What causes "Assembly cannot be have reserved name '{loadedCustomScriptAssembly.Name}'" in Unity-Technologies/UnityCsReference?

Rename the asmdef to a value not present among predefined/framework assembly names (prefix with your company/package name).

Unity-Technologies/UnityCsReference · error · AssemblyDefinitionException

Assembly cannot be have reserved name '{loadedCustomScriptAs

Error message

Assembly cannot be have reserved name '{loadedCustomScriptAssembly.Name}'

What it means

Thrown by CheckPredefinedAssemblyNames (LoadingAssemblyDefinition.cs:316) when an asmdef's 'name' matches an entry in predefinedAssemblyNames. That set is built from EditorBuildRules.PredefinedTargetAssemblyNames (Unity's built-in/predefined assemblies) plus, depending on the target framework, the .NET Framework / .NET Standard 2.1 reference assemblies. These names are reserved so user assemblies cannot shadow or alias the framework's own assemblies.

Source

Thrown at Editor/Mono/Scripting/ScriptCompilation/LoadingAssemblyDefinition.cs:316

            Exceptions = exceptions.ToArray();
        }

        public void ClearCustomScriptAssemblies()
        {
            CustomScriptAssemblies = Array.Empty<CustomScriptAssembly>();
            CustomScriptAssemblyReferences.Clear();
        }

        static void CheckPredefinedAssemblyNames(
            HashSet<string> predefinedAssemblyNames,
            Dictionary<string, CustomScriptAssembly> assemblyLowercaseNamesLookup,
            string lowerCaseName,
            Dictionary<string, List<string>> prefixToFilePathLookup,
            ref CustomScriptAssembly loadedCustomScriptAssembly)
        {
            if (predefinedAssemblyNames.Contains(loadedCustomScriptAssembly.Name))
            {
                throw new AssemblyDefinitionException(
                    $"Assembly cannot be have reserved name '{loadedCustomScriptAssembly.Name}'",
                    loadedCustomScriptAssembly.FilePath);
            }

            if (assemblyLowercaseNamesLookup.TryGetValue(lowerCaseName, out var duplicate))
            {
                var filePaths = new[]
                {
                    loadedCustomScriptAssembly.FilePath,
                    duplicate.FilePath
                };
                var errorMsg = $"Assembly with name '{loadedCustomScriptAssembly.Name}' already exists";
                loadedCustomScriptAssembly = null; // Set to null to prevent it being added.
                throw new AssemblyDefinitionException(errorMsg, filePaths);
            }

            // Check both asmdef and asmref files.
            if (prefixToFilePathLookup.TryGetValue(loadedCustomScriptAssembly.PathPrefix,

View on GitHub (pinned to 225b0fbdb5)

Solutions

Rename the asmdef to a value not present among predefined/framework assembly names (prefix with your company/package name).
If the name was chosen to 'replace' a built-in assembly, that is unsupported; instead reference the built-in assembly and extend it.

Example fix

// before (.asmdef)
{ "name": "System" }
// after
{ "name": "MyCompany.SystemExtensions" }

Defensive patterns

Strategy: validation

Validate before calling

// Reject reserved/framework assembly names before importing
static readonly HashSet<string> Reserved = new HashSet<string>(StringComparer.OrdinalIgnoreCase)
{
    "UnityEngine","UnityEditor","mscorlib","System","System.Core","System.Xml",
    "netstandard","System.Net.Http"
};

if (Reserved.Contains(data.name))
    throw new InvalidOperationException(
        $"'{data.name}' is a reserved/framework assembly name; rename it.");

Prevention

Prefix asmdef names with a company/package namespace.
Never name an asmdef after a BCL or built-in Unity assembly.
Cross-check candidate names against EditorBuildRules.PredefinedTargetAssemblyNames when authoring tooling.

When it happens

Trigger: Naming an asmdef "UnityEngine", "UnityEditor", "mscorlib", "System", "System.Core", "netstandard", or any other predefined/framework assembly name.

Common situations: Accidentally naming a wrapper assembly after the framework it wraps. Package naming collisions with BCL assemblies. Copying internal Unity assembly names into a project.

Related errors

AI-assisted analysis of Unity-Technologies/UnityCsReference@225b0fbdb5 (2026-08-13). Data as JSON: /api/errors/e5c6bf52f623b8ab. Report an issue: GitHub.