Cannot render boxes from WSTP C++

Question

I am attempting to send text from a WSTP C++ backend to the Mathematica kernel which should be parsed and rendered as boxes.

The goal is for the user to see the outputs of things like Echo as:

Frustratingly, when text like this is sent as a packet through WSTP, it does not render, and hides anything else in the string:

I construct the string representation of the boxes in C++ through raw string literals (having also tried explicit character escaping):

std::string x = R"delimiter(\!\(\*SubscriptBox[A,0]\))delimiter"

which, by writing to file, I have confirmed produces precisely the input to the Echo in my first example.

Yet when I send Echo[x] to the kernel through

WSPutFunction(stdlink, "EvaluatePacket", 1);
WSPutFunction(stdlink, "Echo", 1);
WSPutString(stdlink, x.c_str());
WSEndPacket(stdlink);

it doesn't render. What am I doing wrong?

Note that I must embed the boxes into larger strings. Ergo something like this workaround, letting the kernel generate the box strings, is not acceptable:

std::string expr = "Subscript[A,0]";

WSPutFunction(stdlink, "EvaluatePacket", 1);
WSPutFunction(stdlink, "Echo", 1);

// send StringJoin[..., ToString @ StandardForm @ ToExpression @ expr]
WSPutFunction(stdlink, "StringJoin", 2);
    WSPutString(stdlink, "message prefix or something: ");
    WSPutFunction(stdlink, "ToString", 1);
        WSPutFunction(stdlink, "StandardForm", 1);
            WSPutFunction(stdlink, "ToExpression", 1);
                WSPutString(stdlink, expr.c_str());

Answer 1

A compromise is to let the kernel generate the boxes string and pass it back to the WSTP process, for concatenation with other strings as needed.

std::string local_getStandardFormFromMMA(std::string expr) {
    
    WSPutFunction(stdlink, "EvaluatePacket", 1);
    WSPutFunction(stdlink, "ToString", 1);
    WSPutFunction(stdlink, "StandardForm", 1);
    WSPutFunction(stdlink, "ToExpression", 1);
    WSPutString(stdlink, expr.c_str());
    WSEndPacket(stdlink);
    
    // I like to live dangerously
    long success;
    WSCheckFunction(stdlink, "ReturnPacket", &success);
    
    const char* response;
    WSGetString(stdlink, &response);
    std::string ret = std::string(response);
    WSReleaseString(stdlink, response);
    return ret;
}

Needlessly communicating with the kernel to obtain a string is still a small evil I'd like to avoid, so I will keep this question open.