grafana/k6 · error

internal error while auto-attaching to browser pages: %w

Error message

internal error while auto-attaching to browser pages: %w

What it means

Returned by Browser.initEvents (during NewBrowser setup) when the CDP call target.SetAutoAttach(true, true).WithFlatten(true) fails. Auto-attach is what lets k6 receive events for new pages/targets, so its failure during setup aborts browser creation. The 'internal error' prefix signals causes outside the script's control: a broken/closing connection or a Chrome that rejects the call.

Source

Thrown at internal/js/modules/k6/browser/common/browser.go:254

				if ev, ok := event.data.(*target.EventAttachedToTarget); ok {
					b.logger.Debugf("Browser:initEvents:onAttachedToTarget", "sid:%v tid:%v", ev.SessionID, ev.TargetInfo.TargetID)
					if err := b.onAttachedToTarget(ev); err != nil {
						k6ext.Panicf(b.vuCtx, "browser is attaching to target: %w", err)
					}
				} else if ev, ok := event.data.(*target.EventDetachedFromTarget); ok {
					b.logger.Debugf("Browser:initEvents:onDetachedFromTarget", "sid:%v", ev.SessionID)
					b.onDetachedFromTarget(ev)
				} else if event.typ == EventConnectionClose {
					b.logger.Debugf("Browser:initEvents:EventConnectionClose", "")
					return
				}
			}
		}
	}()

	action := target.SetAutoAttach(true, true).WithFlatten(true)
	if err := action.Do(cdp.WithExecutor(b.vuCtx, b.conn)); err != nil {
		return fmt.Errorf("internal error while auto-attaching to browser pages: %w", err)
	}

	// Target.setAutoAttach has a bug where it does not wait for new Targets being attached.
	// However making a dummy call afterwards fixes this.
	// This can be removed after https://chromium-review.googlesource.com/c/chromium/src/+/2885888 lands in stable.
	action2 := target.GetTargetInfo()
	if _, err := action2.Do(cdp.WithExecutor(b.vuCtx, b.conn)); err != nil {
		return fmt.Errorf("internal error while getting browser target info: %w", err)
	}

	return nil
}

// connectionOnAttachedToTarget is called when Connection receives an attachedToTarget
// event. Returning false will stop the event from being processed by the connection.
func (b *Browser) connectionOnAttachedToTarget(eva *target.EventAttachedToTarget) bool {
	// This allows to attach targets to the same browser context as the current
	// one, and to the default browser context.

View on GitHub (pinned to 93accf6570)

Solutions

1. Verify the remote Chrome instance stays alive through the whole connect (no restarts/orchestrator kills)
2. Use a Chrome/Chromium version compatible with your k6 version (modern stable Chrome)
3. Raise K6_BROWSER_TIMEOUT so setup CDP calls are not cut off on slow links
4. Remove WebSocket-hostile proxies between k6 and the browser
5. Retry the launch/connect once; transient mid-setup drops often succeed on retry

Example fix

# before
K6_BROWSER_TIMEOUT=15s k6 run test.js

# after
K6_BROWSER_TIMEOUT=60s k6 run test.js

try {
  const browser = chromium.connectOverCDP(wsUrl);
} catch (e) {
  const m = String(e.message);
  if (m.includes('auto-attaching to browser pages')) {
    // setup-time CDP failure: retry with a fresh connection and a stable browser instance
    console.error('SetAutoAttach failed during setup; check browser stability and timeout:', m);
  }
  throw e;
}

When it happens

Trigger: The CDP WebSocket connection drops between connect and this call (remote Chrome restarting); Chrome version does not support flatten/auto-attach semantics k6 uses; connection closed due to timeout during setup; another debugger client detached the browser.

Common situations: Remote browser pools recycling instances mid-connect; very old Chromium builds predating flattened sessions; proxies killing the WebSocket early; rarely on local stock Chrome — if reproducible there, suspect resource starvation slowing setup past the timeout.

Related errors

• browser connect: %w
• internal error while getting browser target info: %w
• initializing networking %T: %w
• can't fetch the page for unknown reason
• errorText

AI-assisted analysis of grafana/k6@93accf6570 (2026-08-15). Data as JSON: /api/errors/7a902f4fff3b6a78. Report an issue: GitHub.