Custom prompt for Langchain agent

I am trying to custom prompt for an agent but fail, the agent is not listening to the prompt, would someone point me to the right way?

In my case, the fixed_prompt is only a string.

agent = initialize_agent(
agent=AgentType.CHAT_CONVERSATIONAL_REACT_DESCRIPTION,
tools=tools,
llm=turbo_llm,
verbose=True,
early_stopping_method=‘generate’,
SystemMessagePromptTemplate = fixed_prompt,
memory=memory
)