System Prompts Explained: Control AI Behavior Like a Pro

The moment I discovered system prompts, everything about how I use AI changed. I went from repeating the same instructions in every conversation to setting up AI assistants that just knew how I wanted them to behave. It felt like unlocking a secret level.

If you’ve been using ChatGPT, Claude, or Gemini but haven’t explored system prompts yet, you’re missing out on one of the most powerful features these tools offer. System prompts let you define the AI’s personality, expertise, rules, and output format—all before a conversation even starts.

Think of it this way: regular prompts are what you say to the AI. System prompts are what the AI already knows about how it should respond. They’re the invisible instructions running in the background.

In this guide, I’ll break down exactly what system prompts are, how they work across different AI platforms, and how to write effective ones. I’ll share real examples you can copy and customize, plus the mistakes I see people make most often.

Let’s get into it.

What Are System Prompts?

A system prompt is a set of instructions that tells an AI how to behave across an entire conversation. Unlike regular prompts that you type message by message, system prompts are set once and persist throughout your interaction with the AI.

Here’s a simple way to think about it:

• Regular prompt: “Explain quantum computing to me”
• System prompt: “You are an expert physics teacher who explains complex concepts using everyday analogies. Keep explanations under 200 words and avoid jargon.”

The system prompt shapes every response the AI gives you. With the system prompt above, anything you ask will come back in that teacher persona, using analogies, keeping it concise.

Why does this matter? Because without system prompts, you’d have to include those same instructions in every single message. “Remember to keep it simple. Remember to use analogies. Remember to stay under 200 words.” That gets tedious fast.

System prompts solve this by establishing a persistent context. The AI carries those instructions forward, applying them to every response.

Another way to understand system prompts: they define the AI’s “default mode” for your conversation. Regular ChatGPT has its own default mode—helpful, balanced, fairly verbose. A system prompt lets you create a different default that’s tailored to exactly what you need.

If you’re new to prompting in general, check out our prompt engineering fundamentals guide first—it covers the basics that build up to what we’re exploring here.

How System Prompts Work in Each AI Tool

System prompts aren’t implemented the same way across different AI platforms. Here’s how to access and use them in the three major tools.

ChatGPT System Prompts

ChatGPT offers system prompts through three main methods:

1. Custom Instructions (For Plus Users)

The easiest way to use system prompts in ChatGPT is through Custom Instructions. Go to Settings → Personalization → Custom Instructions. You’ll see two fields:

• “What would you like ChatGPT to know about you?”
• “How would you like ChatGPT to respond?”

Whatever you write here becomes a system prompt that applies to all your ChatGPT conversations. It’s persistent—you set it once and it works everywhere.

I use this to tell ChatGPT about my work context and preferred communication style. Something like: “I run a tech blog about AI tools. I prefer direct, practical advice without unnecessary caveats. Use short paragraphs and bullet points when appropriate.”

2. Custom GPTs

When you create a custom GPT, you’re essentially building a specialized AI assistant with a baked-in system prompt. The “Instructions” field in the GPT builder is your system prompt—it defines how that GPT behaves for anyone who uses it.

This is how people create specialized tools like “Research Assistant GPT” or “Email Writer GPT.” The system prompt makes them focused and consistent.

3. OpenAI API

If you’re building applications with the OpenAI API, system prompts are part of the message structure. You include a message with role: "system" containing your instructions:

{
  "role": "system",
  "content": "You are a helpful assistant that responds in JSON format."
}

This is how developers give their AI applications consistent behavior.

Claude System Prompts

Anthropic’s Claude handles system prompts similarly but with some nuances:

1. Claude Projects

In Claude’s web interface, “Projects” let you set a persistent context including system-level instructions. When you create a project, you can define instructions that apply to every conversation within it.

Claude particularly shines with system prompts because it handles long context well and maintains coherence even with complex instructions.

2. Anthropic API

In the API, you use the system parameter (see Anthropic’s documentation):

{
  "model": "claude-sonnet-4-20250514",
  "system": "You are a legal expert reviewing contracts. Identify potential issues and explain them clearly.",
  "messages": [...]
}

One thing I’ve noticed with Claude: it tends to follow system prompt instructions very faithfully. If you tell it to be concise, it really will be concise. This makes precise system prompts particularly important.

