_.role

BAML prompts compile into a messages array that LLM providers use:

BAML Prompt -> [{ role: "system", content: "..." }, { role: "user", content: "..." }]

By default, BAML puts everything into a single message with the system role (or the best default for the selected provider).

Syntax

{{ _.role("system") }}
{{ _.role("user") }}
{{ _.role("assistant") }}

Basic Example

function MyFunction(input: string) -> string {
    client "openai/gpt-4o"
    prompt #"
        {{ _.role("system") }}
        Everything after this will be a system prompt!

        {{ _.role("user") }}
        And everything after this will be a user message.
        {{ input }}
    "#
}

tip

The playground shows the current role for each message section.

note

BAML may change the default role to user for specific APIs that only support user prompts, like when using images.

Using in Loops

Inject a list of messages with different roles:

class Message {
    role string
    message string
}

function ChatWithAgent(input: Message[]) -> string {
    client "openai/gpt-4o"
    prompt #"
        {% for m in input %}
            {{ _.role(m.role) }}
            {{ m.message }}
        {% endfor %}
    "#
}

Alternate roles based on index:

function ChatMessages(messages: string[]) -> string {
    client "openai/gpt-4o"
    prompt #"
        {% for m in messages %}
            {{ _.role("user" if loop.index % 2 == 1 else "assistant") }}
            {{ m }}
        {% endfor %}
    "#
}

Using in Template Strings

template_string YouAreA(name: string, job: string) #"
    {{ _.role("system") }}
    You are an expert {{ name }}. {{ job }}

    {{ ctx.output_format }}
    {{ _.role("user") }}
"#

function CheckJobPosting(post: string) -> bool {
    client "openai/gpt-4o"
    prompt #"
        {{ YouAreA("hr admin", "Your role is to ensure every job posting is bias free.") }}

        {{ post }}
    "#
}

Available Roles

Role	Description
system	System instructions
user	User input
assistant	AI response (for context)

See Also

• Jinja Overview
• Template Strings
• Loops