diff --git src/wp-includes/rest-api/endpoints/class-wp-rest-attachments-controller.php src/wp-includes/rest-api/endpoints/class-wp-rest-attachments-controller.php
index fe7bd06ee0..bc1a280492 100644
--- src/wp-includes/rest-api/endpoints/class-wp-rest-attachments-controller.php
+++ src/wp-includes/rest-api/endpoints/class-wp-rest-attachments-controller.php
@@ -417,6 +417,9 @@ class WP_REST_Attachments_Controller extends WP_REST_Posts_Controller {
 	 * @return array Item schema as an array.
 	 */
 	public function get_item_schema() {
+		if ( $this->schema ) {
+			return $this->add_additional_fields_schema( $this->schema );
+		}
 
 		$schema = parent::get_item_schema();
 
@@ -513,7 +516,8 @@ class WP_REST_Attachments_Controller extends WP_REST_Posts_Controller {
 
 		unset( $schema['properties']['password'] );
 
-		return $schema;
+		$this->schema = $schema;
+		return $this->add_additional_fields_schema( $this->schema );
 	}
 
 	/**
diff --git src/wp-includes/rest-api/endpoints/class-wp-rest-autosaves-controller.php src/wp-includes/rest-api/endpoints/class-wp-rest-autosaves-controller.php
index e55d3a3729..0d015519d3 100644
--- src/wp-includes/rest-api/endpoints/class-wp-rest-autosaves-controller.php
+++ src/wp-includes/rest-api/endpoints/class-wp-rest-autosaves-controller.php
@@ -296,6 +296,10 @@ class WP_REST_Autosaves_Controller extends WP_REST_Revisions_Controller {
 	 * @return array Item schema data.
 	 */
 	public function get_item_schema() {
+		if ( $this->schema ) {
+			return $this->add_additional_fields_schema( $this->schema );
+		}
+
 		$schema = $this->revisions_controller->get_item_schema();
 
 		$schema['properties']['preview_link'] = array(
@@ -306,7 +310,8 @@ class WP_REST_Autosaves_Controller extends WP_REST_Revisions_Controller {
 			'readonly'    => true,
 		);
 
-		return $schema;
+		$this->schema = $schema;
+		return $this->add_additional_fields_schema( $this->schema );
 	}
 
 	/**
diff --git src/wp-includes/rest-api/endpoints/class-wp-rest-block-renderer-controller.php src/wp-includes/rest-api/endpoints/class-wp-rest-block-renderer-controller.php
index 681e519313..18fbe1e036 100644
--- src/wp-includes/rest-api/endpoints/class-wp-rest-block-renderer-controller.php
+++ src/wp-includes/rest-api/endpoints/class-wp-rest-block-renderer-controller.php
@@ -161,7 +161,11 @@ class WP_REST_Block_Renderer_Controller extends WP_REST_Controller {
 	 * @return array Item schema data.
 	 */
 	public function get_item_schema() {
-		return array(
+		if ( $this->schema ) {
+			return $this->schema;
+		}
+
+		$this->schema = array(
 			'$schema'    => 'http://json-schema.org/schema#',
 			'title'      => 'rendered-block',
 			'type'       => 'object',
@@ -174,5 +178,6 @@ class WP_REST_Block_Renderer_Controller extends WP_REST_Controller {
 				),
 			),
 		);
+		return $this->schema;
 	}
 }
diff --git src/wp-includes/rest-api/endpoints/class-wp-rest-blocks-controller.php src/wp-includes/rest-api/endpoints/class-wp-rest-blocks-controller.php
index a523764dc5..36d1573a9a 100644
--- src/wp-includes/rest-api/endpoints/class-wp-rest-blocks-controller.php
+++ src/wp-includes/rest-api/endpoints/class-wp-rest-blocks-controller.php
@@ -68,6 +68,7 @@ class WP_REST_Blocks_Controller extends WP_REST_Posts_Controller {
 	 * @return array Item schema data.
 	 */
 	public function get_item_schema() {
+		// Do not cache this schema because all properties are derived from parent controller.
 		$schema = parent::get_item_schema();
 
