Unity-Technologies/UnityCsReference · error · Exception

Assembly ' ' is a CodeGen assembly and cannot be Auto…

Error message

Assembly '{name}' is a CodeGen assembly and cannot be Auto Referenced

What it means

Thrown by CustomScriptAssemblyData.ValidateFields() (CustomScriptAssembly.cs:132) when autoReferenced is true (its default) AND UnityCodeGenHelpers.IsCodeGen(name) returns true. IsCodeGen returns true for assembly names that start with "Unity." and end with either ".Compiler" or ".CodeGen" (case-insensitive). Those suffixes are reserved for Unity's own source-generator / compiler toolchain assemblies, which must never be auto-referenced into every other assembly because doing so would force all assemblies to depend on codegen infrastructure.

Solutions

1. Rename the assembly so it does not start with "Unity." combined with a ".CodeGen" / ".Compiler" suffix (the "Unity." prefix alone is not enough; both conditions must hold).
2. If the reserved name is intentional, set "autoReferenced": false in the asmdef and have dependents reference it explicitly.

Example fix

// before (.asmdef)
{
    "name": "Unity.MyGen.CodeGen",
    "autoReferenced": true
}
// after
{
    "name": "MyCompany.MyGen.CodeGen",
    "autoReferenced": false
}

// Replicate UnityCodeGenHelpers.IsCodeGen before saving an asmdef
static bool IsReservedCodeGenName(string assemblyName)
{
    var n = System.IO.Path.GetFileNameWithoutExtension(assemblyName);
    bool startsUnity = n.StartsWith("Unity.", StringComparison.OrdinalIgnoreCase);
    return startsUnity &&
           (n.EndsWith(".Compiler", StringComparison.OrdinalIgnoreCase)
            || n.EndsWith(".CodeGen", StringComparison.OrdinalIgnoreCase));
}

if (data.autoReferenced && IsReservedCodeGenName(data.name))
    throw new InvalidOperationException(
        $"'{data.name}' matches a reserved CodeGen name; set autoReferenced=false or rename.");

When it happens

Trigger: Creating an asmdef whose name matches the reserved pattern, e.g. "Unity.MyPackage.CodeGen" or "Unity.Something.Compiler", while leaving "autoReferenced": true (the JSON default). Importing a third-party package whose asmdef reuses the Unity.*.CodeGen naming.

Common situations: A package author building a Roslyn source generator package and copying Unity's internal naming convention. Renaming an existing asmdef into the reserved namespace without clearing autoReferenced.

Related errors

• Assembly cannot be have reserved name
• Assembly with name ' ' already exists
• Assembly has duplicate references
• Both 'excludePlatforms' and 'includePlatforms' are set.
• ' ' is not a valid expression

AI-assisted analysis of Unity-Technologies/UnityCsReference@225b0fbdb5 (2026-08-13). Data as JSON: /api/errors/fe6c4ff2d56df259. Report an issue: GitHub.

Appendix: source

Thrown at Editor/Mono/Scripting/ScriptCompilation/CustomScriptAssembly.cs:132

            if (assemblyData == null)
                throw new System.Exception("Json file does not contain an assembly definition");

            return assemblyData;
        }

        public void ValidateFields()
        {
            if (string.IsNullOrEmpty(name))
                throw new System.Exception("Required property 'name' not set");

            if ((excludePlatforms != null && excludePlatforms.Length > 0) &&
                (includePlatforms != null && includePlatforms.Length > 0))
                throw new System.Exception("Both 'excludePlatforms' and 'includePlatforms' are set.");

            if (autoReferenced && UnityCodeGenHelpers.IsCodeGen(name))
            {
                throw new Exception($"Assembly '{name}' is a CodeGen assembly and cannot be Auto Referenced");
            }
        }

        public static string ToJson(CustomScriptAssemblyData data)
        {
            return UnityEngine.JsonUtility.ToJson(data, true);
        }

        static void UpdateRenamedReferences(CustomScriptAssemblyData data)
        {
            if (data.references == null || data.references.Length == 0)
                return;

            HashSet<string> additionalReferences = null;

            for (int i = 0; i < data.references.Length; ++i)
            {
                var reference = data.references[i];

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