Selenium: InvalidSessionIdException: invalid session id

Every WebDriver command carries a session ID. This exception means the driver no longer recognizes that ID, because the session has ended. Sessions end when you call driver.quit(), when the last browser window is closed with driver.close(), or when the browser crashes or is killed. Any command after that, including a cleanup quit() in some bindings, fails with this message. Newer ChromeDriver versions may add detail such as session deleted as the browser has closed the connection.

In test suites it often points to a shared driver that one test quit while others still use it, or to a browser crash under memory pressure that only shows up as the next command failing. On Selenium Grid, sessions also end when they exceed the node's idle timeout.

What is not wrong: the element or page logic of the failing line. The browser behind the session is gone.

import pytest
from selenium import webdriver

@pytest.fixture
def driver():
    d = webdriver.Chrome()
    yield d
    d.quit()          # one driver per test, quit exactly once

def test_title(driver):
    driver.get("https://example.com/")
    assert "Example" in driver.title

Tie each driver's lifetime to one owner, a fixture or a context manager, and never share drivers across threads. Monitor container memory, because browser crashes surface only as this exception on the next command. Keep Grid timeouts aligned with the longest step in your flows.