What Is Custom Instructions?

The Plain-English Explanation

Instead of repeating the same context at the start of every conversation — "I'm an Australian marketing manager, write in Australian English, keep responses concise, use professional but friendly tone" — custom instructions let you set these preferences once and have them apply automatically to every interaction.

Most major AI tools support some form of custom instructions. ChatGPT has a dedicated Custom Instructions feature. Claude has project-level instructions. These settings sit in the background, shaping every response without you needing to re-state your preferences.

Why It Matters

Custom instructions save time and improve consistency. Instead of getting generic responses that need to be adjusted to your context, you get tailored outputs from the first message. For professionals who use AI daily, this eliminates the repetitive work of setting up context and preferences in every new conversation.

Examples in Practice

A teacher setting custom instructions to always explain concepts at a Year 10 reading level with Australian curriculum references.
A developer specifying their preferred programming language, framework, and coding style so every code generation request matches their standards.
A content marketer configuring tone of voice, brand guidelines, and target audience so AI-generated drafts are closer to publish-ready from the start.

Common Misconceptions

Myth: Custom instructions make the AI remember everything about you.

Reality: They provide persistent context for how you want responses shaped. They don't give the AI memory of previous conversations or personal data beyond what you explicitly include.

Myth: Setting custom instructions once is enough.

Reality: Good custom instructions evolve. As you discover what works and what doesn't, refine them. Review and update your instructions every few weeks to improve your results.

Myth: Custom instructions work the same across all AI tools.

Reality: Each platform implements persistent instructions differently. ChatGPT has a two-part system (about you + how to respond). Claude uses system prompts and project instructions. Understanding the specific mechanism helps you use it effectively.

Learn Custom Instructions in Depth

Module 6 of AI Productivity covers custom instructions and personalisation — teaching you to configure AI tools so they work the way you need them to, every time.

Explore AI Productivity

Frequently Asked Questions

Where do I set custom instructions in ChatGPT?

Click your profile icon in the bottom-left corner, then select 'Custom instructions.' You'll see two fields: one for information about you and one for how you want ChatGPT to respond.

Do custom instructions use up my context window?

Yes — custom instructions are added to every conversation, so they consume tokens from your context window. Keep them concise and focused on the most important preferences to avoid wasting space.

Can I have different custom instructions for different tasks?

ChatGPT has one set of custom instructions, but you can create different Custom GPTs for different tasks, each with their own instructions. Claude allows different project-level instructions.