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Version: 1.x

API04001: Invalid Response Format

What This Means​

The server responded, but the response format is not what the POS expected. The server should return JSON data, but something else was received.

Common Causes​

  • PHP error displayed — A PHP error is being output before JSON
  • Plugin conflict — Another plugin is outputting content
  • Maintenance mode — Site is showing a maintenance page
  • Wrong content type — Server sending HTML instead of JSON
  • Caching issue — A cached error page is being served

How to Fix​

1. Check Your Site​

Visit your WordPress site in a browser:

  • Is it displaying normally?
  • Are there any visible errors?
  • Is it in maintenance mode?

2. Check for PHP Errors​

In wp-config.php, temporarily enable debugging:

define('WP_DEBUG', true);
define('WP_DEBUG_LOG', true);
define('WP_DEBUG_DISPLAY', false);

Check wp-content/debug.log for errors.

3. Test the REST API Directly​

Visit https://yoursite.com/wp-json/ in your browser:

  • Should return JSON data
  • If you see HTML or errors, there's a problem
  • Check for plugin-related output

4. Disable Caching Temporarily​

Caching plugins may serve stale responses:

  • Clear all caches
  • Temporarily disable caching plugins
  • Exclude the REST API from caching

5. Check for Plugin Conflicts​

If a plugin outputs content on every page:

  1. Disable all non-essential plugins
  2. Test the POS
  3. Re-enable plugins one by one