grafana/k6 · error

metric not registered %q

Error message

metric not registered %q

What it means

parseAndEmitWebVitalMetric parsed the web-vitals payload but the reported metric name (e.g. CLS, LCP, FCP, TTFID, INP) is not present in the k6Metrics.WebVitals registry. Either web-vitals emitted a new/unknown metric name or the metric registration step was skipped — an internal consistency guard.

Source

Thrown at internal/js/modules/k6/browser/common/frame_session.go:360

	wv := struct {
		ID             string
		Name           string
		Value          json.Number
		Rating         string
		Delta          json.Number
		NumEntries     json.Number
		NavigationType string
		URL            string
		SpanID         string
	}{}

	if err := json.Unmarshal([]byte(object), &wv); err != nil {
		return fmt.Errorf("json couldn't be parsed: %w", err)
	}

	metric, ok := fs.k6Metrics.WebVitals[wv.Name]
	if !ok {
		return fmt.Errorf("metric not registered %q", wv.Name)
	}

	value, err := wv.Value.Float64()
	if err != nil {
		return fmt.Errorf("value couldn't be parsed %q", wv.Value)
	}

	state := fs.vu.State()
	tags := state.Tags.GetCurrentValues().Tags
	if state.Options.SystemTags.Has(k6metrics.TagURL) {
		tags = handleURLTag(fs.page, wv.URL, http.MethodGet, tags)
	}

	tags = tags.With("rating", wv.Rating)

	now := time.Now()
	pushIfNotDone(fs.vu.Context(), fs.logger, state.Samples, k6metrics.ConnectedSamples{
		Samples: []k6metrics.Sample{

View on GitHub (pinned to 94169daab2)

Solutions

1. Update k6 so the web-vitals names and registered metrics match
2. Report to grafana/k6 with the unknown metric name from the error
3. Disable web-vitals collection if not needed, to avoid the code path

When it happens

Trigger: Thrown at internal/js/modules/k6/browser/common/frame_session.go:360 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of grafana/k6@94169daab2 (2026-08-18). Data as JSON: /api/errors/ae047581f09717fa. Report an issue: GitHub.