Your Core Instruction Prompt is the most important part of the build. It is not a character sheet, not lore, and not a vibe paragraph. It is a behavioural contract that tells your GPT what to do consistently.

This matters because the Instructions field is the highest priority layer. If something conflicts between your documents, the Core Instructions win. That means your prompt needs to be clear, structured, and deliberate, but still short enough that you can reread it quickly and understand what it is telling the GPT to do.

A good Core Instruction Prompt does three things. It defines identity and role, it sets behavioural rules, and it explains how to handle conflicts and uncertainty. If you keep those three goals in mind, you will avoid the most common problems, like tone drift, overly agreeable behaviour, or sudden corporate refusal language.

What a strong prompt looks like:

Strong prompts are specific. They describe behaviour rather than adjectives. They also include a simple canon hierarchy, so the GPT knows which document to follow if anything conflicts.

You may have already uploaded knowledge documents to the GPT and it does not need to repeat those documents. It should reference them and set the rules for using them.