Do I need to file anything before my California small claims hearing?

If you've been served with a small claims summons in California, there's no formal written answer required, which is unusual compared to most civil lawsuits and is exactly why many defendants assume there's nothing to prepare before showing up.

What you may need to file is different: a counterclaim, meaning your own claim against the plaintiff arising from the same dispute, on Judicial Council form SC-120. That form has a real deadline attached to it, generally at least five days before the hearing, or ten days if you were served sixteen or more days before the hearing date. Miss that window and you generally lose the ability to raise your claim inside this same case, leaving you to file it separately later, with all the extra time and cost that involves.