 		/*
diff --git src/wp-includes/rest-api/endpoints/class-wp-rest-comments-controller.php src/wp-includes/rest-api/endpoints/class-wp-rest-comments-controller.php
index b730844fde..8bce803ea9 100644
--- src/wp-includes/rest-api/endpoints/class-wp-rest-comments-controller.php
+++ src/wp-includes/rest-api/endpoints/class-wp-rest-comments-controller.php
@@ -1215,6 +1215,10 @@ class WP_REST_Comments_Controller extends WP_REST_Controller {
 	 * @return array
 	 */
 	public function get_item_schema() {
+		if ( $this->schema ) {
+			return $this->add_additional_fields_schema( $this->schema );
+		}
+
 		$schema = array(
 			'$schema'    => 'http://json-schema.org/draft-04/schema#',
 			'title'      => 'comment',
@@ -1364,7 +1368,8 @@ class WP_REST_Comments_Controller extends WP_REST_Controller {
 
 		$schema['properties']['meta'] = $this->meta->get_field_schema();
 
-		return $this->add_additional_fields_schema( $schema );
+		$this->schema = $schema;
+		return $this->add_additional_fields_schema( $this->schema );
 	}
 
 	/**
diff --git src/wp-includes/rest-api/endpoints/class-wp-rest-controller.php src/wp-includes/rest-api/endpoints/class-wp-rest-controller.php
index cd976f3357..f6991e0d67 100644
--- src/wp-includes/rest-api/endpoints/class-wp-rest-controller.php
+++ src/wp-includes/rest-api/endpoints/class-wp-rest-controller.php
@@ -30,6 +30,14 @@ abstract class WP_REST_Controller {
 	 */
 	protected $rest_base;
 
+	/**
+	 * Cached results of get_item_schema.
+	 *
+	 * @since 5.3.0
+	 * @var array
+	 */
+	protected $schema;
+
 	/**
 	 * Registers the routes for the objects of the controller.
 	 *
diff --git src/wp-includes/rest-api/endpoints/class-wp-rest-post-statuses-controller.php src/wp-includes/rest-api/endpoints/class-wp-rest-post-statuses-controller.php
index 699e8fd2ce..155c8aaa96 100644
--- src/wp-includes/rest-api/endpoints/class-wp-rest-post-statuses-controller.php
+++ src/wp-includes/rest-api/endpoints/class-wp-rest-post-statuses-controller.php
@@ -268,6 +268,10 @@ class WP_REST_Post_Statuses_Controller extends WP_REST_Controller {
 	 * @return array Item schema data.
 	 */
 	public function get_item_schema() {
+		if ( $this->schema ) {
+			return $this->add_additional_fields_schema( $this->schema );
+		}
+
 		$schema = array(
 			'$schema'    => 'http://json-schema.org/draft-04/schema#',
 			'title'      => 'status',
@@ -318,7 +322,8 @@ class WP_REST_Post_Statuses_Controller extends WP_REST_Controller {
 			),
 		);
 
-		return $this->add_additional_fields_schema( $schema );
+		$this->schema = $schema;
+		return $this->add_additional_fields_schema( $this->schema );
 	}
 
 	/**
diff --git src/wp-includes/rest-api/endpoints/class-wp-rest-post-types-controller.php src/wp-includes/rest-api/endpoints/class-wp-rest-post-types-controller.php
index c718a3dfa6..99a3c77071 100644
--- src/wp-includes/rest-api/endpoints/class-wp-rest-post-types-controller.php
+++ src/wp-includes/rest-api/endpoints/class-wp-rest-post-types-controller.php
@@ -242,6 +242,10 @@ class WP_REST_Post_Types_Controller extends WP_REST_Controller {
 	 * @return array Item schema data.
 	 */
 	public function get_item_schema() {
+		if ( $this->schema ) {
+			return $this->add_additional_fields_schema( $this->schema );
+		}
+
 		$schema = array(
 			'$schema'    => 'http://json-schema.org/draft-04/schema#',
 			'title'      => 'type',
@@ -312,7 +316,9 @@ class WP_REST_Post_Types_Controller extends WP_REST_Controller {
 				),
 			),
 		);
-		return $this->add_additional_fields_schema( $schema );
+
+		$this->schema = $schema;
+		return $this->add_additional_fields_schema( $this->schema );
 	}
 
