diff --git a/src/wp-includes/post.php b/src/wp-includes/post.php
index 77a1a49238..fabd68edb2 100644
--- a/src/wp-includes/post.php
+++ b/src/wp-includes/post.php
@@ -6031,7 +6031,7 @@ function get_pages( $args = array() ) {
 					}
 					$post_author = $post_author->ID;
 				}
-				$query_args['author__in'][] = $post_author;
+				$query_args['author__in'][] = (int) $post_author;
 			}
 		}
 	}
@@ -6060,6 +6060,16 @@ function get_pages( $args = array() ) {
 		$query_args['posts_per_page'] = $number;
 	}
 
+	/**
+	 * Filters query arguments passed to WP_Query in get_pages.
+	 *
+	 * @since 6.3.0
+	 *
+	 * @param array $query_args  Array of arguments passed to WP_Query.
+	 * @param array $parsed_args Array of get_pages() arguments.
+	 */
+	$query_args = apply_filters( 'get_pages_query_args', $query_args, $parsed_args );
+
 	$query = new WP_Query( $query_args );
 	$pages = $query->get_posts();
 
diff --git a/tests/phpunit/tests/post/getPages.php b/tests/phpunit/tests/post/getPages.php
index 904de785d9..92de3356fa 100644
--- a/tests/phpunit/tests/post/getPages.php
+++ b/tests/phpunit/tests/post/getPages.php
@@ -325,6 +325,189 @@ class Tests_Post_GetPages extends WP_UnitTestCase {
 		$this->assertSame( $inc, $exc_result );
 	}
 
