Unity-Technologies/UnityCsReference · error · Exception

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

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.

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];

View on GitHub (pinned to 225b0fbdb5)

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 '{loadedCustomScriptAs
• Assembly with name '{loadedCustomScriptAssembly.Name}' alrea
• Both 'excludePlatforms' and 'includePlatforms' are set.
• Invalid Define Constraint: "{DefineConstraints[i]}" at line
• Platform name '{0}' not supported. Supported platform names:

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