Prompt templating

Templates let you inject variables into prompts at runtime. Braintrust supports Mustache and Nunjucks templating:

Mustache (default): Simple variable substitution and basic logic.
Nunjucks: Advanced templating with loops, conditionals, and filters.

Mustache

Mustache is the default templating language. For the complete syntax, see the Mustache documentation.

Basic variable substitution

Use {{variable}} to insert values:

Hello {{name}}! Your account balance is ${{balance}}.

Nested properties

Access nested object properties with dot notation:

User: {{user.name}}
Email: {{user.profile.email}}
City: {{user.profile.address.city}}

Sections and iteration

Use sections to iterate over arrays or conditionally show content:

{{#items}}
- {{name}}: ${{price}}
{{/items}}

{{#user}}
Welcome back, {{name}}!
{{/user}}

Inverted sections

Use ^ to show content when a value is falsy or empty:

{{^items}}
No items found.
{{/items}}

Comments

Use {{! comment }} for comments that won’t appear in output:

{{! This is a comment explaining the template }}
Hello {{name}}!

Preserve special characters

If you want to preserve double curly brackets {{ and }} as plain text when using Mustache, change the delimiter tags:

{{=<% %>=}}
Return the number in the following format: {{ number }}

<% input.formula %>

Strict mode

Mustache supports strict mode, which throws an error when required template variables are missing:

const result = prompt.build(
  { name: "Alice" },
  {
    strict: true, // Throws if any required variables are missing
  },
);

Nunjucks

For more complex templating needs, use Nunjucks, which implements Jinja2 syntax in JavaScript. For the full list of syntax and features, see the Nunjucks templating documentation.

Loops

Process arrays and iterate over data:

{% for item in items %}
- {{ item.name }}: {{ item.description }}
{% endfor %}

Loop variables provide useful metadata:

{% for product in products %}
{{ loop.index }}. {{ product.name }}{% if not loop.last %}, {% endif %}
{% endfor %}

Available loop variables are loop.index (1-indexed), loop.index0 (0-indexed), loop.first, loop.last, and loop.length.

Conditionals

Add logic to your prompts:

{% if user.age >= 18 %}
You are eligible to vote.
{% elif user.age >= 16 %}
You can get a driver's license.
{% else %}
You are a minor.
{% endif %}

Combine conditionals with loops:

{% for product in products %}
  {% if product.inStock %}
Available: {{ product.name }} - ${{ product.price }}
  {% endif %}
{% endfor %}

Filters

Transform data with built-in filters:

Hello {{ name | upper }}!
Your email is {{ email | lower }}.
Items: {{ items | join(", ") }}

Common filters:

upper, lower: Change case.
title, capitalize: Capitalize text.
join(separator): Join array elements.
length: Get array or string length.
default(value): Provide default value.
replace(old, new): Replace text.

String operations

Concatenate strings with ~:

{{ greeting ~ " " ~ name }}!
Full name: {{ firstName ~ " " ~ lastName }}

Nested data access

Access nested properties and array elements:

{{ user.profile.address.city }}
{{ items[0].name }}
{{ data.results[2].score }}

Next steps

Create prompts that use these templates.
Use prompts in code and pass input values for your template variables.

Start

Instrument

Observe

Annotate

Evaluate

Deploy

Admin

Best practices

Mustache

Basic variable substitution

Nested properties

Sections and iteration

Inverted sections

Comments

Preserve special characters

Strict mode

Nunjucks

Loops

Conditionals

Filters

String operations

Nested data access

Next steps

​Mustache

​Basic variable substitution

​Nested properties

​Sections and iteration

​Inverted sections

​Comments

​Preserve special characters

​Strict mode

​Nunjucks

​Loops

​Conditionals

​Filters

​String operations

​Nested data access

​Next steps

Mustache

Basic variable substitution

Nested properties

Sections and iteration

Inverted sections

Comments

Preserve special characters

Strict mode

Nunjucks

Loops

Conditionals

Filters

String operations

Nested data access

Next steps