 	/**
diff --git src/wp-includes/rest-api/endpoints/class-wp-rest-posts-controller.php src/wp-includes/rest-api/endpoints/class-wp-rest-posts-controller.php
index 090ca2a5da..ad53ea6a1e 100644
--- src/wp-includes/rest-api/endpoints/class-wp-rest-posts-controller.php
+++ src/wp-includes/rest-api/endpoints/class-wp-rest-posts-controller.php
@@ -1870,6 +1870,9 @@ class WP_REST_Posts_Controller extends WP_REST_Controller {
 	 * @return array Item schema data.
 	 */
 	public function get_item_schema() {
+		if ( $this->schema ) {
+			return $this->add_additional_fields_schema( $this->schema );
+		}
 
 		$schema = array(
 			'$schema'    => 'http://json-schema.org/draft-04/schema#',
@@ -2229,7 +2232,8 @@ class WP_REST_Posts_Controller extends WP_REST_Controller {
 			$schema['links'] = $schema_links;
 		}
 
-		return $this->add_additional_fields_schema( $schema );
+		$this->schema = $schema;
+		return $this->add_additional_fields_schema( $this->schema );
 	}
 
 	/**
diff --git src/wp-includes/rest-api/endpoints/class-wp-rest-revisions-controller.php src/wp-includes/rest-api/endpoints/class-wp-rest-revisions-controller.php
index 52df0f2206..1b34619130 100644
--- src/wp-includes/rest-api/endpoints/class-wp-rest-revisions-controller.php
+++ src/wp-includes/rest-api/endpoints/class-wp-rest-revisions-controller.php
@@ -606,6 +606,10 @@ class WP_REST_Revisions_Controller extends WP_REST_Controller {
 	 * @return array Item schema data.
 	 */
 	public function get_item_schema() {
+		if ( $this->schema ) {
+			return $this->add_additional_fields_schema( $this->schema );
+		}
+
 		$schema = array(
 			'$schema'    => 'http://json-schema.org/draft-04/schema#',
 			'title'      => "{$this->parent_post_type}-revision",
@@ -682,7 +686,8 @@ class WP_REST_Revisions_Controller extends WP_REST_Controller {
 			$schema['properties']['guid'] = $parent_schema['properties']['guid'];
 		}
 
-		return $this->add_additional_fields_schema( $schema );
+		$this->schema = $schema;
+		return $this->add_additional_fields_schema( $this->schema );
 	}
 
 	/**
diff --git src/wp-includes/rest-api/endpoints/class-wp-rest-search-controller.php src/wp-includes/rest-api/endpoints/class-wp-rest-search-controller.php
index 299029b1c3..c464e0d1b4 100644
--- src/wp-includes/rest-api/endpoints/class-wp-rest-search-controller.php
+++ src/wp-includes/rest-api/endpoints/class-wp-rest-search-controller.php
@@ -211,6 +211,10 @@ class WP_REST_Search_Controller extends WP_REST_Controller {
 	 * @return array Item schema data.
 	 */
 	public function get_item_schema() {
+		if ( $this->schema ) {
+			return $this->add_additional_fields_schema( $this->schema );
+		}
+
 		$types    = array();
 		$subtypes = array();
 		foreach ( $this->search_handlers as $search_handler ) {
@@ -262,7 +266,8 @@ class WP_REST_Search_Controller extends WP_REST_Controller {
 			),
 		);
 
-		return $this->add_additional_fields_schema( $schema );
+		$this->schema = $schema;
+		return $this->add_additional_fields_schema( $this->schema );
 	}
 
