Tell it what "done" looks like
Outcome
By the end, you'll be able to shape Claude's reply — its length, format, and tone — just by asking for it directly.
Concept
Claude doesn't know how you want an answer packaged unless you say so. Want a numbered list instead of a wall of text? A two-sentence summary instead of an essay? Plain language instead of jargon? Ask for exactly that, in the same message as your question — Claude will follow the spec.
Build the prompt, step by step
The ask
Explain what a "for loop" does in programming.
What comes back
A for loop is a control flow statement that allows you to execute a block of code repeatedly based on a specified condition or sequence. It iterates over a range of values, a collection, or until a condition is met, executing the loop body each time. For loops are fundamental in programming and are used in virtually every language, from Python and JavaScript to C++ and Java. They are particularly useful when you know in advance how many times you want to repeat an action, such as processing each item in a list, generating a sequence of numbers, or performing a calculation multiple times. The basic structure typically includes an initialization, a condition check, and an increment or update step.
Try it
Open claude.ai and try this prompt:
Prompt
Explain what a "for loop" does in programming. Use exactly 3 bullet points, keep each under 20 words, and avoid jargon.
What to look for: Check whether Claude actually followed the spec — exactly 3 bullets, each one short, no jargon. Then ask the same question again without the format instructions and compare.
Takeaway
If you care how an answer is shaped — length, format, tone — say so in the prompt; don't leave it to chance.
Your turn
This prompt makes Claude guess. Rewrite it so it has output specification to answer your actual situation — the checklist tracks what you add.
The weak prompt
Explain how photosynthesis works.
What you've added
- Names a formatAsk for a list, a table, steps, JSON.
- Names a lengthCap it: three sentences, under 100 words.
- Names a structureSay what goes where: start with the summary, then the detail.
- Names the tone or audiencePitch it: for a 10-year-old, plain English, formal.