<format> — when and how to use it in Claude prompts

Use to describe a schema, a JSON shape, or a free-form style requirement.

When to use <format>

• Constraining Claude to a specific output structure when prose instructions alone aren't enough.
• Documenting a JSON or XML response shape inline.

When not to use it

• When you're using Claude's native tool-use / JSON mode via the API — let those features enforce structure.

Minimal example

<format>
{
  "sentiment": "positive|neutral|negative",
  "confidence": 0.0
}
</format>

Full example

<instructions>
Analyze the review and return JSON matching the shape in <format>.
Return only the JSON object — no prose, no markdown fences.
</instructions>

<format>
{
  "sentiment": "positive | neutral | negative",
  "confidence": 0.0,
  "keywords": ["string"],
  "would_recommend": true
}
</format>

<review>{{ user_review }}</review>

Common mistakes

• Describing the format in prose and also showing a schema — pick one. Two specs invite disagreement.
• Forgetting to tell Claude "return only the JSON" — by default it may wrap in ```json fences.