System prompt: static, set at agent initialization — defines persona, capabilities, constraints, output format.

System prompt: static, set at agent initialization — defines persona, capabilities, constraints, output format. Doesn't change per run. Runtime instruction: dynamic, passed per invocation — the specific task for this run. Separating them allows: (1) Reuse the same agent for multiple tasks. (2) Cache the system prompt token cost. (3) Cleaner API — callers only pass the task, not re-specify the agent's full context.