Custom instructions versus a style profile

Also called: chatgpt custom instructions, personalization settings

Custom instructions are a free-text field inside one assistant where you state your preferences; a style profile is a measured artifact about your writing that can be pasted into that field, into a different assistant, or into an agent's system prompt.

Custom instructions are a slot; a style profile is something you can put in the slot, and in other slots too.

Framing them as competitors is a category error, and it is the reason the comparison confuses people. One is a storage location owned by a vendor. The other is content whose value is that it is not owned by a vendor.

The practical question is therefore not which to use but what to put in the field. A field filled with adjectives produces the same output as anyone else's adjectives. A field filled with measured constraints from your own corpus produces something only your corpus could have produced.

DimensionCustom instructionsStyle profile
Kind of thingA per-vendor text fieldA portable artifact
Who authors the contentYou, from memoryDerived from your writing
Works in another assistantNo — each vendor has its own fieldYes — paste the same artifact
Survives a vendor changing the featureNoYes
CostIncluded with the assistantCost of building the profile
What each one is, on the dimensions people actually compare

Frequently asked questions

Are custom instructions free?
Yes, the field is included with the assistant. The cost that people miss is the writing of good content for it, which is the part a measured profile replaces.
How long can ChatGPT custom instructions be?
OpenAI has raised the limit over time and does not commit to a figure in product for the long term, so any number quoted in a third-party article has an expiry date. Treat length as a moving constraint and keep the profile itself stored outside the field.
Should I use custom instructions or Projects?
Both, for different scopes. Account-level instructions carry what is true of all your writing; a project or workspace carries what is true of one audience or one piece of work.

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MyWritingTwin. “Custom instructions versus a style profile.” Last updated 2026-09-15. https://www.mywritingtwin.com/glossary/custom-instructions-vs-style-profile