Checks if the current request is a WP REST API request.

is_rest.php

<?php

if ( !function_exists( 'is_rest' ) ) {
	/**
	 * Checks if the current request is a WP REST API request.
	 *
	 * Case #1: After WP_REST_Request initialisation
	 * Case #2: Support "plain" permalink settings and check if `rest_route` starts with `/`
	 * Case #3: It can happen that WP_Rewrite is not yet initialized,
	 *          so do this (wp-settings.php)
	 * Case #4: URL Path begins with wp-json/ (your REST prefix)
	 *          Also supports WP installations in subfolders
	 *
	 * @returns boolean
	 * @author matzeeable
	 */
	function is_rest() {
		if (defined('REST_REQUEST') && REST_REQUEST // (#1)
				|| isset($_GET['rest_route']) // (#2)
						&& strpos( $_GET['rest_route'] , '/', 0 ) === 0)
				return true;

		// (#3)
		global $wp_rewrite;
		if ($wp_rewrite === null) $wp_rewrite = new WP_Rewrite();
			
		// (#4)
		$rest_url = wp_parse_url( trailingslashit( rest_url( ) ) );
		$current_url = wp_parse_url( add_query_arg( array( ) ) );
		return strpos( $current_url['path'] ?? '/', $rest_url['path'], 0 ) === 0;
	}
}

Before calling $prefix = rest_get_url_prefix( ); you can short circuit the return value when REST_REQUEST is defined. No need to call that function until later.

@matzeeable Thank you for a great piece of code, it is very useful. However, I would like to mention that I tested the code with WP 5.8 and plain permalinks. The proper structure of the URL is like that: site.org/?rest_route=/wp/v2/posts, not site.com/?rest_route=/wp-json/wp/v2/posts. That is why the case #2 code does not work for me. Line 20

&& strpos( trim( $_GET['rest_route'], '\\/' ), rest_get_url_prefix( ) , 0 ) === 0)

should be removed entirely. Without line 20 code works properly.

Kindly check and if I am right, please make modifications here and in the https://wordpress.stackexchange.com/a/317041/108721. Your answer is used widely already, for instance, in the Kadence theme.

@kagg-design what is the result of rest_get_url_prefix() in your case?

wp-json. There is no need to get it, as it is not present in the URL: https://prnt.sc/1qgqeut

@kagg-design Thanks for the hint. Strange, at time of creation of this function I did test all scenarios... I have updated the gist and now I am just checking if rest_route starts with /.

Maybe something changed in the WP Core.

We can look at a simple test site with WP 5.8, 2021 theme and no plugins.

http://test.kagg.eu/?rest_route=/wp-json/wp/v2/posts - 404
http://test.kagg.eu/?rest_route=/wp/v2/posts - works

Many thanks for making changes!

Hi, just a heads up regarding your recent update.

&& strpos( $_GET['rest_route'] ), '/', 0 ) === 0) I think should instead be && strpos( $_GET['rest_route'], '/', 0 ) === 0)

Updated. I should definitely not update gist on my smartphone... :D

I don't understand what #3 is doing there!

@gitgetgotgotten Look at the code of rest_url() used in line 28.

@kagg-design Aha, thank you!

thanks it works for me!