 	/**
diff --git src/wp-includes/rest-api/endpoints/class-wp-rest-settings-controller.php src/wp-includes/rest-api/endpoints/class-wp-rest-settings-controller.php
index 95752f8236..b11e4fdca8 100644
--- src/wp-includes/rest-api/endpoints/class-wp-rest-settings-controller.php
+++ src/wp-includes/rest-api/endpoints/class-wp-rest-settings-controller.php
@@ -270,6 +270,10 @@ class WP_REST_Settings_Controller extends WP_REST_Controller {
 	 * @return array Item schema data.
 	 */
 	public function get_item_schema() {
+		if ( $this->schema ) {
+			return $this->add_additional_fields_schema( $this->schema );
+		}
+
 		$options = $this->get_registered_options();
 
 		$schema = array(
@@ -286,7 +290,8 @@ class WP_REST_Settings_Controller extends WP_REST_Controller {
 			);
 		}
 
-		return $this->add_additional_fields_schema( $schema );
+		$this->schema = $schema;
+		return $this->add_additional_fields_schema( $this->schema );
 	}
 
 	/**
diff --git src/wp-includes/rest-api/endpoints/class-wp-rest-taxonomies-controller.php src/wp-includes/rest-api/endpoints/class-wp-rest-taxonomies-controller.php
index 6dc78cb6d0..58ad748132 100644
--- src/wp-includes/rest-api/endpoints/class-wp-rest-taxonomies-controller.php
+++ src/wp-includes/rest-api/endpoints/class-wp-rest-taxonomies-controller.php
@@ -276,6 +276,10 @@ class WP_REST_Taxonomies_Controller extends WP_REST_Controller {
 	 * @return array Item schema data.
 	 */
 	public function get_item_schema() {
+		if ( $this->schema ) {
+			return $this->add_additional_fields_schema( $this->schema );
+		}
+
 		$schema = array(
 			'$schema'    => 'http://json-schema.org/draft-04/schema#',
 			'title'      => 'taxonomy',
@@ -373,7 +377,9 @@ class WP_REST_Taxonomies_Controller extends WP_REST_Controller {
 				),
 			),
 		);
-		return $this->add_additional_fields_schema( $schema );
+
+		$this->schema = $schema;
+		return $this->add_additional_fields_schema( $this->schema );
 	}
 
 	/**
diff --git src/wp-includes/rest-api/endpoints/class-wp-rest-terms-controller.php src/wp-includes/rest-api/endpoints/class-wp-rest-terms-controller.php
index 881afcd41f..52bc9d2b0a 100644
--- src/wp-includes/rest-api/endpoints/class-wp-rest-terms-controller.php
+++ src/wp-includes/rest-api/endpoints/class-wp-rest-terms-controller.php
@@ -856,6 +856,10 @@ class WP_REST_Terms_Controller extends WP_REST_Controller {
 	 * @return array Item schema data.
 	 */
 	public function get_item_schema() {
+		if ( $this->schema ) {
+			return $this->add_additional_fields_schema( $this->schema );
+		}
+
 		$schema = array(
 			'$schema'    => 'http://json-schema.org/draft-04/schema#',
 			'title'      => 'post_tag' === $this->taxonomy ? 'tag' : $this->taxonomy,
@@ -924,7 +928,8 @@ class WP_REST_Terms_Controller extends WP_REST_Controller {
 
 		$schema['properties']['meta'] = $this->meta->get_field_schema();
 
-		return $this->add_additional_fields_schema( $schema );
+		$this->schema = $schema;
+		return $this->add_additional_fields_schema( $this->schema );
 	}
 
 	/**
diff --git src/wp-includes/rest-api/endpoints/class-wp-rest-themes-controller.php src/wp-includes/rest-api/endpoints/class-wp-rest-themes-controller.php
index dbfaa839cb..e04b23594d 100644
--- src/wp-includes/rest-api/endpoints/class-wp-rest-themes-controller.php
+++ src/wp-includes/rest-api/endpoints/class-wp-rest-themes-controller.php
@@ -147,6 +147,10 @@ class WP_REST_Themes_Controller extends WP_REST_Controller {
 	 * @return array Item schema data.
 	 */
 	public function get_item_schema() {
+		if ( $this->schema ) {
+			return $this->add_additional_fields_schema( $this->schema );
+		}
+
 		$schema = array(
 			'$schema'    => 'http://json-schema.org/draft-04/schema#',
 			'title'      => 'theme',
@@ -177,7 +181,8 @@ class WP_REST_Themes_Controller extends WP_REST_Controller {
 			),
 		);
 
