thanos-io/thanos · error

invalid scalar result format

Error message

invalid scalar result format %v, expected timestamp -> value tuple

What it means

convertScalarJSONToVector decodes a scalar/string result, which Prometheus encodes as a two-element array [timestamp, value]. This error fires when the decoded JSON array does not have exactly two elements, so it cannot be interpreted as a timestamp -> value tuple. It is deliberately checked after unmarshaling into a slice without a fixed length so the length remains observable.

Solutions

1. Inspect the raw data.result JSON to see the actual shape returned.
2. Fix the server/mock/test fixture so scalar results are exactly [<unix_ts_ms>, "<value>"] as an array.
3. Remove any proxy/middleware that mutates the response payload.
4. If the server legitimately returns vector results, ensure the query/resultType handling matches (scalar vs vector paths).

Example fix

// before: malformed scalar result
"result": []
// after: valid scalar tuple
"result": [1710000000000, "42"]

// when feeding JSON directly (tests/mocks), check shape first
var raw []json.RawMessage
json.Unmarshal(scalarJSON, &raw)
if len(raw) != 2 {
    return errors.New("scalar result must be a 2-element [timestamp, value] tuple")
}

v, err := convertScalarJSONToVector(raw)
if err != nil {
    // log raw result JSON; fix server/fixture to emit [ts, "value"]
}

When it happens

Trigger: Called from QueryInstant (or TestRule_UnmarshalScalarResponse) when data.result for a scalar response decodes to an array whose length != 2 — e.g. empty array, nested arrays, or an object where a tuple was expected.

Common situations: Nonstandard server implementations or mocks returning malformed scalar results; response rewriting middleware; tests feeding hand-crafted JSON with the wrong shape.

Related errors

• fetch Prometheus flags
• validate relabel config
• decode result into ValueTypeScalar
• unmarshaling scalar time from
• unmarshaling scalar value from

AI-assisted analysis of thanos-io/thanos@35b8b99117 (2026-09-07). Data as JSON: /api/errors/7b16ab1720336ef8. Report an issue: GitHub.

Appendix: source

Thrown at pkg/promclient/promclient.go:618

	}
	return matrixResult, m.Warnings, m.Data.Explanation, nil
}

// Scalar response consists of array with mixed types so it needs to be
// unmarshaled separately.
func convertScalarJSONToVector(scalarJSONResult json.RawMessage) (model.Vector, error) {
	var (
		// Do not specify exact length of the expected slice since JSON unmarshaling
		// would make the length fit the size and we won't be able to check the length afterwards.
		resultPointSlice []json.RawMessage
		resultTime       model.Time
		resultValue      model.SampleValue
	)
	if err := json.Unmarshal(scalarJSONResult, &resultPointSlice); err != nil {
		return nil, err
	}
	if len(resultPointSlice) != 2 {
		return nil, errors.Errorf("invalid scalar result format %v, expected timestamp -> value tuple", resultPointSlice)
	}
	if err := json.Unmarshal(resultPointSlice[0], &resultTime); err != nil {
		return nil, errors.Wrapf(err, "unmarshaling scalar time from %v", resultPointSlice)
	}
	if err := json.Unmarshal(resultPointSlice[1], &resultValue); err != nil {
		return nil, errors.Wrapf(err, "unmarshaling scalar value from %v", resultPointSlice)
	}
	return model.Vector{&model.Sample{
		Metric:    model.Metric{},
		Value:     resultValue,
		Timestamp: resultTime}}, nil
}

// AlertmanagerAlerts returns alerts from Alertmanager.
func (c *Client) AlertmanagerAlerts(ctx context.Context, base *url.URL) ([]*model.Alert, error) {
	u := *base
	u.Path = path.Join(u.Path, "/api/v1/alerts")

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