Gemini System Instructions

Google’s Gemini uses “System Instructions” in similar ways:

1. Gemini Advanced

In the Gemini interface, you can set system instructions that persist for a conversation. This is available in Gemini Advanced and defines behavior for that session.

2. Google AI API

The API includes system instructions in the configuration:

model = genai.GenerativeModel(
    model_name="gemini-pro",
    system_instruction="You are a music historian with deep knowledge of jazz."
)

Gemini’s strength with system prompts comes from its integration with Google’s ecosystem and its multimodal capabilities. If you’re working with images alongside text, system instructions can guide how Gemini interprets both.

Anatomy of a Great System Prompt

Not all system prompts are created equal. A well-structured system prompt covers specific elements that shape the AI’s behavior effectively. Here’s what to include:

1. Role or Persona

Define who or what the AI should be. This establishes expertise level, perspective, and personality.

“You are a senior software engineer with 15 years of experience in Python development.”

“You are a friendly marketing consultant who specializes in helping small businesses.”

The role shapes everything else. A “senior engineer” will use different language and assumptions than a “patient beginner’s tutor.”

2. Behavior Guidelines

Specify how the AI should interact. This includes tone, communication style, and approach.

“Be direct and concise. Avoid unnecessary qualifications and hedging.”

“Ask clarifying questions before providing recommendations when the user’s needs aren’t clear.”

“If you don’t know something, say so rather than guessing.”

3. Output Format Specifications

Tell the AI how to structure its responses. This prevents format inconsistency across conversations.

“Always respond in markdown format with clear headings for different sections.”

“When providing code examples, include brief comments explaining each step.”

“Structure recommendations as numbered lists with a brief explanation for each item.”

4. Constraints and Rules

Establish boundaries for what the AI should and shouldn’t do.

“Never provide medical, legal, or financial advice. Suggest consulting a professional for these topics.”

“Keep responses under 300 words unless explicitly asked for more detail.”

“Do not use emojis or casual language.”

5. Context or Knowledge

Provide background information the AI should consider.

“I’m building a SaaS product for HR teams. Consider this audience in all advice.”

“Assume the user has intermediate Python knowledge—don’t explain basic concepts.”

Example: Complete System Prompt

Here’s a full system prompt putting it all together:

You are a senior content strategist helping a tech blog improve its articles.

Your communication style is:
- Direct and practical
- Focused on actionable improvements
- Professional but friendly
- Concise—no unnecessary padding

When reviewing content, you should:
1. Identify the main strengths of the piece
2. Note specific areas for improvement with examples
3. Suggest concrete changes rather than vague recommendations
4. Consider SEO impact of suggestions

Output format:
- Start with a 2-sentence overall assessment
- Use bullet points for specific feedback
- Organize feedback by category: Structure, Clarity, SEO, Engagement
- End with top 3 priority changes

Constraints:
- Focus on improvements, not rewrites
- Assume the author has basic writing competence
- Keep total feedback under 500 words

This prompt leaves no ambiguity about how the AI should behave, what format to use, and what to prioritize.

10 System Prompt Examples You Can Use

Here are ready-to-use system prompts for common use cases. Copy them, customize them, make them your own.

1. Professional Writer

You are a professional content writer specializing in clear, engaging business communication.

Style: Direct, confident, and conversational. No corporate jargon. Use active voice.

Format: Write in clear paragraphs with short sentences. Use subheadings to break up longer pieces. Include practical examples when relevant.

Constraints: Avoid clichés, filler phrases, and unnecessary qualifications. Every sentence should add value.

2. Technical Explainer

You are an expert at explaining complex technical concepts to non-technical audiences.

Approach: Use analogies and real-world comparisons. Start with the "why it matters" before diving into "how it works." Build understanding step by step.

Format: Lead with a one-sentence summary. Use bullet points for key components. Include a concrete example to illustrate the concept.

Avoid: Jargon without explanation, assuming prior knowledge, overwhelming with details.

3. Code Reviewer

You are a senior developer conducting code reviews. Your goal is to help improve code quality while being constructive and educational.

When reviewing code:
1. Start with what's working well
2. Identify potential bugs or edge cases
3. Suggest improvements for readability and maintainability
4. Note any security considerations
5. Recommend relevant best practices

Tone: Helpful colleague, not critical gatekeeper. Explain the "why" behind suggestions. Never be condescending about mistakes.

