Chainlit can be mounted as a FastAPI sub application.
my_cl_app
import chainlit as cl

@cl.on_chat_start
async def main():
    await cl.Message(content="Hello World").send()
main
from fastapi import FastAPI
from chainlit.utils import mount_chainlit

app = FastAPI()


@app.get("/app")
def read_main():
    return {"message": "Hello World from main app"}

mount_chainlit(app=app, target="my_cl_app.py", path="/chainlit")
In the example above, we have a FastAPI application with a single endpoint /app. We mount the Chainlit application my_cl_app.py to the /chainlit path. Start the FastAPI server: 
uvicorn main:app --host 0.0.0.0 --port 80
When using FastAPI integration, header authentication is the preferred method for authenticating users. This approach allows Chainlit to delegate the authentication process to the parent FastAPI application, providing a more seamless and secure integration.