For new Selenium projects, use WebDriver BiDi: enable BiDi in the browser options, register a network listener before the action under test, then navigate or click and record the events. BiDi can observe requests and responses across browsers that implement the relevant APIs. Use Selenium’s CDP integration when you specifically target Chrome or Edge and need an existing Chromium-specific interceptor, but pin compatible browser and Selenium versions because CDP is version-sensitive.
Decide what “capture” means
Network capture can describe three different jobs. Choose one before writing the test:
- Observe: receive lifecycle events such as an outgoing request, response start, response completion, or a fetch error without changing traffic.
- Intercept a request: inspect or change outgoing headers or the request body, or block the request.
- Intercept a response: inspect or change response headers, status, or content before the page receives it.
Observation is appropriate for logging, assertions, and diagnostics. Interception changes the path being tested, so use it only when the test intentionally needs mocked, blocked, or modified traffic.
Use WebDriver BiDi for a new implementation
WebDriver BiDi is Selenium’s standards-based, bidirectional protocol. It adds a WebSocket through which the driver can receive browser events, including network events. Selenium positions BiDi as the forward-looking replacement for temporary, browser-specific CDP integrations. Actual event support still depends on the browser, driver, binding, and Selenium release.
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Prerequisites
- Install a current Selenium binding and a browser/driver combination that supports the BiDi features you need.
- Enable BiDi in the binding’s browser options before creating the driver. In Python, the option is exposed as
options.enable_bidi = Truein current Selenium releases. - Register listeners before navigation or the click that generates the traffic. Event streams are not a substitute for a request log captured after the fact; an event emitted before subscription can be missed.
- Record the Selenium and browser versions with your test results. BiDi method names and callback signatures vary by language binding and release.
Python: observe request and response lifecycle events
The following example targets the Selenium Python 4.43.0 API shape. It enables BiDi, attaches handlers for the four network event types documented by that release, performs an action, and prints useful fields. If your installed binding exposes the same methods under a slightly different module path, use its generated API reference rather than silently mixing examples from another release.
import json
from selenium import webdriver
from selenium.webdriver.chrome.options import Options
from selenium.webdriver.common.bidi.modules.network import NetworkEvent
options = Options()
options.enable_bidi = True
driver = webdriver.Chrome(options=options)
# Keep handlers registered before the action that creates requests.
def before_request(event):
request = event.get("request", event)
print(json.dumps({
"event": "beforeRequestSent",
"request_id": request.get("request", request.get("requestId")),
"url": request.get("url"),
"method": request.get("method"),
"headers": request.get("headers"),
}, default=str))
def response_started(event):
response = event.get("response", event)
print(json.dumps({
"event": "responseStarted",
"request_id": response.get("request"),
"status": response.get("status"),
"url": response.get("url"),
"headers": response.get("headers"),
}, default=str))
def response_completed(event):
response = event.get("response", event)
print(json.dumps({
"event": "responseCompleted",
"request_id": response.get("request"),
"status": response.get("status"),
"url": response.get("url"),
}, default=str))
def fetch_error(event):
print(json.dumps({"event": "fetchError", "details": event}, default=str))
try:
# The Python 4.43.0 network API accepts an event, callback,
# optional URL patterns, and optional browsing contexts.
driver.network.add_request_handler(
NetworkEvent.BEFORE_REQUEST_SENT, before_request)
driver.network.add_request_handler(
NetworkEvent.RESPONSE_STARTED, response_started)
driver.network.add_request_handler(
NetworkEvent.RESPONSE_COMPLETED, response_completed)
driver.network.add_request_handler(
NetworkEvent.FETCH_ERROR, fetch_error)
driver.get("https://example.com/")
driver.find_element("tag name", "body").click()
finally:
driver.quit()Event objects are binding-defined structures. Some releases provide typed event objects rather than dictionaries, and asynchronous bindings may require an async callback. Treat the event names and registration call as the 4.43.0 reference point, then check the API reference for your installed version. A handler can also receive URL patterns or browsing-context filters when you need to reduce noise.
Capture only the URLs you care about
Filtering at subscription time is preferable to collecting every image, stylesheet, and analytics request. Supply URL patterns where your binding supports them, for example an API hostname or path pattern. If your binding does not expose pattern filtering, filter in the callback and keep the original URL, method, status, and request identifier so related events can be joined later.
def api_request(event):
request = event.get("request", event)
url = request.get("url", "")
if "/api/" not in url:
return
print(request.get("method"), url, request.get("headers"))
# Use the optional pattern argument when supported by your release:
# driver.network.add_request_handler(
# NetworkEvent.BEFORE_REQUEST_SENT, api_request,
# url_patterns=["*://example.com/api/*"])Correlate a request with its response
Store events by their request identifier. A single page action can create redirects, preflight requests, retries, and parallel resources, so URL alone is not a reliable key. Keep a record containing method, URL, timestamps, status, headers, and any error. A completed response is not guaranteed: a blocked request, navigation cancellation, or network failure may produce a fetch-error event instead.
