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To attach Selenium to an already running headless Chrome, start Chrome with a remote DevTools debugging port, then give that host and port to ChromeDriver through the debuggerAddress option. A local example is:
chrome --headless=new --remote-debugging-port=9222 --user-data-dir=/tmp/chrome-debug https://example.comThen connect from Selenium:
from selenium import webdriver
from selenium.webdriver.chrome.options import Options
options = Options()
options.add_experimental_option("debuggerAddress", "127.0.0.1:9222")
driver = webdriver.Chrome(options=options)
print(driver.title)
driver.quit()What the attachment actually does
Normally, webdriver.Chrome() asks ChromeDriver to launch a new Chrome process. With debuggerAddress, Chrome is launched separately and ChromeDriver connects to the DevTools server exposed by that process. ChromeDriver documents the capability as a debugger server address in hostname/ip:port form, such as 127.0.0.1:38947. Selenium’s Chromium options API calls the equivalent property debugger_address and requires a string.
This is useful when another process has already prepared a page, profile, extension, cookies or network state. It is not equivalent to a fully fresh Selenium session: ChromeDriver’s automation extension is loaded at startup, and it is absent when you attach to an existing process. Commands that depend on that extension can therefore fail.
Prerequisites and a safe launch
- Chrome or Chromium installed on the machine running the browser.
- A compatible ChromeDriver/Selenium installation. Keep Chrome and ChromeDriver versions aligned, especially after browser updates.
- A separate user-data directory for the debug process. Do not point a test instance at the profile used by an ordinary Chrome process.
- A reachable debugging host and port. Keep the endpoint on a trusted interface unless you have separately secured it.
For a simple local session, use a fixed port:
chrome --headless=new
--remote-debugging-port=9222
--user-data-dir=/tmp/chrome-debug
https://example.comThe --headless=new flag selects Chrome’s current headless implementation in documentation that uses this example. Command-line behavior is version-sensitive, so verify the flags supported by the Chrome binary installed in your environment.
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Linux, macOS and Windows command details
The flag order is the same on each platform; only the executable path and data-directory syntax differ. On Windows, for example, invoke the installed chrome.exe with a Windows path such as --user-data-dir=C:tempchrome-debug. The directory must be writable and should not be shared by concurrent browser processes.
Find and verify the DevTools endpoint
A fixed port is convenient for one local session. For parallel jobs, ask Chrome to choose an available port:
chrome --headless=new --remote-debugging-port=0
--user-data-dir=/tmp/chrome-debugChrome prints a WebSocket URL when it starts. The DevTools Protocol also exposes browser information at /json/version, including webSocketDebuggerUrl. When Chrome chooses the port, it can write the selected value to a DevToolsActivePort file in the user-data directory.
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Before starting Selenium, check the endpoint from the same machine:
curl http://127.0.0.1:9222/json/versionA working response is JSON describing the browser and its WebSocket debugger URL. If the request is refused, Chrome has not started, the selected port is different, or the endpoint is bound to another interface.
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Fixed versus ephemeral ports
| Choice | Best for | Trade-off |
|---|---|---|
| Fixed port, such as 9222 | Local tutorials, one long-lived process and simple scripts | Another process may already occupy the port; parallel jobs need coordination |
| Port 0 | Workers that start many independent Chrome sessions | Your launcher must discover and pass the selected port to Selenium |
Connect with Selenium Python
Install Selenium in the environment that will run the client:
python -m pip install -U seleniumComplete example:
from selenium import webdriver
from selenium.webdriver.chrome.options import Options
from selenium.common.exceptions import WebDriverException
options = Options()
options.add_experimental_option("debuggerAddress", "127.0.0.1:9222")
try:
driver = webdriver.Chrome(options=options)
print("Title:", driver.title)
print("URL:", driver.current_url)
driver.quit()
except WebDriverException as exc:
print("Could not attach:", exc)The value must be a string containing only the debugger host and port, not an HTTP URL and not the WebSocket path. Use localhost:9222 or 127.0.0.1:9222 when Chrome runs locally. If Chrome runs in a container or another machine, use the address reachable from the Selenium process and configure the network accordingly.
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Selenium language bindings expose Chromium options differently. In Python, the portable form is add_experimental_option("debuggerAddress", "127.0.0.1:9222"). The underlying Selenium Chromium options API describes the corresponding property as debugger_address. Follow the naming used by your binding rather than copying Python syntax into Java, C# or JavaScript.
