A task-instruction prompt tells the agent what to do right now. A job-description prompt tells the agent who it is, what it owns, and how it behaves across all tasks it will ever receive. The distinction matters for agents that run repeatedly on varied input: a developer agent needs consistent behavior across many different tickets, not behavior tuned to one specific task.
Job-description prompts typically include: the agent's name and role, its department and reporting structure, a first-thing-every-run checklist (check inbox, check open issues), the specific skill or workflow it runs, communication protocols (how and when to publish events, comment on PRs), git rules, and escalation criteria. This is closer to an employee onboarding document than a system prompt in the traditional sense.
The advantage of the job-description approach is stability and consistency. The agent behaves like a team member with a defined role rather than a stateless instruction-follower. The disadvantage is prompt length — job-description prompts are longer, consuming more tokens per session. The tradeoff is typically worthwhile for agents that run many times, since the consistency benefit compounds.