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Prompts and Template Strings

BAML uses Jinja templating for dynamic prompt generation.

Prompt Syntax​

Prompts are defined using block strings with Jinja syntax:

function MyFunction(input: string) -> string {
    client "openai/gpt-4o"
    prompt #"
        Process this input: {{ input }}

        {{ ctx.output_format }}
    "#
}

Template Strings​

Template strings are reusable prompt components that accept parameters and return strings.

Syntax​

template_string TemplateName(param: Type) #"
    Template content with {{ param }}
"#

Example​

template_string PrintMessages(messages: Message[]) #"
    {% for m in messages %}
        {{ _.role(m.role) }}
        {{ m.message }}
    {% endfor %}
"#

function ClassifyConversation(messages: Message[]) -> Category[] {
    client "openai/gpt-4o"
    prompt #"
        Classify this conversation:
        {{ PrintMessages(messages) }}

        Use the following categories:
        {{ ctx.output_format }}
    "#
}

Key Features​

  • Can be nested inside each other
  • Can be called multiple times
  • Support all Jinja syntax
  • Allow reusing prompt logic across functions

Using in Tests​

Template strings can provide test data:

template_string EchoTestData(arg: int) ##"
    The number is {{ arg }}
"##

test TemplateTest {
    functions [Echo]
    args {
        input EchoTestData(43)
    }
    @@assert( {{ this == "The number is 43" }} )
}

Special Variables​

VariableDescription
ctx.output_formatAuto-generated format instructions
ctx.clientCurrent client/model info
_.role(name)Set message role

Jinja Features​

Variables​

prompt #"
    Hello {{ name }}!
    Count: {{ count }}
"#

Conditionals​

prompt #"
    {% if verbose %}
        Provide detailed explanation.
    {% else %}
        Be concise.
    {% endif %}
"#

Loops​

prompt #"
    {% for item in items %}
        - {{ item }}
    {% endfor %}
"#

Filters​

prompt #"
    {{ text | upper }}
    {{ items | length }}
"#
warning

The BAML linter may show warnings for template strings due to static analysis limitations. If it renders correctly in the playground, you're good.

See Also​