WEBDRIVER_BIDI_WEBSOCKET_ENDPOINT_REGEX matches a Puppeteer browser-process output line that begins exactly WebDriver BiDi listening on , then captures a suffix beginning with ws://. When Puppeteer’s launcher receives that captured base endpoint, it appends /session before opening the BiDi WebSocket connection. The regex is an internal implementation detail, so check the source for your installed Puppeteer version when debugging a release.
What does WEBDRIVER_BIDI_WEBSOCKET_ENDPOINT_REGEX match?
The current Puppeteer main source defines it as:
export const WEBDRIVER_BIDI_WEBSOCKET_ENDPOINT_REGEX =
/^WebDriver BiDi listening on (ws://.*)$/;It is anchored to the entire output line. The literal prefix, including its final space, must appear at the beginning, and the line must end after the captured text. There are no flags. Puppeteer’s process helper tests output lines against the expression and resolves with capture group 1.
| Part | Meaning |
|---|---|
^ |
Start of the line. |
WebDriver BiDi listening on |
The exact required label and trailing space. |
(ws://.*) |
Capture group 1: text beginning with ws://, followed by any remaining characters on the line. |
$ |
End of the line. |
For example, a line such as WebDriver BiDi listening on ws://127.0.0.1:9222 matches, and group 1 is ws://127.0.0.1:9222. This expression checks the required prefix and captures the suffix; it is not a full URL validator. Because it specifically requires ws://, wss:// does not match. The greedy .* accepts the rest of the line, including trailing text if present.
Why does Puppeteer append /session?
The regex only extracts the base WebSocket endpoint from the browser’s process output. Puppeteer’s createBiDiBrowser flow takes that captured value, appends /session, and passes the resulting URL to its WebSocket transport and BiDi connection flow. The regex itself neither adds nor interprets the session path.
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This aligns with the WebDriver BiDi specification’s construction of a WebSocket URI from the listener details and a resource name; for a null session, the resource name is /session. See the W3C WebDriver BiDi specification and Puppeteer’s BrowserLauncher implementation.
Why is Puppeteer waiting for “WebDriver BiDi listening on”?
In the launch-time discovery path, Puppeteer waits for a browser-process output line that satisfies this exact expression. If no line matches, the helper cannot resolve the endpoint from that output. The expected label is therefore significant: a different prefix, a missing space, or an endpoint beginning with wss:// will not match this regex as written.
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This does not mean every Puppeteer WebSocket connection uses this expression. The regex belongs to a process-output launch path. Puppeteer.connect() can instead use a supplied browserWSEndpoint; consult the ConnectOptions reference for that API.
How does this relate to Puppeteer’s browser protocol?
Puppeteer documents WebDriver BiDi automation support for Chrome and Firefox. Its current documentation says Firefox uses BiDi by default when launched, while Chrome uses CDP by default because not all CDP features are supported over BiDi. Chrome can opt into BiDi with protocol: 'webDriverBiDi'. Some Puppeteer APIs remain unsupported over BiDi and may raise UnsupportedOperation; the documented support table includes CDP-specific APIs such as Page.createCDPSession() and selected emulation, coverage, tracing, accessibility, page, and network methods. Check the support list for your version before switching protocols: Puppeteer WebDriver BiDi support.
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Keep the paths distinct while diagnosing a problem: launch-time endpoint discovery parses process output, whereas explicit connection uses connection options. The documented BiDi capability handling for session.new applies to protocol="webDriverBiDi" and Puppeteer.connect(); it does not make this launch regex a universal endpoint parser.
Troubleshooting a launch that does not find the endpoint
- Check the exact output line. It must begin with
WebDriver BiDi listening onfollowed byws://. Compare the raw browser output with the expected label and spacing. - Check what the regex captures. The endpoint is capture group 1, not the complete log line. The launcher adds
/sessionafterward. - Confirm you are debugging the launch path. A caller-supplied
browserWSEndpointuses the connect path, not necessarily this process-output matcher. - Verify protocol and API compatibility. Firefox’s and Chrome’s documented defaults differ, and BiDi does not support every CDP API. Check Puppeteer’s current support table before enabling BiDi.
- Inspect the installed-version source. The cited repository paths point to the mutable
mainbranch. Source behavior can differ from a released package or browser build; compare the implementation with the version actually installed.
The relevant implementation references are the @puppeteer/browsers launch helper, Puppeteer’s BrowserLauncher, and the browsers API index.
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