The location and implementation of custom instructions differs across platforms, so let me orient you to where this capability sits on the tools you are most likely to be using.
In Claude, custom instructions can be set at the profile level, which applies across all conversations and all projects, and at the project level, where they apply only within that specific workspace. The profile-level instructions are your global defaults. The project-level instructions override or supplement those defaults for a specific piece of work. The two layers work together: profile handles the permanent, project handles the specific.
In ChatGPT, custom instructions are configured in Settings under Personalisation, and there are two fields: one for information about you and one for how you want responses to be formatted. When you build a Custom GPT, the instructions you write for that GPT function as the project-level layer: specific to that workspace and that purpose.
In both cases, the principle is the same: set your global defaults at the platform level, and use project-level instructions to adjust for specific contexts. The companion PDF for this module includes a reference for where these settings live on each platform, so you can find them without searching.
