In case anyone runs into the same thing or even knows how to fix it, I'm posting the contents of my bug report here. I was also having trouble getting the report to submit, so put the repro case on github: https://github.com/angusmf/Test_HLAPI_Package 1. What happened Attempting to use HLAPI package. Can build a project that uses the components within (NetworkManager and NetworkIdentity for example) with no problem. Adding a script that inherits from one of the classes in the HLAPI will ALWAYS produce the following error when building the project: Copying assembly from 'Temp/com.unity.multiplayer-hlapi.Runtime.dll' to 'Library/ScriptAssemblies/com.unity.multiplayer-hlapi.Runtime.dll' failed This happens no matter how the package is added ("normal" reference-by-version from unity package servers, local FS, or git repo) and happens on two Windows system I've tried. 2. How we can reproduce it using the example you attached Build the project. The first build may succeed, but you will get the error and any further builds will fail immediately. The NetworkBehaviour named "NetBehaviour" causes this error. You can get the build to run by changing it to a MonoBehaviour and reloading the project.