What a tool should return

Return a dictionary, not a bare string. The model reads the keys.

# harder for the model to use
return "10:30 AM"

# easier, and it can tell success from failure
return {"status": "success", "city": city, "time": "10:30 AM"}

Include a status field so the agent can tell the difference between a real answer and a failure, and say something useful either way.