For such a small code base, the changes required by the new tests caused a fairly significant change. First, we added an IsComplete property to the Todo model:

internal class Todo
{
  public bool IsComplete { get; set; }
  public string Description { get; set; }
  
  internal void Validate()
  {
    Description = Description ?? throw new DescriptionRequiredException();
  }
}

The rest of the changes affect the TodoList class. A boolean property was added to toggle the visibility of completed items, the Complete method was modified to only mark the item as complete, and a where clause was added to the items retrieved from the list:

internal class TodoList
{
  private readonly List<Todo> _items = new List<Todo>();
         
  public IEnumerable<Todo> Items => _items.Where(t => !t.IsComplete || ShowCompleted);
  
  public bool ShowCompleted { get; set; }
  
  public void AddTodo(Todo item)
  {
    item = item ?? throw new ArgumentNullException();
    item.Validate();
    _items.Add(item);
  }
  
  public void Complete(Todo item)
  {
    item.IsComplete = true;            
  }        
}