Visiting https://example.com/wp-json/tomjn/v1/addition?a=1&b=2 should return the response 3

rest_endpoint.php

<?php


function toms_addition_endpoint( \WP_REST_Request $request ) {
	$result = $request['a'] + $request['b'];
	return rest_ensure_response( $result );
}

function toms_addition_routes() {
	register_rest_route( 'tomjn/v1', '/addition', array(
		'methods'  => WP_REST_Server::READABLE,
		'callback' => 'toms_addition_endpoint',
		'args' => [
			'a' => [
				'description' => 'the first number to add',
				'type' => 'number',
				'validate_callback' => 'is_numeric',
			],
			'b' => [
				'description' => 'the second number to add',
				'type' => 'number',
				'validate_callback' => 'is_numeric',
			],
		],
	));
}
add_action( 'rest_api_init', 'toms_addition_routes' );