System Prompts & Custom Instructions
Set It Once, Use It Always
Tired of repeating the same instructions in every conversation? System prompts and custom instructions let you set persistent rules that apply to every interaction — your preferred writing style, your role, your formatting preferences, things to always do or never do. Set them once and every conversation starts with your preferences already loaded.
What Are Custom Instructions?
Both ChatGPT and Claude offer ways to set persistent instructions that apply to all new conversations. In ChatGPT, it's called Custom Instructions (Settings → Personalization). In Claude, it's built into Projects and profile preferences. These are essentially standing orders: "Always write in British English," "I'm a marketing director at a B2B SaaS company," "Never use bullet points — I prefer numbered lists."
What to put in custom instructions
Your role/profession, your preferred communication style, output format preferences (concise vs. detailed), topics you frequently work on, and things to always avoid (jargon, clichés, certain phrases).
What NOT to put in custom instructions
Task-specific details, context that only applies to one conversation, or extremely long instructions (keep it under 300 words — focused and clear).
System Prompts for Power Users
If you use the API or build custom tools, system prompts are the more powerful version of custom instructions. A system prompt is a hidden instruction that sets the AI's behavior before the user says anything. It's how companies build specialized AI assistants — a customer support bot, a coding assistant, a writing coach — each with different system-level instructions.
Custom instruction example: "I'm a product manager at a fintech startup. I write for executive audiences. I prefer concise, data-driven communication. Always use numbered lists over bullets. Never start responses with 'Great question!' or similar filler. When I ask for analysis, include both quantitative and qualitative perspectives. Default to 200 words or less unless I ask for more."
[Every conversation now starts with this context. The AI knows your role, audience, style preferences, and format defaults without you repeating them.]
Why this works: This 65-word custom instruction eliminates the need to specify role, audience, tone, and format in every single prompt. It's like the AI already knows you.
Projects: The Next Level
In 2026, both Claude and ChatGPT offer project-based workspaces where you can combine custom instructions with uploaded documents. Create a project for each major area of your work: one for client proposals, one for content creation, one for data analysis. Each project has its own instructions, reference documents, and conversation history.
Start with one custom instruction
Don't overthink it. Set your custom instructions to two things: 1) Who you are and what you do. 2) Your biggest output pet peeve (too long? too formal? too many bullet points?). You can always refine later.
A content marketer uses Claude daily for blog posts, email copy, and social media. She wastes time repeating her brand voice guidelines in every conversation.
She creates a Claude Project called "Brand Content" with the brand style guide, tone of voice document, and 5 sample posts uploaded. Her project instructions say: "Always write in our brand voice. Use the uploaded style guide. Suggest headlines in the format [Number] + [Benefit] + [Audience]."
Every piece of content starts on-brand from the first draft. She stops spending 5 minutes at the start of each conversation re-explaining the brand voice. Annual time savings: ~40 hours.
Quick Check
What's the best thing to put in your custom instructions?
Key Takeaway
Custom instructions and Projects eliminate repetitive setup. Define your role, style preferences, and format defaults once — then every conversation starts from a better baseline. Use Projects for different areas of your work.