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A wrapper’s promptTemplate is a Handlebars string. At request time it’s evaluated against the caller’s input, and the result becomes the prompt sent to the base model. This is where a wrapper earns its keep: callers type a few plain fields, and your template assembles them into a consistent, well-engineered prompt. 
{{subject}}, {{lookup maps.mood mood}}, cinematic portrait, shallow depth of field
Given { "subject": "a fox", "mood": "noir" }, this renders: 
a fox, high-contrast black and white, dramatic shadows, cinematic portrait, shallow depth of field

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What’s available in the template

ReferenceResolves to
{{fieldName}}Any field from the caller’s input — every input property is a top-level variable.
mapsYour templateContext.maps — reusable lookup tables.
constantsYour templateContext.constants — static strings reused across renders.
A few behaviors to keep in mind:
  • Missing variables render empty. A reference to a field the caller didn’t send produces an empty string, not an error. Design templates that read cleanly when optional fields are absent.
  • Output is not HTML-escaped (noEscape is on). Quotes, ampersands, and angle brackets pass through verbatim — exactly what you want for a prompt string.
  • Templates are capped at 50 KB. More than enough for a prompt recipe; if you’re approaching it, move data into templateContext.maps.

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Helpers

Only these five helpers are available. Anything else is unsupported.
HelperSignatureReturns
lookup{{lookup maps.group key}}The value at group[key], or empty if absent. Safe against prototype keys.
eq{{#if (eq a b)}}…{{/if}}true when a === b.
and{{#if (and x y)}}…{{/if}}true when every argument is truthy.
or{{#if (or x y)}}…{{/if}}true when any argument is truthy.
exists{{#if (exists field)}}…{{/if}}true when the value is not null and not an empty string.

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Conditionals

Combine helpers with Handlebars’ built-in #if to branch: 
A portrait of {{subject}}{{#if (exists outfit)}} wearing {{outfit}}{{/if}}.
{{#if (eq mood "noir")}}Shot on black-and-white film.{{/if}}
{{#if (and lighting (or (eq style "studio") (eq style "editorial")))}}{{lighting}} key light.{{/if}}
exists is the one to reach for when guarding optional fields — #if alone treats an empty string as truthy in some cases, while exists correctly skips both null and "".

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Lookup maps and constants

templateContext holds data your template references — keeping the template itself short and readable. 
{
  "maps": {
    "mood": {
      "warm": "golden-hour warmth, soft rim light",
      "moody": "low-key lighting, deep shadows",
      "noir": "high-contrast black and white, dramatic shadows"
    }
  },
  "constants": {
    "negative": "blurry, lowres, watermark"
  }
}
Read them in the template with lookup (for maps) or a direct reference (for constants): 
{{subject}}, {{lookup maps.mood mood}}. Avoid: {{constants.negative}}.
Use maps when an enum field should expand into richer phrasing — moods, styles, shot types — especially when the same template structure serves many enum values. Use constants for static text you repeat, like a negative-prompt suffix. Skip templateContext for one-off values that read fine inline.
A lookup map is the idiomatic way to translate a small, user-friendly enum ("noir") into a long, model-friendly phrase ("high-contrast black and white, dramatic shadows"). Keep the enum in your input schema, keep the phrasing in maps.

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What’s not allowed

These Handlebars constructs are rejected when you create or update the wrapper — they compile but would silently render empty, so the engine fails loudly instead:
ConstructExampleDo this instead
Partials{{> shared}}Inline the text, or use a constants entry.
Inline partials / decorators{{#*inline "x"}}…{{/inline}}Inline the text directly.
Raw blocks{{{{raw}}}}…{{{{/raw}}}}Write the literal text without the raw wrapper.
The common mistake is hardcoding a big lookup table as inline conditionals or a partial. Don’t — put enumerated values in templateContext.maps and read them with {{lookup maps.group key}}.

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Test before you ship

Use preview_wrapper (from your AI assistant, or the studio’s live preview) to render the template against a sample input and confirm the prompt looks right — including how it reads when optional fields are omitted. Preview is the fastest way to catch a missing comma, an unmatched mood key, or a conditional that fires when it shouldn’t.
The template produces the prompt string only. Non-prompt fields the base model needs (counts, sizes, formats) come from your field mappings, not the template.