You have probably stopped writing one-line prompts. You name the task, you give some background, you say what you want. And the output still arrives slightly off: technically correct, wrong register, wrong shape, wrong length for the person who has to read it.
That is not vagueness. Vagueness you can feel while you are typing. This is partial specification, and it is invisible, because everything you wrote was good. The problem is what you did not write.
Watch how it happens. You describe the task carefully and forget to say how the prose should be built, so you get the model's house style. You name the audience and forget the format, so you get five paragraphs where you needed four sections. You get the tone right and forget the objective, so you get something pleasant that does not ask for the thing you actually needed to ask for. Every one of those is a prompt that was 80% written.
COSTAR is not a clever prompt. It is a checklist that makes a partial prompt impossible.
It won Singapore's national GPT-4 prompt engineering competition, and it is worth being clear about why, because it was not cleverness. There is no trick in COSTAR, no phrasing that unlocks anything. It won because it forces completeness, and completeness beats cleverness on real work almost every time.
The contractor test
Think about what you would say to a builder. "Build me a kitchen" is a real instruction. It is also the reason you end up with a kitchen you did not want, because the builder had to decide the layout, the worktop material, the budget and the timeline on your behalf, and they decided all of it silently.
Hand over blueprints, material specs, a timeline, a budget and a note about the look you are after, and you get your kitchen. Nothing about the builder changed. You just stopped delegating decisions you cared about.
COSTAR is the blueprint. Six dimensions, each closing off one category of decision the model would otherwise make for you:
- Context. The situation this output exists inside.
- Objective. The exact thing you want produced.
- Style. How the prose is constructed.
- Tone. The register it lands in.
- Audience. Who reads it and what they already know.
- Response format. The structure it has to arrive in.