import random

def unreliable_tool(task: str) -> str:
    """
    Simulate a tool that fails 30% of the time.
    """
    if random.random() < 0.3:
        raise ValueError("Tool failed due to random error.")
    # Simple implementation: if task contains arithmetic, compute it
    if "2+2" in task:
        return "4"
    return "unknown"