Selenium invalid session id (tested)

InvalidSessionIdException: invalid session id means the browser behind your WebDriver session is gone: Chrome crashed or was killed ("session deleted as the browser has closed the connection"), or close() closed its last window. Start a new driver, and fix the cause: memory or /dev/shm in Docker, or code that closes too early.

Setup used in every block

common.py writes a small HTML page to a local file and starts headless Chrome. Exception messages are cut to their first line.

import os, pathlib
from selenium import webdriver
CHROME = os.path.expanduser("~/.cache/ms-playwright/chromium-1248/chrome-linux64/chrome")
def page(html):
    p = pathlib.Path("/tmp/sl383/page.html"); p.write_text(html); return p.as_uri()
def driver():
    o = webdriver.ChromeOptions()
    o.binary_location = CHROME
    o.add_argument("--headless=new"); o.add_argument("--no-sandbox"); o.add_argument("--window-size=1000,700")
    return webdriver.Chrome(options=o)

1. Reproduce it: the browser died under the session

import os, signal, subprocess
from common import *
d = driver()
d.get(page("<p>hi</p>"))
# simulate a browser crash (out of memory, /dev/shm full in Docker, ...): kill the Chrome process
pid = d.service.process.pid                                   # chromedriver
chrome = subprocess.run(["pgrep", "-P", str(pid)], capture_output=True, text=True).stdout.split()
for p in chrome:
    os.kill(int(p), signal.SIGKILL)
for attempt in (1, 2):
    try:
        d.title
    except Exception as e:
        print(f"call {attempt}:", type(e).__name__ + ":", e.msg.splitlines()[0])
d.quit()

Output:

call 1: InvalidSessionIdException: invalid session id: session deleted as the browser has closed the connection
call 2: InvalidSessionIdException: invalid session id; For documentation on this error, please visit: https://www.selenium.dev/documentation/webdriver/troubleshooting/errors#invalidsessionidexception

Killing Chrome (as the OS does when it runs out of memory) gives exactly the error people search for: the first call says invalid session id: session deleted as the browser has closed the connection, every later call just invalid session id. chromedriver is still running; the browser behind the session is gone, so the only fix is a new driver. Find out why Chrome died: in Docker the usual cause is the 64 MB default /dev/shm (run the container with --shm-size=2g, or add the Chrome argument --disable-dev-shm-usage); otherwise check memory (dmesg for the OOM killer) and very long sessions that accumulate tabs.

2. close() on the last window ends the session

from common import *
d = driver()
d.get(page("<p>hi</p>"))
d.close()                        # closes the only window
try:
    d.title
except Exception as e:
    print("after close() of the last window:", type(e).__name__ + ":", e.msg.splitlines()[0])
try:
    d.quit()
    print("quit() after that: OK")
except Exception as e:
    print("quit():", type(e).__name__)

Output:

after close() of the last window: InvalidSessionIdException: invalid session id; For documentation on this error, please visit: https://www.selenium.dev/documentation/webdriver/troubleshooting/errors#invalidsessionidexception
quit() after that: OK

driver.close() closes the current window; when it is the only one, chromedriver ends the session, and the next command raises InvalidSessionIdException. Use close() only for extra tabs (then switch_to.window() back to one that is still open), and quit() to finish. Calling quit() afterwards was still fine.

3. After quit(): not InvalidSessionId but MaxRetryError

from common import *
d = driver()
d.get(page("<p>hi</p>"))
d.quit()
try:
    d.title
except Exception as e:
    print("after quit():", type(e).__module__ + "." + type(e).__name__)
    print(str(e).split(" (Caused by")[0])

Output:

after quit(): urllib3.exceptions.MaxRetryError
HTTPConnectionPool(host='localhost', port=42099): Max retries exceeded with url: /session/f0fdee7053df2b89a105408a159aaeca/title

After quit(), chromedriver itself has exited, so the next call does not even get an "invalid session id": the HTTP request to chromedriver's local port is refused, and you get urllib3.exceptions.MaxRetryError ... Max retries exceeded with url: /session/<id>/... caused by Connection refused (only the first line is printed here). Seeing this error means some code still uses a driver after quit(), typically a test fixture that quits in teardown while another test, thread or atexit handler still holds it.

4. Checklist

• One driver per test or per thread; never share one driver between threads.
• Call quit() exactly once, in a finally block or fixture teardown, and do not touch the driver after that.
• Docker: --shm-size=2g or --disable-dev-shm-usage; watch memory.
• Recover from a crash by creating a new driver; the old session cannot be revived.

Related: Selenium Unable to obtain driver for chrome, This version of ChromeDriver only supports Chrome version.

Written with AI assistance. Measured on 2026-10-23 with Selenium 4.51.0 (Python 3.13.5) and headless Chrome for Testing 156.0.8078.4. Each block was run verbatim and the output pasted unedited. PlainHeaders is our extension; Python, Selenium and Chrome are free.