Legal Question in Real Estate Law in California

Yes, you must send copies of all pleadings to every attorney involved.

I agree with Mr. Shers.

You must serve all papers in the case on all the parties.

So, the question boils down (based on the previous answers) to what you mean by "countersuit." There is no such legal term in California statutes. If you have filed a cross-complaint in the same legal action, the answer is yes. If your "countersuit: is a separate case with a distinct case number, the answer is probably no. Also, if the cases are now separate, I'd be expecting a motion to consolidate them.

A follow-up on my earlier answer:

Discovery inquiries are not pleadings. However, discovery does have to be served on all parties.

When a party is represented by more than one law firm in the same action, as A for the complaint and B for the cross-complaint, I think it would make good sense to serve both counsel with the discovery demands. In the case of responses, I would serve them on the asking party's attorney and attorney(s) for all other parties. However, I do not find any law or rule that requires service on each law firm; the requirement is only to serve each party.

yes you must send copies of all discovery requests and responses to all attorneys and parties representing themselves.