100644
--- a/tests/unit/EntityRegistryIndexesTest.php
+++ b/tests/unit/EntityRegistryIndexesTest.php
@@ -14,7 +14,9 @@ class EntityRegistryIndexesTest extends TestCase {
 
 	protected function setUp(): void {
 		TMDO_Entity_Registry::init();
-		TMDO_Entity_Registry::register_adapter( 'post', $this->createMock( TMDO_Entity_Adapter_Interface::class ) );
+		$adapter = $this->createMock( TMDO_Entity_Adapter_Interface::class );
+		$adapter->method( 'get_entity_id_column' )->willReturn( 'post_id' );
+		TMDO_Entity_Registry::register_adapter( 'post', $adapter );
 	}
 
 	private function fields(): array {
@@ -37,6 +39,16 @@ class EntityRegistryIndexesTest extends TestCase {
 		$this->assertSame( array( 'ok' => array( 'x_price' ) ), TMDO_Entity_Registry::get_group_indexes( 'post', 'x_core' ) );
 	}
 
+	/**
+	 * A sort index ends with the entity ID, so a flat-first search ordered by a
+	 * column with the ID tie-break reads the index in order (2.4 PR-118).
+	 */
+	public function test_an_index_may_end_with_the_entity_id_column(): void {
+		TMDO_Entity_Registry::register_group( 'post', 'x_core', $this->fields(), array( 'indexes' => array( 'price_id' => array( 'x_price', 'post_id' ), 'id_first' => array( 'post_id', 'x_price' ) ) ) );
+
+		$this->assertSame( array( 'price_id' => array( 'x_price', 'post_id' ) ), TMDO_Entity_Registry::get_group_indexes( 'post', 'x_core' ), 'the ID only as the last column' );
+	}
+
 	public function test_indexes_leave_the_field_definition_and_hash_alone(): void {
 		TMDO_Entity_Registry::register_group( 'post', 'plain', $this->fields() );
 		TMDO_Entity_Registry::register_group( 'post', 'indexed', $this->fields(), array( 'indexes' => array( 'flag_price' => array( 'x_flag', 'x_price' ) ) ) );