Stored only in your browser's localStorage — never sent anywhere except directly to Google's API at generativelanguage.googleapis.com.
Get a key at aistudio.google.com

Sets the assistant's persona and behavior for the entire conversation. Cleared when you clear the conversation.

0 = focused / deterministic  ·  1 = balanced  ·  2 = creative / random

2.0 Flash supports up to 8192. 1.5 Pro supports up to 8192. 2.5 Pro exp supports up to 65536.