# Writing prompts with variables

> How prompts are built in Prompt Verse: typed pieces, the four prompt types, wrappers such as role and constraint, {{variables}} and rendering in 11 formats.

Source: https://prompt-verse.io/docs/writing-prompts/

A prompt in Prompt Verse is not one long string. It is a tree of **pieces**, rendered into text when you need it.

## Pieces

Every piece has a type:

| Type | Use it for |
|---|---|
| `text` | A paragraph of instruction |
| `section` | A titled group of pieces. Sections nest |
| `module` | A reusable content module |
| `block` | A group of modules, for large prompts |
| `wrapper` | A directive around other pieces |

A piece can also point at a **library piece**, so a rule written once is reused everywhere, or nest **another prompt**.

## Prompt types

The type decides what the root of the tree may contain.

- **Simple**: one text body.
- **Structured**: nestable, titled sections. This is the default.
- **Modular**: reusable content modules.
- **Advanced**: blocks that group modules.

The render tells you if the structure breaks the type you chose.

## Wrappers

A wrapper changes how the pieces inside it are processed. Typical ones:

- **Role** (AS): “a support agent who has read the whole thread”.
- **Constraint** (CONSTRAIN): “never invent a refund, a date or a policy”.
- **Format** (FORMAT): “respond only in JSON”.
- **Examples** (EXAMPLES): a few worked cases.
- **Conditional**, **Repeat**, **For each**, **Chain**, **Retry**, **Fallback**, **Guard**, **Temperature** and **Parallel** for larger designs.

## Variables

Write a variable as `{{name}}`. Names are letters, digits and underscores, for example `{{account_age}}`. Variables can appear in any text piece.

```prompt title="A structured prompt with variables"
# The message
{{message}}

# What we know
Order: {{order}}
Account age: {{account_age}}

# Write the reply
Answer the question that was actually asked, in at most six sentences.
```

When you render, you give values. Anything you leave out is reported as **missing**, so you or your AI can ask for it.

## Rendering

Rendering is plain code and does not use an AI. It resolves library pieces and nested prompts, applies wrappers, fills variables and writes the result in a format:

`markdown` (default), `text`, `xml`, `json`, `yaml`, `html`, `csv`, `toml`, `jsonl`, `messages` and `prompty`.

The `messages` format gives a chat message list for APIs that take one. Over MCP, call `prompts` with action `render`, see the [reference](/docs/reference/prompts/).

## Good habits

- Name variables for what they hold, not where they go.
- Put rules in wrappers, not buried in sentences.
- Commit before you rewrite. See [Versions](/docs/versions/).