# <format> tag — Claude XML prompting reference

> How to use the <format> XML tag in Claude prompts: when to use it, when not to, minimal and full examples, common mistakes.
>
> Source: https://claudexml.com/tags/format/ · Last updated 2026-05-25

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

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

    Describes the required shape of the output — a schema, a JSON skeleton, a style spec.


## 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


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


## Full example


```xml
<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.


Cite this page

`<format> — When and How to Use in Claude Prompts. claudexml.com. https://claudexml.com/tags/format/`