# `prompt.placeholder_unused`

A prompt's type has a field that its prompt file never writes.

Area 21, [AI](/docs/ai.md); not declinable; the next step is usually the agent's.

## Why Jadpo refuses it

A prompt is two files: a type whose fields are the values it is given,
and a `.prompt` file of text, where each `{field}` placeholder writes one
of them. The model is sent only that text, so what a reviewer reads in
the `.prompt` file is exactly what the model gets.

A field the text never writes is never sent. Either the text forgot it,
and the model is missing something the code meant it to have, or the
field isn't needed. Jadpo can't tell which, so it refuses the field
until the text writes it or the field is removed.

## What it looks like


domains/support/prompts/answer_question.prompt:

```jadpo
# Instructions

Answer the customer's question in the language of {reader_locale}.

# Request

{question}
```

domains/support/prompts/answer_question.jadpo:

```jadpo
/// Answers a customer's question.
type AnswerQuestion = Prompt {
    question: Text
    reader_locale: Locale
    order_reference: Text
}
```

```text
error: `AnswerQuestion.order_reference` is written nowhere in `answer_question.prompt`
 --> domains/support/prompts/answer_question.jadpo:5:5
  |
5 |     order_reference: Text
  |     ^^^^^^^^^^^^^^^ never written
  reason: Each field of a prompt is a placeholder its text writes, so a model is sent only what a reviewer reads in `answer_question.prompt`.
  next: Write `{order_reference}` in `answer_question.prompt`, or remove the field (agent)
  rule: prompt.placeholder_unused
  help: https://jadpo.dev/docs/errors/prompt.placeholder_unused

check failed: 1 error, 2 files
```

`(agent)` at the end of the `next:` line means your coding agent can make
the fix itself.

## How to fix it

The question is whether the model should be given the field.

**If the model needs it**, write its placeholder in the prompt file. An
order reference is free text, so it goes under `# Request`, not in the
instructions. In `domains/support/prompts/answer_question.prompt`:

```text
{order_reference}
```




The model is now sent the order reference with the question, as a data
block tagged `<order_reference>`.

**If it doesn't**, remove the field. `answer_question.jadpo` becomes:

```jadpo
/// Answers a customer's question.
type AnswerQuestion = Prompt {
    question: Text
    reader_locale: Locale
}
```



Code that builds an `AnswerQuestion` stops giving it an
`order_reference`, and the prompt's text is unchanged.

## Related

- [Prompts as files](/docs/prompts-as-files.md)
- [`prompt.free_text_in_instructions`](/docs/errors/prompt.free_text_in_instructions.md)