requiredUnless()

requiredUnless($field, $value) makes a field required when the other field does not strictly equal the given value.

$rules->string('taxId')
    ->optional()
    ->requiredUnless('customerType', 'individual');

The comparison is strict and the field is required when the condition does not match. Use this in a RuleSet registry factory, with Rule::on(), or with Rule::for() so Fynix can inspect the owning DTO.

$customer = new Customer();
$customer->customerType = 'company';
$customer->taxId = '';

$error = Rule::for($customer)
    ->string('taxId')
    ->optional()
    ->requiredUnless('customerType', 'individual')
    ->validate();