Generally if a court, asked to permit the filing of an amended complaint, rules that the defendant may file the amended complaint, the plaintiff must answer or otherwise responsively plead to the complaint. The time within which the plaintiff must file an answer or other responsive pleading is set forth in the state's rules or code of civil procedure. The time in some states to answer or otherwise responsively plead may be as little as 20 days from the date the amended complaint is deemed filed. That time may also be enlarged by reason of a "mailing rule" which allows a certain number of days to be added where service on the party has been accomplished by mail. If in doubt, check your state's rules or code of civil procedure as well as the court's local rules, if the court has local rules.
Responsive pleadings other than an answer may include a motion to dismiss for failure to state a claim (which I believe in California is called a demurrer); a motion to dismiss for failure to join an indispensable party; a motion for a more definite statement; a motion for judgment on the pleadings; a motion to dismiss for improper venue; a motion to dismiss for lack of subject matter jurisdiction; and a motion for summary judgment. The determination whether to answer or file a different type of responsive pleading should be made with care, and warrants consultation with competent counsel.
Not legal advice as I don't practice law in California. It's just my two cents on the facts you describe in light of general principles of law. If you need legal advice, please consult a lawyer who holds California licensure. That's not me as I practice in Vermont ONLY.
"In the mean time I received a proposed amended complaint." -- It appears from your question the amended complaint has not been filed. It may have been forwarded to you for your consideration re a stipulation allowing its filing, or perhaps it accompanied a motion for leave to amend. Or, perhaps it was filed before you filed your answer. You should contact opposing counsel and clear up any confusion and enter into an appropriate stipulation, if necessary.
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Yes, you need to answer an amended complaint, but as this is just a proposed amended complaint, it could be a possible resolution to the dispute. Hope this perspective helps!