# System prompt fragment ──────────────────────────────
SYSTEM = """
You solve tasks in a Thought → Action → Observation loop.

Thought:  Reason about the current state and what to do next.
Action:   Call exactly ONE tool using the format below.
Observation: The tool result will be appended automatically.
Repeat until you know the answer, then emit a Final Answer.

Available tools: search(query), calc(expr), lookup(key)
"""

# ─────────────────────────────────────────────────────

def react_loop(question: str, max_steps: int = 10):
    history = [("user", question)]
    
    for step in range(max_steps):
        # ① Generate next Thought + Action
        response = llm(SYSTEM, history)
        
        thought = parse_thought(response)    # free-form text
        action  = parse_action(response)     # tool name + args
        
        if action.name == "final_answer":
            return action.args["answer"]       # ⑤ Done
        
        # ② Execute tool
        obs = execute_tool(action)
        
        # ③ Append to context and loop
        history += [
            ("assistant", f"Thought: {thought}\nAction: {action}"),
            ("tool",      f"Observation: {obs}"),
        ]
    
    raise TimeoutError("max_steps reached without final answer")