Unity-Technologies/UnityCsReference · error · AssemblyDefinitionException

Assembly with name '{loadedCustomScriptAssembly.Name}' alrea

Error message

Assembly with name '{loadedCustomScriptAssembly.Name}' already exists

What it means

Thrown by CheckPredefinedAssemblyNames (LoadingAssemblyDefinition.cs:330) when the lowercased form of a newly loaded assembly's name already exists in assemblyLowercaseNamesLookup, i.e. another asmdef already registered that name. Name uniqueness is enforced case-insensitively because assembly resolution on case-insensitive filesystems must be deterministic. The message reports both colliding file paths.

Source

Thrown at Editor/Mono/Scripting/ScriptCompilation/LoadingAssemblyDefinition.cs:330

            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,
                out var duplicateFilePaths))
            {
                var filePaths = new List<string> {loadedCustomScriptAssembly.FilePath};
                filePaths.AddRange(duplicateFilePaths);

                throw new AssemblyDefinitionException(
                    $"Folder '{loadedCustomScriptAssembly.PathPrefix}' contains multiple assembly definition files",
                    filePaths.ToArray());
            }
        }

        public static CustomScriptAssembly LoadCustomScriptAssemblyFromJson(string path, string json, string guid)
        {
            try

View on GitHub (pinned to 225b0fbdb5)

Solutions

1. Rename one of the two asmdefs so both names are unique (case-insensitively), prefixing with a namespace/package identifier.
2. If the duplicate is unintentional, delete the stray .asmdef file.

Example fix

// before: Assets/A/A.asmdef { "name": "Shared" }
//         Assets/B/B.asmdef { "name": "shared" }
// after: Assets/B/B.asmdef { "name": "Shared.B" }

// Enforce case-insensitive name uniqueness across all asmdefs
var byLowerName = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);
foreach (var (path, name) in allAsmdefPathsWithNames)
{
    if (byLowerName.TryGetValue(name.ToLowerInvariant(), out var existing))
        throw new InvalidOperationException(
            $"Duplicate assembly name '{name}' in {path} (already in {existing})");
    byLowerName[name.ToLowerInvariant()] = path;
}

When it happens

Trigger: Two .asmdef files whose 'name' fields match ignoring case, e.g. "MyLib" and "mylib". Copying an asmdef to a new folder to fork it without renaming.

Common situations: Duplicating an asmdef to create a variant and forgetting to rename. Cross-platform casing differences. Package A and Package B both shipping an asmdef with the same generic name.

Related errors

• Assembly cannot be have reserved name '{loadedCustomScriptAs
• Folder '{loadedCustomScriptAssembly.PathPrefix}' contains mu
• Both 'excludePlatforms' and 'includePlatforms' are set.
• Assembly '{name}' is a CodeGen assembly and cannot be Auto R
• Invalid Define Constraint: "{DefineConstraints[i]}" at line

AI-assisted analysis of Unity-Technologies/UnityCsReference@225b0fbdb5 (2026-08-13). Data as JSON: /api/errors/23fd328084abeaf7. Report an issue: GitHub.