Transcoding is working for the non-error use case.

I had to configure nginx differently to enable CORS (which is not clear in the docs IMO).

I use grpc java and handle an error like this for the simple hello message:

@Override
public void sayHello(UserOuterClass.HelloRequest request, StreamObserver<UserOuterClass.HelloReply> responseObserver) {
    log.info("there is a hello request");

    if(request.getName().equals("coucou")){

        log.info("coucou is forbidden");

        responseObserver.onError(Status.NOT_FOUND.asException());
        return;
    }

    UserOuterClass.HelloReply.Builder replyBuilder = UserOuterClass.HelloReply.newBuilder();

    replyBuilder.setMessage("coucou " + request.getName());

    responseObserver.onNext(replyBuilder.build());
    responseObserver.onCompleted();
    log.info("there has been a response");
}

Now when a request is sent from chrome there is a preflight call which seems ok. But the GET itself corresponding to sayHello function above fails with a CORS issue:

Failed to load http://xxxxxx:8080/v1/hello?name=coucou: No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'http://localhost:3000' is therefore not allowed access. The response had HTTP status code 404.

If the response is ok, the cors headers are present. If it's an error like here, then the cors header are not there.

Moreover here is the response returned by endpoints:

{
 "code": 5,
 "message": "",
 "details": [
  {
   "@type": "type.googleapis.com/google.rpc.DebugInfo",
   "stackEntries": [],
   "detail": "internal"
  }
 ]
}