You can define a summarizer agent with a cheap LLM such as GPT-3.5 and have a custom function to use that agent for message summarization. Something like this:

import autogen
from autogen import ConversableAgent

api_key = open('api.txt').read().strip()

llm_config = { "config_list": [
    { "model": "gpt-4o", "api_key": api_key },
    { "model": "gpt-3.5-turbo", "api_key": api_key },
] } 

cheap_filter = {"model": ["gpt-3.5-turbo"]}

john = ConversableAgent(
    name="John",
    system_message="You are a comedian in a play role.",
    llm_config=llm_config,
    human_input_mode="NEVER",
    max_consecutive_auto_reply=6,
)

david = ConversableAgent(
    name="David",
    system_message="You are a comedian in a play role.",
    llm_config=llm_config,
    human_input_mode="NEVER",
    max_consecutive_auto_reply=3,
    description="Writes Frontend code for the website and start each task.",
)

summarizer = ConversableAgent(
    name="Summarizer",
    system_message="You are a chat summarizer. You receive one chat message at a time and summarize it.",
    llm_config={'config_list': autogen.filter_config(llm_config['config_list'], cheap_filter)},
    human_input_mode="NEVER",
    max_consecutive_auto_reply=1,
)

def summarizeChat(sender, receiver, summary_args):
    summarizer = summary_args['summarizer']
    result = receiver.initiate_chat(
                summarizer, 
                message=receiver.last_message(sender)['content'],
                max_turns=1,
                silent=True
                )
    return result

david.initiate_chat(john,
                    message="Do you know why the computer went to the doctor?",
                    summary_method=summarizeChat,
                    summary_args={"summarizer": summarizer})