Unity-Technologies/UnityCsReference · error · AssemblyDefinitionException

Folder ' ' contains multiple assembly definition files

Error message

Folder '{loadedCustomScriptAssemblyReference.PathPrefix}' contains multiple assembly definition files

What it means

Thrown during assembly-reference graph loading (LoadingAssemblyDefinition.cs:252) when a loaded asmref's PathPrefix collides with one already registered in prefixToFilePathLookup. PathPrefix is the folder path that 'owns' an assembly definition. Unity requires that a given folder prefix hold at most one assembly-definition file across both .asmdef and .asmref types, so it can deterministically map a script's folder to a single assembly.

Solutions

1. Move the new .asmref (or the duplicate .asmdef) into its own subfolder so its PathPrefix no longer collides.
2. Delete the redundant assembly-definition file if it was created by mistake.
3. Confirm the asmref's 'reference' field points at the intended target asmdef elsewhere rather than implying co-location.

Example fix

// before: both Assets/MyLib/A.asmdef and Assets/MyLib/B.asmref exist
// after: move the asmref into a subfolder
//   Assets/MyLib/A.asmdef
//   Assets/MyLib/Editor/B.asmref

// Enforce one assembly-definition file per folder prefix before reimport
static string PrefixOf(string asmdefPath) =>
    System.IO.Path.GetDirectoryName(asmdefPath)?.Replace('\\', '/');

var byPrefix = new Dictionary<string, List<string>>(StringComparer.OrdinalIgnoreCase);
foreach (var f in allAsmdefAndAsmrefPaths)
    (byPrefix[PrefixOf(f)] ??= new List<string>()).Add(f);

var collisions = byPrefix.Where(kv => kv.Value.Count > 1).ToList();
if (collisions.Any())
    throw new InvalidOperationException(
        "Multiple assembly-definition files in folder(s): " +
        string.Join(", ", collisions.Select(c => c.Key)));

When it happens

Trigger: Adding an .asmref file into a folder whose prefix already maps to an .asmdef (or another .asmref). Two assembly-definition files resolving to the same folder prefix.

Common situations: Placing an asmref next to an existing asmdef to 'extend' it instead of putting it in a subfolder. Copying an asmdef + asmref pair into a folder that already has one.

Related errors

• Folder ' ' contains multiple assembly definition files
• Assembly cannot be have reserved name
• Assembly has duplicate references
• Assembly with name ' ' already exists
• Both 'excludePlatforms' and 'includePlatforms' are set.

AI-assisted analysis of Unity-Technologies/UnityCsReference@225b0fbdb5 (2026-08-13). Data as JSON: /api/errors/35e121ca90aa43af. Report an issue: GitHub.

Appendix: source

Thrown at Editor/Mono/Scripting/ScriptCompilation/LoadingAssemblyDefinition.cs:252

                    if (contents != null)
                    {
                        var jsonContents = contents[i];
                        loadedCustomScriptAssemblyReference = LoadCustomScriptAssemblyReferenceFromJson(fullPath, jsonContents);
                    }
                    else
                    {
                        loadedCustomScriptAssemblyReference = LoadCustomScriptAssemblyReferenceFromJsonPath(fullPath);
                    }

                    if (!m_SkipCustomScriptAssemblyGraphValidation)
                    {
                        // Check both asmdef and asmref files.
                        if (prefixToFilePathLookup.TryGetValue(loadedCustomScriptAssemblyReference.PathPrefix, out var duplicateFilePaths))
                        {
                            var filePaths = new List<string> {loadedCustomScriptAssemblyReference.FilePath};
                            filePaths.AddRange(duplicateFilePaths);

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

                    // Convert GUID references to assembly names
                    if (GUIDReference.IsGUIDReference(loadedCustomScriptAssemblyReference.Reference))
                    {
                        // Generate the guid to assembly lookup?
#pragma warning disable UA2001 // The Banned API Analyzer produces compile errors for any new Linq code. This pre-existing usage has been suppressed, but should be rewritten if possible.
                        guidsToAssemblies = guidsToAssemblies ?? CustomScriptAssemblies.ToDictionary(x => x.GUID);
#pragma warning restore UA2001

                        var guid = Utility.FastToLower(GUIDReference.GUIDReferenceToGUID(loadedCustomScriptAssemblyReference.Reference));
                        if (guidsToAssemblies.TryGetValue(guid, out var foundAssembly))
                        {
                            loadedCustomScriptAssemblyReference.Reference = foundAssembly.Name;
                        }
                    }

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