grafana/k6 · error

failed to marshal the message: %w

Error message

failed to marshal the message: %w

What it means

protojson failed to marshal a dynamic response message back to JSON during conversion. k6 converts dynamic protobuf messages to plain Go values via a JSON round-trip so JS property access works; this fires when the message contains data protojson refuses to serialize (e.g. google.protobuf.Any with an unresolvable type or invalid UTF-8 in a string field).

Source

Thrown at internal/lib/netext/grpcext/stream.go:98

//
//	message Point {
//	   double x = 1;
//		  double y = 2;
//		  double z = 3;
//	}
//
// and a value like this:
// msg := Point{X: 6, Y: 4, Z: 0}
// would result in JSON output:
// {"x":6,"y":4}
// rather than the desired:
// {"x":6,"y":4,"z":0}
func convert(marshaler protojson.MarshalOptions, msg *dynamicpb.Message) (any, error) {
	// TODO(olegbespalov): add the test that checks that message is not nil

	raw, err := marshaler.Marshal(msg)
	if err != nil {
		return nil, fmt.Errorf("failed to marshal the message: %w", err)
	}

	var back any

	err = json.Unmarshal(raw, &back)
	if err != nil {
		return nil, fmt.Errorf("failed to unmarshal the message: %w", err)
	}

	return back, err
}

// CloseSend closes the stream
func (s *Stream) CloseSend() error {
	return s.raw.CloseSend()
}

// BuildMessage builds a message from the input

View on GitHub (pinned to 01ffac6f24)

Solutions

1. Check that response string fields contain valid UTF-8 and Any fields reference resolvable types
2. Ensure the client's type resolver knows all message types involved (use reflection from the live server or a complete proto bundle)
3. Capture the actual response server-side and validate it against the schema

When it happens

Trigger: Thrown at internal/lib/netext/grpcext/stream.go:98 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/139cd196c5eec0c6. Report an issue: GitHub.