Should a style profile go in custom instructions or in a project?

Split it. The part that is true of all your writing belongs in the account-wide field; the part that is specific to one audience or one medium belongs in the project or Gem, where it will not leak into unrelated messages.

Will the same profile text work in every assistant?

The content transfers; the length budget and field names do not. Keep the canonical profile outside the vendor and paste the slice that fits each field.

What about agents that never open a chat window?

They need the profile as data rather than as pasted prose. A runtime block loaded into the system prompt at session start is the equivalent installation step.

Comparison

Where to put a style profile in each AI assistant

A style profile goes in the highest-scope field that matches how widely the voice applies: account-level settings for what is true of all your writing, and project or Gem instructions for what is true of one audience.

Every assistant has at least two places to put style information, and the choice between them is a scope decision rather than a preference. Account-level settings apply everywhere, which is right for habits that do not change with the reader. Workspace-level settings apply to one body of work, which is right for register.

The failure people hit is putting a full multi-register profile in an account-level field. It then applies to messages written to the wrong audience, and the assistant follows it faithfully.

Assistant	Account-wide field	Workspace field	Best use of each
ChatGPT	Custom instructions, under Personalization	Project instructions	Account for cross-cutting habits; project per audience
Claude	Instructions for Claude, plus Styles	Project custom instructions	Styles for register presets; project for one body of work
Gemini	Saved info	Gem instructions	Saved info for stable facts; a Gem per recurring task
Microsoft Copilot	Personalization settings	Limited	Account-level only; expect less scope control
Agents and connectors	System prompt	Per-tool config	Load the runtime block at session start

Installation target by assistant and scope, verified 2026-09-15

Method notes

Field names change between vendor releases; the scope distinction has been stable longer than any of the names.

Frequently asked questions

Should a style profile go in custom instructions or in a project?: Split it. The part that is true of all your writing belongs in the account-wide field; the part that is specific to one audience or one medium belongs in the project or Gem, where it will not leak into unrelated messages.
Will the same profile text work in every assistant?: The content transfers; the length budget and field names do not. Keep the canonical profile outside the vendor and paste the slice that fits each field.
What about agents that never open a chat window?: They need the profile as data rather than as pasted prose. A runtime block loaded into the system prompt at session start is the equivalent installation step.

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MyWritingTwin. “Where to put a style profile in each AI assistant.” Last updated 2026-09-15. https://www.mywritingtwin.com/reference/where-to-put-a-style-profile