CO-STAR
Context · Objective · Style · Tone · Audience · Response — Six slots, three of them about voice. The framework to reach for when who is reading this matters as much as what it says.
Use it for
Published writing, marketing copy, customer communication, and anything with a house style. Style, Tone and Audience as separate slots is the most precise voice control of any common framework.
Avoid it for
Technical or analytical work, where three voice slots is wasted effort and Objective ends up carrying the whole prompt.
The slots
Each letter is a question the prompt has to answer.Context
What is the background?
The situation the writing sits inside.
Example: We are a Dutch bicycle insurer launching a claims app after two years of complaints about paper forms.
Objective
What must the text achieve?
The job the words have to do, not the topic they cover.
Example: Get existing customers to install the app before their next renewal.
Style Optional
Whose craft?
The writing discipline: journalistic, technical documentation, direct-response, academic. Style is structure and technique.
Example: Plain-language service writing — short sentences, concrete nouns, no metaphors.
Tone Optional
What attitude?
The emotional register. Tone is attitude; style is technique. Keep them distinct.
Example: Apologetic about the old process without dwelling on it, then matter-of-fact.
Audience
Who reads this?
Be specific about what they already know and what they are worried about.
Example: Customers aged 45–75, comfortable with online banking, sceptical that an app will be simpler than a phone call.
Response
What exact output?
Format, length, and structure. Say it mechanically.
Example: One email under 120 words, plus three subject-line options, plus the app-store link as a single call to action.
Strengths
- Separating Style from Tone is a real distinction: one is craft, the other is attitude.
- Audience as its own slot changes vocabulary and assumed knowledge more than any other single field.
- Response is an explicit output contract.
Weaknesses
- Six slots is a lot; in practice people leave two blank and lose the framework’s advantage.
- Style versus Tone genuinely confuses people, and a muddled pair is worse than one clear field.
- No slot for source material or examples.
Sources and grounding
- 1How I Won Singapore’s GPT-4 Prompt Engineering CompetitionSheila Teo, Towards Data ScienceThe original published description of CO-STAR by its author, who developed it on GovTech Singapore’s data science team.
- 2Mastering the art of prompt engineeringGovTech SingaporeInstitutional background on the competition CO-STAR came out of.