thanos-io/thanos · error

bad response status from

Error message

bad response status %v from %q

What it means

After a successful alert POST, postAlerts checks the response status and requires a 2xx code. Any non-2xx (404 wrong path, 401 auth, 429 rate limit, 5xx) yields this error carrying the status line and target URL.

Solutions

1. Read the status code in the message: 404 means wrong URL/path, 401/403 means configure credentials, 429 means reduce alert volume, 5xx means check Alertmanager logs.
2. Verify the alertmanager URL includes the correct path (e.g. /api/v2/alerts is handled internally) and scheme.
3. Configure the dispatcher's RoundTripper with the required auth (basic auth, TLS client certs).
4. Check target Alertmanager logs/metrics for the corresponding request failure to see why it rejected the batch.

Example fix

// before
urls: http://alertmanager:9093/alertmanager
// after (correct base URL without extra path)
urls: http://alertmanager:9093

// verify endpoint before posting
resp, err := http.Head(strings.TrimSuffix(amURL, "/") + "/-/ready")
if err != nil || resp.StatusCode/100 != 2 {
    return fmt.Errorf("alertmanager %s not healthy", amURL)
}

if err := sender.postAlerts(ctx, alerts); err != nil {
    if strings.Contains(err.Error(), "status 429") {
        // back off and retry later
    } else if strings.Contains(err.Error(), "status 404") {
        // fix URL config; do not retry
    }
    return err
}

When it happens

Trigger: The Alertmanager (or proxy in front of it) returned e.g. 404 because the URL path is wrong, 401/403 because auth is required, or 5xx because it failed to process the alert batch.

Common situations: Alertmanager behind a reverse proxy requiring basic auth or mTLS the sender doesn't provide; URL configured with wrong path prefix; target Alertmanager rejecting payloads due to limits; version mismatch between API groups.

Understand the failure class

Background: "API error: {status}" and "HTTP 401/403/404/429/5xx" errors: non-2xx HTTP responses explained — this error's family across 27 libraries.

Related errors

• send request to
• got non-200 response code
• creating request to downstream URL
• error starting web server
• http.Error(w, err.Error(), http.StatusInternalServerError)

AI-assisted analysis of thanos-io/thanos@35b8b99117 (2026-09-07). Data as JSON: /api/errors/fd23dd66ced485a5. Report an issue: GitHub.

Appendix: source

Thrown at pkg/alert/alert.go:399

func (a *Alertmanager) postAlerts(ctx context.Context, u url.URL, r io.Reader) error {
	req, err := http.NewRequest("POST", u.String(), r)
	if err != nil {
		return err
	}
	ctx, cancel := context.WithTimeout(ctx, a.timeout)
	defer cancel()
	req = req.WithContext(ctx)
	req.Header.Set("Content-Type", contentTypeJSON)

	resp, err := a.dispatcher.Do(req)
	if err != nil {
		return errors.Wrapf(err, "send request to %q", u.String())
	}
	defer runutil.ExhaustCloseWithLogOnErr(a.logger, resp.Body, "send one alert")

	if resp.StatusCode/100 != 2 {
		return errors.Errorf("bad response status %v from %q", resp.Status, u.String())
	}
	return nil
}

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