Stop the Submit! Preventing Full Form Submission with Drupal AJAX Buttons

Using AJAX buttons in Drupal forms is great for dynamic interfaces, but sometimes they cause an unexpected headache: triggering a full form validation and submission process when you only wanted a partial update. Why does this happen, and what's the proper Drupal fix?

The Problem Briefly Explained

When an AJAX request from a form button hits the server, Drupal often runs the form's validation handlers before executing your specific AJAX callback. This link between the AJAX action and the validation step can inadvertently push the form down the submission path, especially if validation errors occur on other parts of the form or if the validation step itself implies a submission intent in Drupal's workflow.

The Solution: Use #limit_validation_errors

Instead of resorting to JavaScript hacks like event.preventDefault(), the Drupal Form API provides a clean, server-side solution: the #limit_validation_errors property.

Add this directly to your AJAX button's definition in your form array. By setting it to an empty array ([]), you tell Drupal not to run any validation when this specific button is clicked.

'#limit_validation_errors' => []

How it Works:

By skipping the validation step for this button's AJAX request, you effectively decouple it from the main form submission lifecycle. No validation means Drupal won't proceed to the submit handlers based on this button's action. Your AJAX callback will still run as intended.

Code Example:

Here’s the essential structure for your AJAX button in PHP:

$form['my_ajax_button'] = [
  '#type' => 'button', // Use 'button', not 'submit' for non-submitting actions
  '#value' => t('Perform AJAX Action'),
  '#ajax' => [
    'callback' => 'my_ajax_callback_function', // Your callback function name
    'wrapper' => 'my-ajax-wrapper-id',       // The HTML ID of the element to update
    'event' => 'click',
    'progress' => ['type' => 'throbber'],
  ],
  // Optional: Add attributes like classes if needed
  '#attributes' => ['class' => ['my-ajax-button-class']],
  // Ensures no submit handlers *specific* to this button run
  '#submit' => [],

  // --- THE SOLUTION ---
  // Add this line to prevent validating the whole form
  // and triggering the main form submission process.
  '#limit_validation_errors' => [],
  // --- End Solution ---
];

// Remember: Your 'my_ajax_callback_function' must exist
// and return a \Drupal\Core\Ajax\AjaxResponse object.

Conclusion

For AJAX buttons within Drupal forms that are intended only for partial updates or specific actions (and not submitting the main form), using '#limit_validation_errors' => [] is the standard, reliable, and Drupal-native way to prevent unwanted validation and submission. Keep your AJAX clean!