+	/**
+	 * @ticket 12821
+	 * @covers ::get_pages
+	 */
+	public function test_get_pages_test_filter() {
+		register_post_type( 'wptests_pt', array( 'hierarchical' => true ) );
+
+		$posts              = self::factory()->post->create_many(
+			2,
+			array(
+				'post_type' => 'wptests_pt',
+			)
+		);
+		$query_args_values  = array();
+		$parsed_args_values = array();
+
+		// Filter the query to return the wptests_pt post type.
+		add_filter(
+			'get_pages_query_args',
+			static function( $query_args, $parsed_args ) use ( &$query_args_values, &$parsed_args_values ) {
+				$query_args['post_type'] = 'wptests_pt';
+				$query_args_values       = $query_args;
+				$parsed_args_values      = $parsed_args;
+				return $query_args;
+			},
+			10,
+			2
+		);
+
+		$pages    = get_pages();
+		$page_ids = wp_list_pluck( $pages, 'ID' );
+		$this->assertSameSets( $posts, $page_ids, 'The return post ids should match the post type wptests_pt.' );
+
+		$query_args = array(
+			'orderby'                => array( 'post_title' => 'ASC' ),
+			'order'                  => 'ASC',
+			'post__not_in'           => array(),
+			'meta_key'               => '',
+			'meta_value'             => '',
+			'posts_per_page'         => -1,
+			'offset'                 => 0,
+			'post_type'              => 'wptests_pt',
+			'post_status'            => array( 'publish' ),
+			'update_post_term_cache' => false,
+			'update_post_meta_cache' => false,
+			'ignore_sticky_posts'    => true,
+			'no_found_rows'          => true,
+		);
+
+		$this->assertSameSets( $query_args, $query_args_values, 'Query arguments should match expected values' );
+
+		$parsed_args = array(
+			'child_of'     => 0,
+			'sort_order'   => 'ASC',
+			'sort_column'  => 'post_title',
+			'hierarchical' => 1,
+			'exclude'      => array(),
+			'include'      => array(),
+			'meta_key'     => '',
+			'meta_value'   => '',
+			'authors'      => '',
+			'parent'       => -1,
+			'exclude_tree' => array(),
+			'number'       => '',
+			'offset'       => 0,
+			'post_type'    => 'page',
+			'post_status'  => 'publish',
+		);
+
+		$this->assertSameSets( $parsed_args, $parsed_args_values, 'Parsed arguments should match expected values' );
+	}
+
+	/**
+	 * @ticket 12821
+	 * @covers ::get_pages
+	 * @dataProvider data_get_pages_args
+	 */
+	public function test_get_pages_args_test_filter( $args, $expected_query_args ) {
+		$filter = new MockAction();
+		add_filter( 'get_pages_query_args', array( $filter, 'filter' ), 10, 2 );
+
+		$results = get_pages( $args );
+
+		$this->assertIsArray( $results, 'get_pages should result an array' );
+
+		$filter_args = $filter->get_args();
+
+		$default_args = array(
+			'orderby'                => array( 'post_title' => 'ASC' ),
+			'order'                  => 'ASC',
+			'post__not_in'           => array(),
+			'meta_key'               => '',
+			'meta_value'             => '',
+			'posts_per_page'         => -1,
+			'offset'                 => 0,
+			'post_type'              => 'page',
+			'post_status'            => array( 'publish' ),
+			'update_post_term_cache' => false,
+			'update_post_meta_cache' => false,
+			'ignore_sticky_posts'    => true,
+			'no_found_rows'          => true,
+		);
+
+		$query_args = wp_parse_args( $expected_query_args, $default_args );
+
+		$this->assertSameSets( $query_args, $filter_args[0][0], 'Unexpected $query_args for get_pages_query_args filter' );
+
+		$defaults = array(
+			'child_of'     => 0,
+			'sort_order'   => 'ASC',
+			'sort_column'  => 'post_title',
+			'hierarchical' => 1,
+			'exclude'      => array(),
+			'include'      => array(),
+			'meta_key'     => '',
+			'meta_value'   => '',
+			'authors'      => '',
+			'parent'       => -1,
+			'exclude_tree' => array(),
+			'number'       => '',
+			'offset'       => 0,
+			'post_type'    => 'page',
+			'post_status'  => 'publish',
+		);
+
+		$parsed_args = wp_parse_args( $args, $defaults );
+		$this->assertSameSets( $parsed_args, $filter_args[0][1], 'Unexpected $parsed_args for get_pages_query_args filter' );
+	}
+
+	public function data_get_pages_args() {
+		return array(
+			'default'            => array(
+				'args'                => array(),
+				'expected_query_args' => array(),
+			),
+			'exclude'            => array(
+				'args'                => array( 'exclude' => array( 1, 2, 4 ) ),
+				'expected_query_args' => array( 'post__not_in' => array( 1, 2, 4 ) ),
+			),
+			'post status'        => array(
+				'args'                => array( 'post_status' => 'draft' ),
+				'expected_query_args' => array( 'post_status' => array( 'draft' ) ),
+			),
+			'number'             => array(
+				'args'                => array( 'number' => 99 ),
+				'expected_query_args' => array( 'posts_per_page' => 99 ),
+			),
+			'meta query'         => array(
+				'args'                => array(
+					'meta_key'   => 'foo',
+					'meta_value' => 'bar',
+				),
+				'expected_query_args' => array(
+					'meta_key'   => 'foo',
+					'meta_value' => 'bar',
+				),
+			),
+			'post parent number' => array(
+				'args'                => array( 'parent' => 5 ),
+				'expected_query_args' => array( 'post_parent' => 5 ),
+			),
+			'post parent array'  => array(
+				'args'                => array( 'parent' => array( 5 ) ),
+				'expected_query_args' => array( 'post_parent__in' => array( 5 ) ),
+			),
+			'offset'             => array(
+				'args'                => array( 'offset' => 2 ),
+				'expected_query_args' => array( 'offset' => 2 ),
+			),
+			'authors'            => array(
+				'args'                => array( 'authors' => 2 ),
+				'expected_query_args' => array( 'author__in' => array( 2 ) ),
+			),
+			'sort order'         => array(
+				'args'                => array( 'sort_order' => 'DESC' ),
+				'expected_query_args' => array(
+					'order'   => 'DESC',
+					'orderby' => array( 'post_title' => 'DESC' ),
+				),
+			),
+		);
+	}
+
 	/**
 	 * @ticket 12821
 	 */