The short version

Formatting works best as an output contract. Instead of listing everything the response must avoid, describe the form the finished answer should take: its structure, length, voice, and required fields.

Why positive instructions help

A prohibition marks a boundary but does not identify the destination. “Do not be verbose” leaves the desired length and level of detail unresolved. “Write three concise paragraphs under 300 words” gives Claude a target it can follow and you can evaluate.

Four parts of an output contract

  1. Structure: prose, sections, table, XML, or another useful shape.
  2. Length: a word limit, paragraph count, row count, or other practical bound.
  3. Style: executive, plain-language, technical, or conversational.
  4. Field rules: exact headings, labels, keys, or columns when downstream use requires them.

Specify only the dimensions that matter. Excessive rules can make a simple response harder to produce and harder to read.

Example

A vague request says: “Review these project updates. Do not be long or use too many bullets.”

A stronger request says: “Write a management brief in three sections: Status as one paragraph; Risks and decisions as no more than three bullets; Next actions as owner, action, and timing. Use plain business language and stay under 250 words.”

The second prompt defines a result that can move directly into the intended workflow.

When exact formats matter

Use explicit tags or keys when another system will parse the response. For human reading, prefer a natural format that supports the decision. The format should serve the next action rather than display prompt complexity.

Practical checklist

  • State what Claude should produce.
  • Set a length bound only when it improves usability.
  • Name the audience and desired voice.
  • Define exact fields when a template or system requires them.
  • Test the same contract across several inputs.

Try it

Rewrite a prompt built mostly from “do not” instructions. Replace each prohibition with a positive description of the finished answer, then remove any rule that does not affect usability.