# utils.py
from typing import List

def build_conversation_prompt(history: List[str], user_input: str, system_prompt: str = None) -> str:
    """
    Build a single string prompt for the causal LM from conversation history and the new user input.

    history: list of previous lines (alternating user/assistant) or full conversation pieces.
    user_input: current user message.
    system_prompt: optional introductory prompt at beginning.
    """
    parts = []
    if system_prompt:
        parts.append(system_prompt.strip())
    for i, h in enumerate(history):
        parts.append(h.strip())
    parts.append("User: " + user_input.strip())
    parts.append("Assistant:")
    return "\n".join(parts)