thanos-io/thanos · warning

ErrFlagEndpointNotFound

ErrFlagEndpointNotFound

Error message

no flag endpoint found

What it means

ErrFlagEndpointNotFound is returned by promclient.ConfiguredFlags when Prometheus's /flags endpoint answers 404. The client uses this sentinel to signal that the connected Prometheus does not expose the flags API (old version or flag disabled), letting callers skip flag-dependent logic instead of failing.

Solutions

1. Upgrade Prometheus to a version exposing /api/v1/status/flags (>= 2.13)
2. Compare errors with errors.Is and skip flag-dependent logic when this sentinel is returned (sidecar already does this)
3. Verify promURL points at the correct Prometheus instance
4. Check whether a proxy/ingress strips the status endpoints

Example fix

// before
if flags, flagErr = client.ConfiguredFlags(ctx, m.promURL); flagErr != nil {
    return errors.Wrapf(flagErr, "fetch Prometheus flags")
}
// after
if flags, flagErr = client.ConfiguredFlags(ctx, m.promURL); flagErr != nil && !errors.Is(flagErr, promclient.ErrFlagEndpointNotFound) {
    return errors.Wrapf(flagErr, "fetch Prometheus flags")
}

func isFlagEndpointNotFound(err error) bool {
    return errors.Is(err, promclient.ErrFlagEndpointNotFound)
}

flags, err := client.ConfiguredFlags(ctx, promURL)
if errors.Is(err, promclient.ErrFlagEndpointNotFound) {
    log.Warn("Prometheus does not expose /flags; skipping flag validation")
    return nil
}

When it happens

Trigger: ConfiguredFlags(ctx, promURL) receiving HTTP 404 from Prometheus's /api/v1/status/flags endpoint; sidecar retry loop explicitly ignores this sentinel (err != promclient.ErrFlagEndpointNotFound).

Common situations: Sidecar pointed at Prometheus < 2.13 (no /flags endpoint); monitoring scrape configs disabling the status API; misconfigured promURL pointing at a non-Prometheus service.

Understand the failure class

Background: 'Could not be found', 'does not exist', 'not found in database': the resource-not-found family when an ID, slug, key, or URI lookup comes back empty — this error's family across 20 libraries.

Related errors

• failed to validate prometheus flags
• failed to get prometheus version
• initial external labels query
• perform request against
• expected 2xx response, got

AI-assisted analysis of thanos-io/thanos@35b8b99117 (2026-09-07). Data as JSON: /api/errors/e3a2d1fd4b1b9915. Report an issue: GitHub.

Appendix: source

Thrown at pkg/promclient/promclient.go:49

	"github.com/prometheus/prometheus/model/timestamp"
	"github.com/prometheus/prometheus/promql"
	"github.com/prometheus/prometheus/promql/parser"
	"google.golang.org/grpc/codes"
	"gopkg.in/yaml.v2"

	"github.com/thanos-io/thanos/pkg/clientconfig"
	"github.com/thanos-io/thanos/pkg/exemplars/exemplarspb"
	"github.com/thanos-io/thanos/pkg/metadata/metadatapb"
	"github.com/thanos-io/thanos/pkg/rules/rulespb"
	"github.com/thanos-io/thanos/pkg/runutil"
	"github.com/thanos-io/thanos/pkg/status/statuspb"
	"github.com/thanos-io/thanos/pkg/store/storepb"
	"github.com/thanos-io/thanos/pkg/targets/targetspb"
	"github.com/thanos-io/thanos/pkg/tracing"
)

var (
	ErrFlagEndpointNotFound = errors.New("no flag endpoint found")

	statusToCode = map[int]codes.Code{
		http.StatusBadRequest:          codes.InvalidArgument,
		http.StatusNotFound:            codes.NotFound,
		http.StatusUnprocessableEntity: codes.Internal,
		http.StatusServiceUnavailable:  codes.Unavailable,
		http.StatusInternalServerError: codes.Internal,
	}
)

const (
	SUCCESS = "success"
)

// HTTPClient sends an HTTP request and returns the response.
type HTTPClient interface {
	Do(*http.Request) (*http.Response, error)
}

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