-		return $this->add_additional_fields_schema( $schema );
+		$this->schema = $schema;
+		return $this->add_additional_fields_schema( $this->schema );
 	}
 
 	/**
diff --git src/wp-includes/rest-api/endpoints/class-wp-rest-users-controller.php src/wp-includes/rest-api/endpoints/class-wp-rest-users-controller.php
index bd09b70b32..ec29c66974 100644
--- src/wp-includes/rest-api/endpoints/class-wp-rest-users-controller.php
+++ src/wp-includes/rest-api/endpoints/class-wp-rest-users-controller.php
@@ -1178,6 +1178,10 @@ class WP_REST_Users_Controller extends WP_REST_Controller {
 	 * @return array Item schema data.
 	 */
 	public function get_item_schema() {
+		if ( $this->schema ) {
+			return $this->add_additional_fields_schema( $this->schema );
+		}
+
 		$schema = array(
 			'$schema'    => 'http://json-schema.org/draft-04/schema#',
 			'title'      => 'user',
@@ -1334,7 +1338,8 @@ class WP_REST_Users_Controller extends WP_REST_Controller {
 
 		$schema['properties']['meta'] = $this->meta->get_field_schema();
 
-		return $this->add_additional_fields_schema( $schema );
+		$this->schema = $schema;
+		return $this->add_additional_fields_schema( $this->schema );
 	}
 
 	/**
diff --git tests/phpunit/tests/rest-api/rest-posts-controller.php tests/phpunit/tests/rest-api/rest-posts-controller.php
index f03eee8e83..ba174c448e 100644
--- tests/phpunit/tests/rest-api/rest-posts-controller.php
+++ tests/phpunit/tests/rest-api/rest-posts-controller.php
@@ -3919,6 +3919,12 @@ class WP_Test_REST_Posts_Controller extends WP_Test_REST_Post_Type_Controller_Te
 
 		remove_post_type_support( 'post', 'author' );
 
+		// Re-initialize the controller to cache-bust schemas from prior test runs.
+		$GLOBALS['wp_rest_server']->override_by_default = true;
+		$controller                                     = new WP_REST_Posts_Controller( 'post' );
+		$controller->register_routes();
+		$GLOBALS['wp_rest_server']->override_by_default = false;
+
 		$response = rest_get_server()->dispatch( new WP_REST_Request( 'OPTIONS', '/wp/v2/posts' ) );
 		$data     = $response->get_data();
 		$schema   = $data['schema'];
diff --git tests/phpunit/tests/rest-api/rest-users-controller.php tests/phpunit/tests/rest-api/rest-users-controller.php
index 02ef87c04b..28755a97d1 100644
--- tests/phpunit/tests/rest-api/rest-users-controller.php
+++ tests/phpunit/tests/rest-api/rest-users-controller.php
@@ -2522,6 +2522,13 @@ class WP_Test_REST_Users_Controller extends WP_Test_REST_Controller_Testcase {
 
 	public function test_get_item_schema_show_avatar() {
 		update_option( 'show_avatars', false );
+
+		// Re-initialize the controller to cache-bust schemas from prior test runs.
+		$GLOBALS['wp_rest_server']->override_by_default = true;
+		$controller                                     = new WP_REST_Users_Controller();
+		$controller->register_routes();
+		$GLOBALS['wp_rest_server']->override_by_default = false;
+
 		$request    = new WP_REST_Request( 'OPTIONS', '/wp/v2/users' );
 		$response   = rest_get_server()->dispatch( $request );
 		$data       = $response->get_data();