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A session can carry its own answer instructions, a system prompt that shapes how it answers every turn: the role to take, the tone to use, or how to format the result. These layer on top of a context’s default instructions rather than replacing them. The context default always applies, and the session prompt adds to it, except where it only repeats the default, which Nexus skips. You pass system_prompt on the query that starts the session, and it stays pinned for the rest of the session.
1

Start a session with a system prompt

Pass system_prompt alongside scope and ask on the opening query. These fields are set only when a session starts.
curl
The response carries a session_id and the query’s id.
2

Continue the session

Reference the previous turn with previous_query_id. Follow-up turns keep the system prompt, so you don’t resend it, and the context default still applies.
curl
In the console, open Advanced options in the new session composer and set the System prompt.