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Use a request handler when the test must change outgoing traffic. Typical examples include adding a deterministic authorization header, replacing a request body, or blocking a third-party URL. Use a response handler when the test must inspect or alter an incoming status, header, or body. The exact continuation and mutation methods differ between Selenium bindings, so do not copy a JavaScript, Java, and Python handler interchangeably.
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- Enable BiDi in browser options.
- Register the request or response handler before navigation.
- Limit the handler to URL patterns or a browsing context where possible.
- Explicitly continue unmodified traffic; an interception that never resumes can make the page hang.
- Remove the handler after the test or close the driver so it cannot affect another case.
Interception is not passive capture. Assertions made after a modified request describe the modified scenario, not the application’s unaltered behavior. Keep separate tests for observation and mocking.
When CDP is the practical fallback
Selenium exposes Chrome DevTools Protocol functionality and documents a NetworkInterceptor example. CDP is appropriate when your test is intentionally Chrome- or Edge-only, when an existing Chromium interceptor already does the job, or when the BiDi feature you need is not implemented by your browser yet.
- Browser reach: BiDi is the cross-browser direction; CDP is tied to Chromium’s DevTools protocol.
- Maintenance: Selenium describes CDP access as temporary while BiDi is implemented and warns that support depends heavily on browser version.
- Migration: Pin Selenium, the browser, and the driver together, and expect CDP code to need changes as versions move.
Do not select CDP merely because a snippet is older or easier to find. For a new cross-browser test, start with BiDi and use CDP only for a documented compatibility reason.
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Make captures useful in real tests
Register before the trigger
Create the driver, enable the protocol, and register handlers before get(), clicking a login button, submitting a form, or opening a new tab. If you attach after the action, you may see only later requests.
Keep the log bounded
Write newline-delimited JSON or a structured test attachment instead of printing entire bodies to standard output. Filter static assets, redact authorization and cookie headers, and cap body size. Network logs can contain passwords, session tokens, personal data, and proprietary responses.
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Allow asynchronous delivery
Event delivery is asynchronous. Wait for the specific request or response your assertion needs rather than sleeping for an arbitrary number of seconds. A robust wait checks request URL and method, then status or error, with a timeout that fails the test clearly.
Account for browser behavior
- CORS can create an OPTIONS preflight before the actual request.
- Service workers may satisfy a request without a normal network trip.
- Redirects create multiple lifecycle records.
- Cached resources may produce fewer network events than a cold profile.
- Opening a new tab or frame may require a browsing-context filter or separate correlation key.
- Headless and headed modes can differ in timing, caching, and browser feature support.
Troubleshooting
No events appear
Confirm that BiDi was enabled before driver creation, the browser and driver support the requested event, and the listener was registered before navigation. Verify that your callback is still alive and that an overly narrow URL pattern is not excluding the request. Try a simple uncached page and log every event temporarily.
The code fails with an unknown option or method
Your Selenium binding or release has a different API surface. Check the installed version, consult that binding’s BiDi network reference, and do not assume Python 4.43.0 names apply to Java, JavaScript, .NET, or an older Python package.
Only some requests are captured
Check for preflight, redirects, service-worker responses, cache hits, and requests from another browsing context. Remove filters, record request identifiers, and compare the event sequence with the browser’s own network panel while diagnosing.
The page hangs after interception
The handler probably did not continue the request or response on every code path. Add an explicit pass-through branch, keep mutation logic small, and remove the interceptor after the assertion.
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CDP works locally but fails in CI
Align browser, driver, and Selenium versions in the CI image. A CDP domain or command can change with the browser version. If the required capability exists in BiDi, migrate instead of adding more version-specific branching.
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If your actual deliverable is a clean visual capture rather than request-level assertions, ScreenshotNeo provides a single HTTP request. It is a screenshot API and MCP server, not a replacement for Selenium network telemetry: use Selenium when you must inspect or modify traffic. ScreenshotNeo is useful when you need a page image or PDF without maintaining browser startup and cleanup.
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FAQ
Does Selenium save a HAR file automatically?
Not from the BiDi event subscription alone. Build the HAR-like structure your test needs from request and response events, or use a separate proxy when a standards-compliant HAR export is a requirement.
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Browser automation receives network metadata after the browser processes TLS; it does not require you to decrypt traffic with a separate man-in-the-middle proxy. The exact response-body and certificate details available depend on the browser and binding implementation.
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Should I use performance logs instead?
Performance-log techniques are Chromium-specific and provide a different, older integration surface. For a new cross-browser design, prefer BiDi; retain an existing performance-log or CDP implementation only when its browser constraints are acceptable.
Frequently Asked Questions
Does Selenium save a HAR file automatically?
Not from BiDi subscriptions alone. Assemble the records you need from events or use a dedicated proxy when HAR export is required.
Can I capture HTTPS requests?
The browser exposes supported network metadata after TLS processing; available fields vary by browser and Selenium binding.
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Performance logs are Chromium-specific. Prefer BiDi for new cross-browser work, keeping older integrations only when their constraints are acceptable.
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