diff --git a/includes/query/class-tmdo-post-query-router.php b/includes/query/class-tmdo-post-query-router.php
index 922277a..7e82377 100644
--- a/includes/query/class-tmdo-post-query-router.php
+++ b/includes/query/class-tmdo-post-query-router.php
@@ -778,22 +778,58 @@ class TMDO_Post_Query_Router {
 		if ( empty( $copy ) ) {
 			return $request;
 		}
+		return self::ties_on_post_id( self::without_group_by( $request ), self::table_alias( $copy['post_type'], $copy['group'] ), '' );
+	}
+
+	/**
+	 * ORDER BY terms that are posts.ID alone (with or without a direction)
+	 * read as the equal flat post_id — in $direction, or each in its own when
+	 * $direction is '' — and only once. posts.ID inside an expression a plugin
+	 * added (FIELD(), IN …) is left alone: a direction inserted there breaks
+	 * the SQL.
+	 *
+	 * @param string $request   SELECT statement.
+	 * @param string $alias     Flat table alias.
+	 * @param string $direction 'ASC', 'DESC' or '' (keep each term's own).
+	 * @return string
+	 */
+	private static function ties_on_post_id( string $request, string $alias, string $direction ): string {
 		global $wpdb;
-		$tie = '`' . self::table_alias( $copy['post_type'], $copy['group'] ) . '`.`post_id` ';
-		return (string) preg_replace_callback(
-			'/\bORDER BY\b(?!.*\bORDER BY\b).*$/s',
-			static function ( $m ) use ( $wpdb, $tie ) {
-				$order = (string) preg_replace_callback(
-					'/`?' . preg_quote( $wpdb->posts, '/' ) . '`?\.`?ID`?(?!\w)(?:\s+(ASC|DESC)\b)?/i',
-					static fn( $id ) => $tie . ( 'DESC' === strtoupper( $id[1] ?? '' ) ? 'DESC' : 'ASC' ),
-					$m[0]
-				);
-				return (string) preg_replace( '/(,\s*' . preg_quote( $tie, '/' ) . '(ASC|DESC))(\s*,\s*' . preg_quote( $tie, '/' ) . '\2)+/', '$1', $order );
-			},
-			self::without_group_by( $request )
+		$tie = "`{$alias}`.`post_id` ";
+		$id  = '`?' . preg_quote( $wpdb->posts, '/' ) . '`?\.`?ID`?';
+		return self::replaced(
+			preg_replace_callback(
+				'/\bORDER BY\b(?!.*\bORDER BY\b).*$/s',
+				static function ( $m ) use ( $id, $tie, $direction ) {
+					$order = self::replaced(
+						preg_replace_callback(
+							'/(?<=ORDER BY |,)(\s*)' . $id . '(?:\s+(ASC|DESC)\b)?(?=\s*(?:,|\bLIMIT\b|$))/i',
+							static fn( $t ) => $t[1] . $tie . ( '' !== $direction ? $direction : ( 'DESC' === strtoupper( $t[2] ?? '' ) ? 'DESC' : 'ASC' ) ),
+							$m[0]
+						),
+						$m[0]
+					);
+					return self::replaced( preg_replace( '/(,\s*' . preg_quote( $tie, '/' ) . '(ASC|DESC))(\s*,\s*' . preg_quote( $tie, '/' ) . '\2)+/', '$1', $order ), $order );
+				},
+				$request
+			),
+			$request
 		);
 	}
 
+	/**
+	 * A preg_* result, or the text it was given when PCRE failed (backtrack
+	 * or JIT stack limit on a very long request): the query stays unrouted
+	 * in shape rather than empty.
+	 *
+	 * @param string|null $result   preg_* return value.
+	 * @param string      $fallback Input text.
+	 * @return string
+	 */
+	private static function replaced( ?string $result, string $fallback ): string {
+		return $result ?? $fallback;
+	}
+
 	/**
 	 * WordPress groups by posts.ID for any tax or meta query. With a flat row
 	 * (one per post) as the only join there is nothing to collapse, and the
@@ -808,7 +844,7 @@ class TMDO_Post_Query_Router {
 		if ( 1 !== preg_match_all( '/\bJOIN\b/i', $request ) ) {
 			return $request;
 		}
-		return (string) preg_replace( '/\s+GROUP BY\s+`?' . preg_quote( $wpdb->posts, '/' ) . '`?\.`?ID`?(?=\s)/', ' ', $request, 1 );
+		return self::replaced( preg_replace( '/\s+GROUP BY\s+`?' . preg_quote( $wpdb->posts, '/' ) . '`?\.`?ID`?(?=\s)/', ' ', $request, 1 ), $request );
 	}
 
