Add Custom Validation to an ACF Field

Use the acf/validate_value filter to add custom server-side validation with error messages.

Validate a specific field by name — enforce a minimum word count on a textarea:

<?php
add_filter( 'acf/validate_value/name=description', function ( $valid, $value, $field, $input_name ) {
    if ( $valid !== true ) {
        return $valid;
    }

    $word_count = str_word_count( strip_tags( $value ) );

    if ( $word_count < 20 ) {
        return 'Description must be at least 20 words. Currently: ' . $word_count . '.';
    }

    return $valid;
}, 10, 4 );

Enforce unique values across posts:

<?php
add_filter( 'acf/validate_value/name=employee_id', function ( $valid, $value, $field, $input_name ) {
    if ( $valid !== true || empty( $value ) ) {
        return $valid;
    }

    $existing = get_posts( [
        'post_type'    => 'staff',
        'meta_key'     => 'employee_id',
        'meta_value'   => $value,
        'post__not_in' => [ $_POST['post_ID'] ?? 0 ],
        'fields'       => 'ids',
    ] );

    if ( ! empty( $existing ) ) {
        return 'This employee ID is already in use.';
    }

    return $valid;
}, 10, 4 );

Validate by field type — require all email fields to use a company domain:

<?php
add_filter( 'acf/validate_value/type=email', function ( $valid, $value, $field, $input_name ) {
    if ( $valid !== true || empty( $value ) ) {
        return $valid;
    }

    if ( ! str_ends_with( $value, '@example.com' ) ) {
        return 'Please use your @example.com email address.';
    }

    return $valid;
}, 10, 4 );

Return true to pass validation, or a string error message to fail. Always check $valid !== true first to respect earlier validation (e.g. the built-in “required” check).