diff --git a/wp-includes/rest-api.php b/wp-includes/rest-api.php
index 536efea696..6eed4e2b08 100644
--- a/wp-includes/rest-api.php
+++ b/wp-includes/rest-api.php
@@ -3182,3 +3182,57 @@ function rest_get_endpoint_args_for_schema( $schema, $method = WP_REST_Server::C
 
 	return $endpoint_args;
 }
+
+
+/**
+ * Converts an error to a response object.
+ *
+ * This iterates over all error codes and messages to change it into a flat
+ * array. This enables simpler client behaviour, as it is represented as a
+ * list in JSON rather than an object/map.
+ *
+ * @since 5.7.0
+ *
+ * @param WP_Error $error WP_Error instance.
+ *
+ * @return WP_REST_Response List of associative arrays with code and message keys.
+ */
+function rest_convert_error_to_response( $error ) {
+	$status = array_reduce(
+		$error->get_all_error_data(),
+		function ( $status, $error_data ) {
+			return is_array( $error_data ) && isset( $error_data['status'] ) ? $error_data['status'] : $status;
+		},
+		500
+	);
+
+	$errors = array();
+
+	foreach ( (array) $error->errors as $code => $messages ) {
+		$all_data  = $error->get_all_error_data( $code );
+		$last_data = array_pop( $all_data );
+
+		foreach ( (array) $messages as $message ) {
+			$formatted = array(
+				'code'    => $code,
+				'message' => $message,
+				'data'    => $last_data,
+			);
+
+			if ( $all_data ) {
+				$formatted['additional_data'] = $all_data;
+			}
+
+			$errors[] = $formatted;
+		}
+	}
+
+	$data = $errors[0];
+	if ( count( $errors ) > 1 ) {
+		// Remove the primary error.
+		array_shift( $errors );
+		$data['additional_errors'] = $errors;
+	}
+
+	return new WP_REST_Response( $data, $status );
+}
diff --git a/wp-includes/rest-api/class-wp-rest-request.php b/wp-includes/rest-api/class-wp-rest-request.php
index e972c5fa25..267ce4e01d 100644
--- a/wp-includes/rest-api/class-wp-rest-request.php
+++ b/wp-includes/rest-api/class-wp-rest-request.php
@@ -802,7 +802,8 @@ class WP_REST_Request implements ArrayAccess {
 
 		$order = $this->get_parameter_order();
 
-		$invalid_params = array();
+		$invalid_params  = array();
+		$invalid_details = array();
 
 		foreach ( $order as $type ) {
 			if ( empty( $this->params[ $type ] ) ) {
@@ -825,10 +826,12 @@ class WP_REST_Request implements ArrayAccess {
 					continue;
 				}
 
+				/** @var mixed|WP_Error $sanitized_value */
 				$sanitized_value = call_user_func( $param_args['sanitize_callback'], $value, $this, $key );
 
 				if ( is_wp_error( $sanitized_value ) ) {
-					$invalid_params[ $key ] = $sanitized_value->get_error_message();
+					$invalid_params[ $key ]  = implode( ' ', $sanitized_value->get_error_messages() );
+					$invalid_details[ $key ] = rest_convert_error_to_response( $sanitized_value )->get_data();
 				} else {
 					$this->params[ $type ][ $key ] = $sanitized_value;
 				}
@@ -841,8 +844,9 @@ class WP_REST_Request implements ArrayAccess {
 				/* translators: %s: List of invalid parameters. */
 				sprintf( __( 'Invalid parameter(s): %s' ), implode( ', ', array_keys( $invalid_params ) ) ),
 				array(
-					'status' => 400,
-					'params' => $invalid_params,
+					'status'  => 400,
+					'params'  => $invalid_params,
+					'details' => $invalid_details,
 				)
 			);
 		}
@@ -894,13 +898,15 @@ class WP_REST_Request implements ArrayAccess {
 		 *
 		 * This is done after required checking as required checking is cheaper.
 		 */
-		$invalid_params = array();
+		$invalid_params  = array();
+		$invalid_details = array();
 
 		foreach ( $args as $key => $arg ) {
 
 			$param = $this->get_param( $key );
 
 			if ( null !== $param && ! empty( $arg['validate_callback'] ) ) {
+				/** @var bool|\WP_Error $valid_check */
 				$valid_check = call_user_func( $arg['validate_callback'], $param, $this, $key );
 
 				if ( false === $valid_check ) {
@@ -908,7 +914,8 @@ class WP_REST_Request implements ArrayAccess {
 				}
 
 				if ( is_wp_error( $valid_check ) ) {
-					$invalid_params[ $key ] = $valid_check->get_error_message();
+					$invalid_params[ $key ]  = implode( ' ', $valid_check->get_error_messages() );
+					$invalid_details[ $key ] = rest_convert_error_to_response( $valid_check )->get_data();
 				}
 			}
 		}
@@ -919,8 +926,9 @@ class WP_REST_Request implements ArrayAccess {
 				/* translators: %s: List of invalid parameters. */
 				sprintf( __( 'Invalid parameter(s): %s' ), implode( ', ', array_keys( $invalid_params ) ) ),
 				array(
-					'status' => 400,
-					'params' => $invalid_params,
+					'status'  => 400,
+					'params'  => $invalid_params,
+					'details' => $invalid_details,
 				)
 			);
 		}
diff --git a/wp-includes/rest-api/class-wp-rest-server.php b/wp-includes/rest-api/class-wp-rest-server.php
index d7f8ae9a41..ed94151228 100644
--- a/wp-includes/rest-api/class-wp-rest-server.php
+++ b/wp-includes/rest-api/class-wp-rest-server.php
@@ -196,41 +196,13 @@ class WP_REST_Server {
 	 * list in JSON rather than an object/map.
 	 *
 	 * @since 4.4.0
+	 * @since 5.7.0 Converted to a wrapper of {@see rest_convert_error_to_response()}.
 	 *
 	 * @param WP_Error $error WP_Error instance.
 	 * @return WP_REST_Response List of associative arrays with code and message keys.
 	 */
 	protected function error_to_response( $error ) {
-		$error_data = $error->get_error_data();
-
-		if ( is_array( $error_data ) && isset( $error_data['status'] ) ) {
-			$status = $error_data['status'];
-		} else {
-			$status = 500;
-		}
-
-		$errors = array();
-
-		foreach ( (array) $error->errors as $code => $messages ) {
-			foreach ( (array) $messages as $message ) {
-				$errors[] = array(
-					'code'    => $code,
-					'message' => $message,
-					'data'    => $error->get_error_data( $code ),
-				);
-			}
-		}
-
-		$data = $errors[0];
-		if ( count( $errors ) > 1 ) {
-			// Remove the primary error.
-			array_shift( $errors );
-			$data['additional_errors'] = $errors;
-		}
-
-		$response = new WP_REST_Response( $data, $status );
-
-		return $response;
+		return rest_convert_error_to_response( $error );
 	}
 
 	/**
diff --git a/wp-includes/version.php b/wp-includes/version.php
index af2610ec52..57090f1b66 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.7-alpha-50149';
+$wp_version = '5.7-alpha-50150';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.