Term
System prompt
An instruction message that establishes role, behaviour, constraints, and tools available to the model before any user input. Sent as a separate message type at the start of the conversation and given higher priority by the model than user messages.
Background
The system prompt is where applications encode their persona and policies. For coding agents, system prompts contain rules like "only edit files I list", "always run tests before committing", "prefer the existing style over reformatting." Models trained for tool use give system-prompt instructions stronger weight than user messages, but a long system prompt still competes for context-window budget. Best practice in 2026: keep system prompts focused, factual, and under a few thousand tokens.