I'm using Flask to run a local web server and using Selenium with Chrome in headless mode to automate a task via Python.

However, when I run the Selenium script in headless mode, it times out when trying to connect to the local Flask server. I get a timeout error during page loading or element wait.

Interestingly, if I first run the same script in headed mode (i.e., without the --headless flag), it works fine. And once it works in headed mode, running it again in headless mode also works — as if the first headed execution somehow "unlocks" the headless behavior.

My Environment:

Selenium Version : 4.28.1

Python Version : 3.10.11

Python script module import list:

from selenium import webdriver
from selenium.webdriver.support.ui import Select
from selenium.webdrivermon.alert import Alert
from selenium.webdrivermon.keys import Keys
from selenium.webdriver.support.ui import WebDriverWait
from selenium.webdrivermon.action_chains import ActionChains
from selenium.webdriver.support import expected_conditions as EC
from selenium.webdrivermon.by import By
from selenium.webdriver.remote.webelement import WebElement
from seleniummon.exceptions import NoAlertPresentException
from selenium.webdriver.chrome.options import Options
from fake_useragent import UserAgent

Python driver Option cmd:

op = webdriver.ChromeOptions()
op.add_argument("start-fullscreen")
op.add_argument("window-size=1920x1080")
op.add_argument('headless=new')

My question:

1. Why does headless mode fail on the first run but work after using headed mode once?

2. Is this related to Chrome caching, user data directories, or network behavior?

3. What can I do to ensure that headless mode works reliably from the first run?