example.go

// example.proto

service Users {
  rpc GetUser (GetUserRequest) returns (GetUserResponse) {}
}

message GetUserResponse {
  User user = 1;
  Error error = 2;
}

// endpoints.go

func makeGetUserEndpoint(userService user.Service) endpoint.Endpoint {
    return func(ctx context.Context, request interface{}) (interface{}, error) {
        req := request.(*GetUserRequest)
        user, err := userService.GetUser(req.User)
        if err != nil {
            return &GetUserResponse{Error: err.(*Error)}, nil
        }
        return &GetUserResponse{User: user}, nil
    }
}

// transport.go

r.Methods("GET").Handle("/users/{id}", httptransport.NewServer(
    endpoints.GetUserEndpoint,
    decodeGetUserRequest,
    encodeResponse,
    options...,
))

type errorer interface {
    GetError() *example.Error
}

func encodeResponse(ctx context.Context, w http.ResponseWriter, response interface{}) error {
    if e, ok := response.(errorer); ok {
        if err := e.GetError(); err != nil {
            w.WriteHeader(int(err.Code))
        }
    }
    w.Header().Set("Content-Type", "application/json; charset=utf-8")
    return json.NewEncoder(w).Encode(response)
}