A higher-priority instruction supplied to a large language model by the application developer, separate from end-user input, that sets persona, scope, tone, refusal rules, and output format for the session.
In practiceThe system prompt is the principal lever an application has over model behaviour short of fine-tuning. It typically asserts the assistant's role, lists permitted and forbidden topics, fixes the output schema, and stipulates how to handle ambiguous, hostile, or out-of-scope requests. In production it is versioned, reviewed, and protected against extraction attempts, because leakage of the system prompt can reveal both intellectual property and the rule-set an adversary needs to defeat. It forms part of the auditable model configuration.
An internal legal-research assistant ships with a system prompt that fixes the assistant as a non-advisory research tool, mandates citation of the supplied source passages, forbids commentary on pending matters, and requires a standard disclaimer at the foot of every answer.
This definition is maintained by Moweb partners and used in live client engagements. For how System prompt applies to your estate, or to challenge a working definition, speak to a partner.