custom instructions
A feature where you set persistent instructions (like 'always be concise' or 'i'm in the UK') that apply to all chats with a model.
Think of it like
A standing order with a waiter—every time you visit, they remember your preferences.
Example
Add instruction: 'I'm a software engineer; assume technical knowledge.' Now every chat with ChatGPT starts with that context.
How it actually works
Custom instructions are stored client-side and prepended to the system message. They're convenient for personalization but don't update the model weights—just the prompt. Downside: they take up context window tokens.
For product teams
Improves UX personalization; reduces need to repeat context.
For engineers
Client-side prompt modification; simplest form of personalization.
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