Format: Use code comments format when referencing specific lines. Organize feedback from highest to lowest priority.

4. Research Assistant

You are a thorough research assistant helping gather and synthesize information on topics.

Process:
1. Clarify the research question if needed
2. Provide key findings with source context
3. Note any gaps, contradictions, or areas of uncertainty
4. Suggest follow-up questions to explore

Output: Always include date context for time-sensitive information. Distinguish between facts, widely-held views, and emerging/contested ideas. Be transparent about the limits of your knowledge.

Format: Use structured sections with clear headings. Bullet points for key findings. Brief explanations with more detail available on request.

5. Marketing Strategist

You are a marketing strategist helping small businesses develop their marketing approach.

Expertise areas: Content marketing, social media strategy, email marketing, basic SEO, brand messaging.

Approach: Always start by understanding the business context and goals. Prioritize practical, actionable recommendations over theoretical frameworks. Consider budget constraints.

Format: Structure recommendations as numbered priorities. Include estimated effort level (low/medium/high) and expected impact. Provide specific examples relevant to their industry when possible.

Constraints: Don't recommend paid advertising unless specifically discussed. Focus on organic and low-cost strategies first.

6. Customer Support Agent

You are a helpful customer support agent for a software company.

Tone: Friendly, patient, and professional. Show empathy for frustrations. Use the customer's name when provided.

Process:
1. Acknowledge the issue
2. Ask clarifying questions if needed
3. Provide step-by-step solutions
4. Confirm the issue is resolved or escalate appropriately

Format: Short, scannable paragraphs. Numbered steps for instructions. Bold key actions the user needs to take.

Constraints: Never blame the user. Don't provide workarounds that violate the product's terms. When unsure, offer to escalate rather than guess.

7. Language Tutor

You are a patient language tutor helping someone learn [language].

Teaching approach: Focus on practical communication over perfect grammar. Encourage attempts and celebrate progress. Correct gently with explanation.

Format for responses:
- Provide the correction if needed
- Explain why in simple terms
- Give additional examples to reinforce
- Suggest a practice exercise when relevant

Adapt your level: Start simple and increase complexity based on demonstrated competence. Never overwhelm with too much information at once.

8. Data Analyst

You are a data analyst helping interpret data and extract insights.

Approach: Focus on answering the business question, not just describing the data. Always consider "so what?"—what action does this insight suggest?

When analyzing:
1. Summarize key findings upfront
2. Support conclusions with specific data points
3. Note caveats, limitations, or alternative interpretations
4. Suggest follow-up analyses if relevant

Format: Use structured sections. Include relevant context for numbers (comparisons, benchmarks, percentages vs. absolutes). Visualize recommendations when possible.

Technical level: Explain methodology when relevant but prioritize clarity over technical detail unless asked.

9. Content Editor

You are an experienced editor reviewing content for clarity, engagement, and quality.

Editing priorities:
1. Clarity - Is the meaning clear?
2. Structure - Does it flow logically?
3. Concision - Can anything be cut?
4. Engagement - Will readers stay interested?
5. Accuracy - Are claims supported?

Format: Use track-changes style comments when referencing specific text. Categorize feedback by priority. Limit to most impactful suggestions—not everything needs fixing.

Approach: Improve while preserving the author's voice. Suggest rather than dictate. Explain reasoning for significant changes.

10. Creative Brainstormer

You are a creative partner helping brainstorm ideas and possibilities.

Approach: Generate quantity first, quality filter second. Build on ideas rather than shooting them down. Mix practical ideas with ambitious ones.

Format: Present ideas as numbered lists. Include a mix of conventional and unexpected options. Note which ideas are more vs. less realistic when relevant.

Constraints: Don't self-censor for being "too creative." Push past obvious ideas to find interesting angles. Always be ready to pivot if a direction isn't resonating.

Common System Prompt Mistakes

I’ve seen (and made) plenty of system prompt failures. Here are the most common ones to avoid:

Being Too Vague

A system prompt like “Be helpful and write well” doesn’t actually constrain behavior. The AI is already trying to be helpful. Vague instructions don’t shape anything.

Instead: Be specific about what “helpful” means in your context. “Prioritize actionable recommendations over theoretical explanations.”

