Spawning Dynamically Created Object (Network)

Hi all,

I'm having trouble spawning dynamically created objects onto the server/client. I know that my issue is related to the registration of the "prefab" (ClientScene.RegisterPrefab) but I can't figure out how to solve it. I've seen a couple of questions and answers on this topic, but they don't seem to help.

At the moment I've got this:

 [Command]
     void Cmd_SpawnAnObject()
     {
          GameObject spawned_object = GameObject.CreatePrimitive(PrimitiveType.Cube);
         spawned_object.AddComponent<NetworkIdentity>();
         spawned_object.AddComponent<NetworkTransform>();
         ClientScene.RegisterPrefab(spawned_object, NetworkHash128.Parse(spawned_object.name));
         NetworkServer.Spawn(spawned_object);
     }

This successfully creates the object on the server, but raises a Failed to spawn server object, assetId=0000000000000000000000000000c0be netId=2 error on the client. My understanding is that calling ClientScene.RegisterPrefab in the CMD doesn't make sense because that's running on the server, right? So it can't get to the ClientScene.

I've tried sending the GameObject back to the client via RPC and registering that before Spawn runs but it looks like that doesn't work. E.g

 [Command]
     void Cmd_SpawnAnObject()
     {
         GameObject spawned_object = GameObject.CreatePrimitive(PrimitiveType.Cube);
         spawned_object.AddComponent<NetworkIdentity>();
         spawned_object.AddComponent<NetworkTransform>();
         Rpc_Register(spawned_object);
         NetworkServer.Spawn(spawned_object);
     }
 
     [ClientRpc]
     void Rpc_Register(GameObject go)
     {
         ClientScene.RegisterPrefab(go, NetworkHash128.Parse(go.name));
     }

I also tried creating the prefab on the client and then sending it to the command, but then I end up with the correct object on the client and none on the server. What I think would solve it is creating the prefab on the client but not an instance and then sending that "blueprint" to the server to spawn it, but I don't know if that's possible.

Everything works fine if I register these in the GUI etc, but the whole point is that I don't know what these objects are until some point during runtime (they're dynamically loaded in from another service). Am I just missing something simple?

Thanks!