Do you need leave to amend?
Five questions, and you have your answer. Runs entirely in your browser. Nothing you click is saved or sent anywhere.
Start here
What it is working from
Code of Civil Procedure section 472(a) gives one amendment as of right, and the right is keyed to filing events rather than to how far along the case is.
There are three windows. Before any answer, demurrer or motion to strike is filed. After a demurrer or motion to strike is filed but before it is heard, if the amended pleading is filed and served no later than the opposition date, which is nine court days before the hearing under CCP 1005(b). And after that, on stipulation of the parties, which the statute names.
A special motion to strike under CCP 425.16 does not open the second window. CCP 472(b).
Two limits sit on top. Facts that arose after filing go in a supplemental complaint and always need leave, CCP 464(a). And the right attaches to the pleading as originally filed, so once you have amended once, every later version needs leave or a stipulation. Hedwall v. PCMV, LLC (2018) 22 Cal.App.5th 564, 574.
Where it stops being reliable
Two situations where the walkthrough gives you an answer that is contested rather than settled.
A co-defendant's answer. If one defendant has answered and another has a live demurrer, whether you keep the right to amend against the demurring defendant is genuinely unresolved. Barton v. Khan (2007) 157 Cal.App.4th 1216, 1221 says you do. Alden v. Hindin (2003) 110 Cal.App.4th 1502, 1508-1509 says you do not. No later published decision has settled it, and both were decided under the pre-2016 version of the statute. The walkthrough routes you to the cautious answer and tells you the argument exists.
A limitations date. Adding a party inside the first window is permitted, Gross v. Department of Transportation (1986) 180 Cal.App.3d 1102, 1105-1106, but older authority reads narrower on new parties. If the statute is close, get an order rather than relying on the window.
This walkthrough is a starting point for your own check, not a substitute for reading section 472 and your department's standing order. The full reasoning, the case law and the failure modes are in the guide.