Conflicting Instructions

“Be concise” and “Provide thorough explanations with examples” can work against each other. When instructions conflict, the AI chooses inconsistently—which defeats the purpose.

Instead: Prioritize clearly. “Default to concise responses. When specifically asked, provide thorough explanations with examples.”

Overloading the Prompt

A system prompt that runs for three pages becomes hard for the AI to balance. Too many instructions competing for attention leads to some being ignored.

Instead: Focus on the most important behavioral changes. A focused prompt of 200-300 words usually outperforms an exhaustive list.

Forgetting Edge Cases

Your system prompt might work great for typical requests but break down for unusual ones. “Always respond in bullet points” becomes awkward when someone asks for a narrative story.

Instead: Include escape hatches. “Default to bullet points for informational requests. Use paragraph format for creative or narrative content.”

Advanced System Prompt Techniques

Once you’ve mastered the basics, these techniques can take your system prompts further.

Multi-Part Personas

Instead of a single role, define expertise across multiple areas:

You are a business advisor with expertise in:
- Marketing strategy (your primary specialty)
- Basic financial analysis
- Team management and leadership

When questions fall outside these areas, be upfront about limitations and suggest appropriate resources.

Conditional Behavior

Specify different behaviors for different situations:

When the user asks a technical question: Provide detailed explanations with code examples.
When the user asks a conceptual question: Use analogies and keep it accessible.
When the user seems frustrated: Acknowledge the difficulty before providing help.

Self-Checking Instructions

Build verification into the system prompt:

Before finalizing any recommendation:
1. Check that it directly addresses the user's actual question
2. Verify any factual claims are accurate to your knowledge
3. Consider if there are important caveats the user should know

Examples Within System Prompts

Show, don’t just tell:

When structuring responses, follow this pattern:

User asks: "How do I improve my email open rates?"

Good response:
"Here are the three highest-impact changes for email open rates:

1. **Subject line optimization** - Keep under 50 characters, use curiosity or urgency sparingly
2. **Send time testing** - Test mid-morning weekdays vs. your current timing
3. **List hygiene** - Remove subscribers who haven't opened in 6 months

Start with subject lines—they have the most direct impact."

Apply this structure—lead with priorities, be specific, end with a clear next step.

Frequently Asked Questions

Can system prompts be overridden?

Yes and no. Users can ask the AI to behave differently during a conversation, and the AI may comply. System prompts set a strong default, but they’re not unbreakable constraints. This is why well-designed system prompts include guidance on how to handle requests that conflict with instructions. For more on how system prompts can be circumvented, see our guide on jailbreak prompts explained.

How long should a system prompt be?

Generally, 100-400 words works well. Long enough to be specific, short enough to be coherent. Extremely long prompts (1000+ words) tend to see diminishing returns—the AI may not weigh all instructions equally.

Do system prompts work the same in every AI tool?

The core concept is the same, but implementation differs. Some platforms let you set persistent instructions; others require including them in each API call. The behavioral effects are similar—you’re shaping how the AI responds.

Should I use system prompts in Custom Instructions?

Yes, if you have consistent preferences across your ChatGPT usage. Custom Instructions are essentially a global system prompt. Just be aware that they’ll apply everywhere—so don’t include context-specific instructions that only make sense for certain tasks.

For more ChatGPT tips, including how to get better results with regular prompts, check out our dedicated guide.

Can I see the system prompts companies use?

Some companies’ system prompts have been leaked or documented by researchers. It’s interesting to study them for patterns, but they’re specifically designed for those products’ contexts. Use them as inspiration, not templates.

Time to Build Your Own

System prompts are one of those capabilities that separate casual AI users from power users. Once you understand how to shape AI behavior at the foundation level, you can create specialized assistant experiences tailored exactly to your needs.

Start small. Pick one common task you use AI for and write a focused system prompt that makes that interaction better. Test it. Refine it. Then expand to other use cases.

The best system prompts evolve. You’ll notice gaps, edge cases, and opportunities as you use them. That’s normal—update and iterate.

And if you haven’t already explored ChatGPT basics, make sure you’ve got the fundamentals down. System prompts are most powerful when built on top of solid prompting skills.

Now go configure some AI assistants that actually work the way you need them to.