System Prompt

Why System Prompt Is Needed

In every conversation, you might want the model to have different "personalities" and "behavior patterns."

For example:

• Customer service scenario: you want the model professional, patient, polite
• Code assistant scenario: you want the model rigorous, direct, providing runnable code
• Creative writing scenario: you want the model imaginative, with varied styles

These "global settings" shouldn't be repeated every time a user asks a question. System Prompt solves this — set it once at the start of the conversation, and it applies globally.

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What Is System Prompt

One-line definition: System Prompt is the global rule layer set for the model before a conversation starts, defining role, goals, boundaries, and default behavior.

Analogy: System Prompt is like giving the model a "employee handbook" — it's not answering a specific question, but telling the model "what role you are, how to do things, what you can't do."

Three layers of Prompt:

• System Prompt: role settings, behavior rules — highest priority
• User Prompt: the actual question the user is asking this turn
• Assistant history: previous conversation context — affects style and continuity

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How to Do It: How to Write Good System Prompts

A good System Prompt typically includes:

• Role: what identity the model has now
• Goal: what the main task of this session is
• Style: how to answer, tone, structure
• Constraints: what not to do, how to handle exceptions

Practical advice:

• Write the most important rules first — key constraints buried too late are easily ignored
• Use executable language over abstract slogans, e.g., "give conclusions first, then reasons"
• Write clear strategies for exceptional situations, e.g., "state uncertainty when unsure"
• If the system calls tools, it's best to also specify when to call and when to stop

For systematic Prompt optimization, see the Prompt Engineering entry.

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Common Pitfalls

• Too long System Prompt consumes Context space, and middle rules are more easily ignored
• Vague instructions like "answer naturally" are usually hard to execute consistently
• Permissions, sensitive operation confirmations, and data validation can't be guaranteed by Prompt alone — need program control

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Remember this: System Prompt is the model's "employee handbook" — set once at the start of the conversation, applies globally, defining role, rules, and boundaries.

Related terms: Prompt · Prompt Engineering