grafana/k6 · error

encoding metrics as Protobuf write request failed: %w

Error message

encoding metrics as Protobuf write request failed: %w

What it means

In newRequestBody, proto.Marshal failed to serialize the pbcloud.MetricSet into a Protobuf wire-format message before it is Snappy-compressed and sent to the cloud. This indicates the in-memory MetricSet is malformed or contains a field the marshaller rejects — an internal serialization failure, not a network problem.

Source

Thrown at output/cloud/expv2/metrics_client.go:96

		return io.NopCloser(bytes.NewReader(b)), nil
	}

	req.Header.Set("Content-Type", "application/x-protobuf")
	req.Header.Set("Content-Encoding", "snappy")
	req.Header.Set("K6-Metrics-Protocol-Version", "2.0")

	err = mc.httpClient.Do(req, nil)
	if err != nil {
		return err
	}

	return nil
}

func newRequestBody(data *pbcloud.MetricSet) ([]byte, error) {
	b, err := proto.Marshal(data)
	if err != nil {
		return nil, fmt.Errorf("encoding metrics as Protobuf write request failed: %w", err)
	}
	// TODO: use the framing format
	// https://github.com/google/snappy/blob/main/framing_format.txt
	// It can be done replacing the encode with
	// https://pkg.go.dev/github.com/klauspost/compress/snappy#NewBufferedWriter
	if snappy.MaxEncodedLen(len(b)) < 0 {
		return nil, fmt.Errorf("the Protobuf message is too large to be handled by Snappy encoder; "+
			"size: %d, limit: %d", len(b), 0xffffffff)
	}
	return snappy.Encode(nil, b), nil
}

View on GitHub (pinned to 01ffac6f24)

Solutions

1. Update k6 to the latest version in case of a serialization regression
2. Check for mixed k6 versions between the binary and any custom builds/extensions
3. Report the issue to grafana/k6 with the error details if it persists

When it happens

Trigger: Thrown at output/cloud/expv2/metrics_client.go:96 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/21ca179e9a58eb5a. Report an issue: GitHub.