grafana/k6 · error

unable to serialise request object to protocol buffer: %w

Error message

unable to serialise request object to protocol buffer: %w

What it means

protojson failed to unmarshal the client.invoke() message bytes into a dynamic message built from the method's input descriptor. Despite saying 'serialise', the failing step is decoding the request JSON: unknown fields, wrong types for proto fields, or malformed JSON that does not match the .proto schema. Raised per-call after the request has passed the descriptor and non-empty-message guards.

Source

Thrown at internal/lib/netext/grpcext/conn.go:153

	if req.MethodDescriptor == nil {
		return nil, fmt.Errorf("request method descriptor is required")
	}
	if len(req.Message) == 0 {
		return nil, fmt.Errorf("request message is required")
	}

	if req.Timeout != time.Duration(0) {
		var cancel context.CancelFunc
		ctx, cancel = context.WithTimeout(ctx, req.Timeout)
		defer cancel()
	}

	ctx = metadata.NewOutgoingContext(ctx, req.Metadata)

	reqdm := dynamicpb.NewMessage(req.MethodDescriptor.Input())
	if err := (protojson.UnmarshalOptions{Resolver: c.types}).Unmarshal(req.Message, reqdm); err != nil {
		return nil, fmt.Errorf("unable to serialise request object to protocol buffer: %w", err)
	}

	ctx = withRPCState(ctx, &rpcState{tagsAndMeta: req.TagsAndMeta})

	var resp *dynamicpb.Message
	if req.DiscardResponseMessage {
		resp = dynamicpb.NewMessage((&emptypb.Empty{}).ProtoReflect().Descriptor())
	} else {
		resp = dynamicpb.NewMessage(req.MethodDescriptor.Output())
	}

	header, trailer := metadata.New(nil), metadata.New(nil)

	copts := make([]grpc.CallOption, 0, len(opts)+2)
	copts = append(copts, opts...)
	copts = append(copts, grpc.Header(&header), grpc.Trailer(&trailer))

	err := c.raw.Invoke(ctx, req.Method, reqdm, resp, copts...)

View on GitHub (pinned to 01ffac6f24)

Solutions

1. Match the message object exactly to the method's request schema: correct field names, types, and nesting
2. Enum values must be valid names; int64 fields should be strings or numbers within range
3. Remove unknown/extra fields, or verify the service was reflected from the up-to-date proto definition

When it happens

Trigger: Thrown at internal/lib/netext/grpcext/conn.go:153 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/611085c37a9f9976. Report an issue: GitHub.