grafana/k6 · error

HTTP POST request failed: %w

Error message

HTTP POST request failed: %w

What it means

WriteClient.Store builds the snappy-compressed protobuf POST to the Prometheus remote-write endpoint; http.NewRequestWithContext failed here, which essentially only happens with an invalid/unparsable server URL in the output config. The remote-write sample batch is not sent and the output errors out.

Source

Thrown at internal/output/prometheusrw/remote/client.go:105

	}
	if c.cfg.BasicAuth != nil {
		req.SetBasicAuth(c.cfg.BasicAuth.Username, c.cfg.BasicAuth.Password)
	}

	if len(c.cfg.Headers) > 0 {
		req.Header = c.cfg.Headers.Clone()
	}

	req.Header.Set("User-Agent", "k6-prometheus-rw-output")

	// They are mostly defined by the specs
	req.Header.Set("Content-Encoding", "snappy")
	req.Header.Set("Content-Type", "application/x-protobuf")
	req.Header.Set("X-Prometheus-Remote-Write-Version", "0.1.0")

	resp, err := c.hc.Do(req) //nolint:gosec
	if err != nil {
		return fmt.Errorf("HTTP POST request failed: %w", err)
	}
	defer func() {
		err = resp.Body.Close()
		if err != nil {
			panic(err)
		}
	}()

	_, err = io.Copy(io.Discard, resp.Body)
	if err != nil {
		return err
	}

	return validateResponseStatus(resp.StatusCode)
}

func newWriteRequestBody(series []*prompb.TimeSeries) ([]byte, error) {
	b, err := proto.Marshal(&prompb.WriteRequest{

View on GitHub (pinned to 01ffac6f24)

Solutions

1. Verify the remote write URL and that Prometheus is reachable
2. Check TLS settings (K6_PROMETHEUS_RW_INSECURE_SKIP_TLS_VERIFY, CA/cert config)
3. Inspect the wrapped error for timeout or DNS details and fix network/firewall issues

When it happens

Trigger: Thrown at internal/output/prometheusrw/remote/client.go:105 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of grafana/k6@01ffac6f24 (2026-08-18). Data as JSON: /api/errors/e1df3584977e842d. Report an issue: GitHub.