Amendment as of right

In federal court, a person who files a lawsuit can amend the complaint once within 21 days of serving it, or within 21 days after the other side responds. During that window, no court order is required and the other side cannot block the change. After that window closes, amendments require either the other side's written consent or the court's permission. The amendment-as-of-right window is often used to fix problems pointed out in an early motion to dismiss before the court rules. Many state courts have similar early-amendment rules with their own timelines.