System prompts — make AI behave exactly how you want
The problem with starting every conversation from scratch
Every time you open a new AI conversation, it knows nothing about you. You have to re-explain your context, your preferences, your tone. It’s repetitive and it degrades output quality.
System prompts solve this. A system prompt is a set of instructions that runs silently before every conversation — telling the AI who it’s talking to, how to behave, and what to prioritise. It’s how you turn a generic AI into your personal assistant.
What a system prompt actually is
Think of it as the briefing you’d give a new employee on their first day. Before they take a single call or write a single email, you explain: here’s who we are, here’s how we communicate, here’s what’s important to us, here are things to avoid.
A regular prompt is like asking someone a question. A system prompt is like training someone before they answer any questions. The same underlying question gets very different answers depending on how the person was trained.
Where to find system prompts
Claude: Go to claude.ai → click your name → “User preferences” — you can add a custom system prompt here that applies to all conversations.
ChatGPT: Go to chatgpt.com → Settings → “Personalization” → “Custom instructions” — two fields: what you want ChatGPT to know about you, and how you want it to respond.
When building with AI APIs: System prompts are a dedicated field in every API call, separate from the conversation itself.
A practical example
Here’s a system prompt for someone who uses AI primarily for professional writing:
"You are my writing assistant. I work in B2B SaaS sales. My writing should be direct, confident, and specific — never vague or corporate. Avoid phrases like 'leverage', 'synergy', 'circle back', or 'touch base.' When I ask you to write something, always produce the full version rather than an outline unless I ask for an outline. If something I ask for could be improved by a small structural change, suggest it briefly after delivering what I asked for. My default email tone is professional but warm."
This prompt means every subsequent conversation will produce output in this voice without any reminders.
Building your own system prompt
Start with three sections:
Who you are and what you do. Give AI enough context to produce relevant answers. Your role, your industry, who your audience typically is.
How you want it to communicate. Tone, length preferences, formatting preferences, things to avoid. If you hate bullet points, say so. If you always want examples, say so.
What it should do differently from default. What does the default AI get wrong for your use case? Correct it here.
What system prompts cannot do
They set defaults, not hard rules. AI can still deviate from them in edge cases, and they don’t override the model’s core safety guidelines. But for the vast majority of everyday use, a well-written system prompt makes a noticeable difference from the first response.
Open whichever AI tool you use most and find the custom instructions or system prompt setting. Write 3-5 sentences covering: who you are, what you mainly use AI for, and one specific thing you want it to always do (or never do). Save it, then start a new conversation and notice how the tone and format have shifted without any instruction from you.