Inspect a running headless page visually
Headless Chrome has no ordinary window, but Chrome DevTools can still show its targets. In a regular Chrome window:
- Open
chrome://inspect. - Click Configure.
- Enter the headless browser’s host and port, for example
127.0.0.1:9222. - Find the target tab and click Inspect.
DevTools then provides a live page view and the usual inspection panels for the remote target. This is useful for checking whether a selector exists, watching console errors and examining network requests while Selenium remains attached.
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Programmatic inspection with CDP
Selenium Python exposes driver.execute_cdp_cmd(command, args) for Chrome DevTools Protocol operations. For example, enable network events before navigating:
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driver.get("https://example.com")
CDP commands and event behavior depend on the Chrome version and protocol domain. Use CDP for browser capabilities that Selenium does not directly expose, but keep ordinary element actions in Selenium where possible.
What may not work after attachment
ChromeDriver states that its automation extension is loaded only when ChromeDriver starts Chrome itself. An attached process does not have that startup extension. As a result, some WebDriver commands can return an error such as operation not supported when using remote debugging. ChromeDriver’s documented example is resizing the browser window.
| Need | Recommended session | Reason |
|---|---|---|
| Reuse an existing page, cookies or profile | Attach with debuggerAddress |
The separately launched browser already owns that state |
| Maximum WebDriver command coverage, including startup-extension features | Let ChromeDriver launch Chrome | The automation extension is installed during startup |
| Low-level network or performance control | Attach or launch, then use CDP where supported | CDP exposes protocol domains independently of ordinary WebDriver calls |
If your test requires a command that fails only in attached mode, remove debuggerAddress, configure the desired Chrome arguments in Options, and create a fresh webdriver.Chrome(options=options) session.
Lifecycle, concurrency and reliability
Wait for Chrome before attaching
Starting a process and immediately constructing WebDriver creates a race. Poll /json/version or the port until it responds, with a finite timeout. Do not retry forever: a bad executable path or invalid profile directory will never become ready.
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Keep ownership clear
Decide which process closes Chrome. driver.quit() ends the WebDriver connection, but the separately launched Chrome process may need to be terminated by the launcher or a process supervisor. Clean up temporary profiles after the browser exits.
Isolate parallel jobs
Each concurrent browser needs its own user-data directory and debugger port, or a reliable port-discovery mechanism. Reusing either can attach a test to the wrong tab or produce profile-lock errors.
Protect the endpoint
The debugging interface is powerful. Binding it beyond the trusted host can expose browser tabs and control capabilities. Restrict firewall and container networking rules, avoid publishing the port publicly, and use an authenticated or otherwise protected tunnel when remote access is unavoidable.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Troubleshooting attachment failures
“cannot connect to chrome” or connection refused
- Confirm Chrome is still running.
- Check
curl http://HOST:PORT/json/version. - Verify the Selenium process can reach the same host and port, especially across containers.
- Check that another process did not take the fixed port.
“debuggerAddress must be a string”
Pass a value such as "127.0.0.1:9222". Do not pass an integer, a dictionary or the full http:// URL.
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Recheck the host and port in Configure, then query /json and /json/version. A browser can expose the endpoint while having no navigable page target if the startup command exited or the tab was closed.
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“operation not supported when using remote debugging”
This indicates a command depends on ChromeDriver’s startup automation extension. Use a fresh ChromeDriver-managed session for that operation, or replace it with a supported CDP command when an equivalent exists.
Profile lock or corrupted state
Stop other Chrome processes using the directory and launch with a new, dedicated --user-data-dir. Never run parallel jobs against one normal user profile.
Version mismatch or unexpected flags
Record the Chrome and ChromeDriver versions, update the Selenium binding, and consult the current Chrome headless and ChromeDriver documentation for the installed release. Headless flags and supported behavior can change between versions.
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Frequently Asked Questions
Can I attach Selenium to a Chrome process running on another machine?
Yes, if the Selenium process can reach that machine’s DevTools host and port. Treat the debugging endpoint as privileged, restrict network exposure, and use a protected tunnel rather than publishing it openly.
Does attaching preserve the existing tab and cookies?
It connects to the already running browser session, so the existing profile and targets remain available. Use a dedicated profile when you need deterministic test state.
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Should I use port 9222 or port 0?
Use a fixed port for a single local process. Use port 0 for concurrent workers, then read the selected port from Chrome’s startup output or the DevToolsActivePort file.
Why does a fresh Selenium session support a command that an attached session rejects?
ChromeDriver loads its automation extension only when it launches Chrome. An attached browser did not receive that startup extension, so some commands are unavailable.
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