ExampleFormBuilder.rb

class ExampleFormBuilder < ActionView::Helpers::FormBuilder
  def error(method, error_text = nil, options = {})
    return if errors_blank?(method) && error_text.blank?
    
    text = if error_text
             error_text
           elsif object.errors.custom_messages_for(method)
             object.errors.custom_messages_for(method).to_sentence
           else
             object.errors.full_messages_for(method).to_sentence
           end

    @template.content_tag(:p, text, options)
  end
  
  private
  def errors_present?(method)
    object.errors.where(method).present?
  end

  def errors_blank?(method)
    !errors_present?(method)
  end
end

new.html.erb

# Version 1: Allow a form builder method to decide which error to use

<%= form_with(model: @user, builder: ExampleFormBuilder) do |form| %>
  <%= form.text_field :name %>
  <%= form.errors :name %>
<% end %>

# Version 2: Pass the error explicitly from the template

<%= form_with(model: @user, builder: ExampleFormBuilder) do |form| %>
  <%= form.text_field :name %>
  <%= form.errors :name, form.object.custom_errors_for(:name).to_sentence || form.object.custom_errors_for(:name).to_sentence %>
<% end %>

user.rb

class User < ApplicationRecord
  validates :name, presence: true
end

users_controller.rb

class UsersController < ApplicationController
  def new
    @user = User.new
  end
  
  def create
    @user = User.new(user_params)
    if @user.save
      redirect_to @user
    else
      render :new, status: :unprocessable_entity
    end
  end
end