Does using the plugin send my prompt to my harness’s model provider?
It depends on the command. A plainask runs inside your harness conversation, so your harness can see that prompt. private is the command built to keep a turn out of your harness’s upstream entirely. See Ask and Private.
What does Private actually guarantee?
Aprivate turn is not sent to your harness’s own model provider. What it does not do is turn off local disk writes or act as a sandbox by itself. The Private page spells out exactly what is and is not covered.

