How do you design a consistent error model across multiple Go microservices?

// Shared proto for rich error details (google.rpc.Status)
// Add error_details.proto to your project

import (
    spb "google.golang.org/genproto/googleapis/rpc/status"
    "google.golang.org/grpc/status"
    errdetails "google.golang.org/genproto/googleapis/rpc/errdetails"
)

// Return rich error from gRPC handler
func (s *userServiceServer) CreateUser(
    ctx context.Context, req *pb.CreateUserRequest,
) (*pb.User, error) {
    if req.Email == "" {
        // Rich validation error with field-level detail
        st := status.New(codes.InvalidArgument, "validation failed")
        detail := &errdetails.BadRequest{
            FieldViolations: []*errdetails.BadRequest_FieldViolation{
                {Field: "email", Description: "email is required"},
            },
        }
        st, _ = st.WithDetails(detail)
        return nil, st.Err()
    }

    user, err := s.repo.Save(ctx, &User{Email: req.Email})
    if err != nil {
        if errors.Is(err, ErrDuplicate) {
            st := status.New(codes.AlreadyExists, "email already registered")
            info := &errdetails.ErrorInfo{
                Reason: "EMAIL_ALREADY_EXISTS",
                Domain: "user.service",
            }
            st, _ = st.WithDetails(info)
            return nil, st.Err()
        }
        return nil, status.Errorf(codes.Internal, "internal error")
    }
    return toProto(user), nil
}

// Client: extract rich error details
_, err := client.CreateUser(ctx, req)
if err != nil {
    st := status.Convert(err)
    for _, detail := range st.Details() {
        switch d := detail.(type) {
        case *errdetails.BadRequest:
            for _, v := range d.FieldViolations {
                log.Printf("field %s: %s", v.Field, v.Description)
            }
        }
    }
}