grafana/k6 · error

can't serialise request object to protocol buffer: %w

Error message

can't serialise request object to protocol buffer: %w

What it means

Stream.Send could not unmarshal the provided message bytes into a dynamic message for the stream's input type. The payload JS object was serialized and then protojson-decoded against the method descriptor; unknown fields, wrong field types, or invalid enum names that do not match the input schema cause this. The message is never put on the wire.

Source

Thrown at internal/lib/netext/grpcext/stream.go:120

	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
func (s *Stream) buildMessage(b []byte) (*dynamicpb.Message, error) {
	msg := dynamicpb.NewMessage(s.methodDescriptor.Input())
	if err := s.unmarshaler.Unmarshal(b, msg); err != nil {
		return nil, fmt.Errorf("can't serialise request object to protocol buffer: %w", err)
	}

	return msg, nil
}

// Send sends the message to the stream
func (s *Stream) Send(b []byte) error {
	msg, err := s.buildMessage(b)
	if err != nil {
		return err
	}

	return s.raw.SendMsg(msg)
}

View on GitHub (pinned to 01ffac6f24)

Solutions

1. Shape the payload to match the stream's request message schema exactly (names, nesting, types)
2. Validate enum names and required fields against the .proto definition
3. Log the serialized payload before calling send() to spot mismatches

When it happens

Trigger: Thrown at internal/lib/netext/grpcext/stream.go:120 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/13a735ae59c22e96. Report an issue: GitHub.