 	/**
@@ -882,16 +918,9 @@ class TMDO_Post_Query_Router {
 		// 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 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 );
+		$direction  = 'ASC' === strtoupper( (string) $sort['order'] ) ? 'ASC' : 'DESC';
+		$flat_first = self::ties_on_post_id( $flat_first, $alias, $direction );
+		return self::replaced( preg_replace( '/^\s*SELECT\s/', 'SELECT STRAIGHT_JOIN ', $flat_first, 1 ), $request );
 	}
 
 	/**
@@ -901,7 +930,7 @@ class TMDO_Post_Query_Router {
 	 * @return string
 	 */
 	private static function without_found_rows( string $request ): string {
-		return (string) preg_replace( '/\bSQL_CALC_FOUND_ROWS\s+/', '', $request, 1 );
+		return self::replaced( preg_replace( '/\bSQL_CALC_FOUND_ROWS\s+/', '', $request, 1 ), $request );
 	}
 
 	/**
diff --git a/tests/integration/PostQueryRouterCopyTest.php b/tests/integration/PostQueryRouterCopyTest.php
index 629b928..ccfc1b5 100644
--- a/tests/integration/PostQueryRouterCopyTest.php
+++ b/tests/integration/PostQueryRouterCopyTest.php
@@ -100,6 +100,13 @@ class PostQueryRouterCopyTest extends TestCase {
 		$this->assertStringEndsWith( 'ORDER BY ' . self::ALIAS . '.`post_date` DESC LIMIT 0, 10', $this->request( '', '' ) );
 	}
 
+	public function test_copy_route_leaves_posts_id_inside_expressions_alone(): void {
+		global $wpdb;
+		$sql = $this->request( '', ', FIELD({posts}.ID,5,9) DESC, ({posts}.ID IN (3,4)) DESC, {posts}.ID DESC' );
+
+		$this->assertStringEndsWith( "`post_date` DESC, FIELD({$wpdb->posts}.ID,5,9) DESC, ({$wpdb->posts}.ID IN (3,4)) DESC, " . self::ALIAS . '.`post_id` DESC LIMIT 0, 10', $sql );
+	}
+
 	public function test_copy_route_keeps_group_by_with_another_join(): void {
 		global $wpdb;
 		$sql = $this->request( " LEFT JOIN wp_term_relationships ON ({$wpdb->posts}.ID = wp_term_relationships.object_id)" );
diff --git a/tests/integration/PostQueryRouterFoundRowsTest.php b/tests/integration/PostQueryRouterFoundRowsTest.php
index 3dd49db..661f12e 100644
--- a/tests/integration/PostQueryRouterFoundRowsTest.php
+++ b/tests/integration/PostQueryRouterFoundRowsTest.php
@@ -272,6 +272,25 @@ class PostQueryRouterFoundRowsTest extends TestCase {
 		$this->assertSame( array( '2', '3' ), $wpdb->get_col( $sql ) );
 	}
 
+	/**
+	 * Only standalone posts.ID terms are tie-breaks: inside an expression a
+	 * plugin added (pinning posts with FIELD() or IN) the direction must not be
+	 * inserted, or the SQL breaks (2.4 correctness review).
+	 */
+	public function test_flat_first_leaves_posts_id_inside_expressions_alone(): void {
+		global $wpdb;
+		list( $router, $query, $request ) = $this->sorted();
+		$pinned = str_replace( ' LIMIT 0, 10', ", FIELD({$wpdb->posts}.ID,3,2) DESC, ({$wpdb->posts}.ID IN (2)) DESC LIMIT 0, 10", $request );
+
+		$sql = $router->posts_request( $pinned, $query );
+
+		$this->assertStringStartsWith( 'SELECT STRAIGHT_JOIN ', $sql );
+		$this->assertStringContainsString( "FIELD({$wpdb->posts}.ID,3,2) DESC, ({$wpdb->posts}.ID IN (2)) DESC LIMIT", $sql );
+		$wpdb->last_error = '';
+		$this->assertSame( $wpdb->get_col( str_replace( 'SQL_CALC_FOUND_ROWS ', '', $pinned ) ), $wpdb->get_col( $sql ) );
+		$this->assertSame( '', $wpdb->last_error );
+	}
+
 	public function test_date_ordered_filtered_request_keeps_wordpress_join_order(): void {
 		list( $router, $query, $request ) = $this->routed();