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System Prompt Templating & Dynamic Composition

Built-in support: PromptTemplate and ChatPromptTemplate

Lauren-AI ships PromptTemplate (single message) and ChatPromptTemplate (multi-turn) in lauren_ai._prompts. Use them for simple {variable} interpolation:

# NOTE: future annotations are allowed, but tool signature types must resolve at import time.
from lauren_ai import PromptTemplate, ChatPromptTemplate

tpl = PromptTemplate(template="You are a {role}. Context: {context}")
msg = tpl.render(role="financial analyst", context="User is a portfolio manager")
# msg.content == "You are a financial analyst. Context: User is a portfolio manager"

Custom builder for richer composition

When you need conditional sections, structured constraints, or a fluent API, implement a SystemPromptBuilder:

class PromptTemplate:
    def __init__(self, template: str):
        self._template = template

    def render(self, **kwargs) -> str:
        result = self._template
        for key, value in kwargs.items():
            result = result.replace(f"{{{key}}}", str(value))
        return result


class SystemPromptBuilder:
    def __init__(self):
        self._sections: list[str] = []

    def add_role(self, role: str) -> "SystemPromptBuilder":
        self._sections.append(f"You are {role}.")
        return self

    def add_context(self, context: str) -> "SystemPromptBuilder":
        self._sections.append(f"Context: {context}")
        return self

    def add_instruction(self, instruction: str) -> "SystemPromptBuilder":
        self._sections.append(instruction)
        return self

    def add_constraints(self, *constraints: str) -> "SystemPromptBuilder":
        if constraints:
            self._sections.append("Constraints:\n" + "\n".join(f"- {c}" for c in constraints))
        return self

    def build(self) -> str:
        return "\n\n".join(self._sections)

Full example

system = (
    SystemPromptBuilder()
    .add_role("a financial analyst specializing in risk assessment")
    .add_context("User is a portfolio manager at a hedge fund")
    .add_instruction("Always provide quantitative estimates with confidence intervals.")
    .add_constraints(
        "Never give specific stock picks",
        "Always mention risks",
    )
    .build()
)

from lauren_ai import agent

@agent(model="claude-opus-4-6", system=system)
class FinancialAgent: ...

Conditional sections

import os

builder = SystemPromptBuilder().add_role("a helpful assistant")
if os.getenv("VERBOSE_MODE"):
    builder.add_instruction("Explain each step in detail.")
if user_tier == "enterprise":
    builder.add_context("User has access to premium data feeds.")
system = builder.build()

Template variables with PromptTemplate

tpl = PromptTemplate(template="You are {role} working for {company}.")
system = tpl.render(role="a legal advisor", company="Acme Corp")

Pitfalls

• build() joins sections with "\n\n" — sections should not have trailing newlines themselves or you get extra blank lines.
• Template {key} replacement is plain string substitution — curly braces in the template for non-variable purposes must be avoided or escaped.
• Prefer the built-in PromptTemplate for simple cases; the custom builder adds no overhead when sections are unconditional.