feat(project-system): recover malformed element files without ever rewriting them

src/Beutl.AgentToolkit/Sessions/FileEditingSession.cs

@@ -153,10 +153,36 @@ private void SetProjectPathCore(string projectPath)
                 Path.Combine(projectDirectory, projectName),
                 sceneName,
                 usedDirs);
+            string? previousSceneDirectory = scene.Uri is { IsFile: true } previousSceneUri
+                ? Path.GetDirectoryName(previousSceneUri.LocalPath)
+                : null;
             scene.Uri = new Uri(scenePath);
+            string sceneDirectory = Path.GetDirectoryName(scenePath)!;
             foreach (Element element in scene.Children)
             {
-                element.Uri = null;
+                // Keep each sidecar's relative path across Save As: a recovered element's stable
+                // fallback identity is derived from its scene-relative path, so a regenerated
+                // path would change the element's Id when the copy is reopened.
+                if (element.Uri is { IsFile: true } previousUri)
+                {
+                    string relativePath = previousSceneDirectory != null
+                        ? Path.GetRelativePath(previousSceneDirectory, previousUri.LocalPath)
+                        : Path.GetFileName(previousUri.LocalPath);
+                    string sceneRoot = Path.TrimEndingDirectorySeparator(Path.GetFullPath(sceneDirectory));
+                    string resolvedPath = Path.GetFullPath(Path.Combine(sceneRoot, relativePath));
+                    if (!resolvedPath.StartsWith(
+                            sceneRoot + Path.DirectorySeparatorChar,
+                            PathComparison.ForCurrentPlatform))
+                    {
+                        resolvedPath = Path.Combine(sceneRoot, Path.GetFileName(previousUri.LocalPath));
+                    }
+
+                    element.Uri = new Uri(resolvedPath);
+                }
+                else
+                {
+                    element.Uri = null;
+                }
             }
 
             index++;

src/Beutl.AgentToolkit/Tools/SessionTools.cs

@@ -1,12 +1,16 @@
-using System.ComponentModel;
+using System.Collections;
+using System.ComponentModel;
 using System.Globalization;
 using Beutl.AgentToolkit.Common;
 using Beutl.AgentToolkit.Reconciliation;
 using Beutl.AgentToolkit.Rendering;
 using Beutl.AgentToolkit.Sessions;
 using Beutl.AgentToolkit.Workspace;
+using Beutl.Animation;
 using Beutl.Editor;
+using Beutl.Engine;
 using Beutl.ProjectSystem;
+using Beutl.Serialization;
 using ModelContextProtocol.Server;
 
 namespace Beutl.AgentToolkit.Tools;
@@ -15,7 +19,10 @@ public sealed record SceneSummary(string SceneId, string Name, int Width, int He
 
 public sealed record SessionSummary(IReadOnlyList<SceneSummary> Scenes);
 
-public sealed record OpenProjectResponse(string Session, string Source, SessionSummary Summary);
+public sealed record OpenProjectResponse(string Session, string Source, SessionSummary Summary)
+{
+    public IReadOnlyList<string> Warnings { get; init; } = [];
+}
 
 public sealed record CreateProjectResponse(string Session, string SavedPath, SessionSummary Summary);
 
@@ -71,10 +78,84 @@ public ValueTask<ToolResult<OpenProjectResponse>> OpenProject(string path, Cance
             return new OpenProjectResponse(
                 result.Session.SessionId,
                 result.Session.Source.ToString(),
-                CreateSummary(result.Session, result.Project));
+                CreateSummary(result.Session, result.Project))
+            {
+                // The traversal walks the live graph, so it must run on the session thread — a live
+                // editor may mutate the collections concurrently.
+                Warnings = result.Session.ReadOnSession(() => CollectDeserializationWarnings(result.Project))
+            };
         });
     }
 
+    private static IReadOnlyList<string> CollectDeserializationWarnings(Project project)
+    {
+        var warnings = new List<string>();
+        foreach (Scene scene in project.Items.OfType<Scene>())
+        {
+            foreach (Element element in scene.Children)
+            {
+                var fallbacks = new List<IFallback>();
+                var visited = new HashSet<object>(ReferenceEqualityComparer.Instance);
+                foreach (EngineObject obj in element.Objects)
+                {
+                    CollectFallbacks(obj, visited, fallbacks);
+                }
+
+                string elementFile = element.Uri is { IsFile: true } uri
+                    ? Path.GetFileName(uri.LocalPath)
+                    : element.Name;
+                foreach (IFallback fallback in fallbacks)
+                {
+                    string error = string.IsNullOrWhiteSpace(fallback.ErrorMessage)
+                        ? fallback.Reason.ToString()
+                        : fallback.ErrorMessage;
+                    warnings.Add(
+                        $"Element file '{elementFile}' contains content that could not be deserialized: {error}");
+                }
+            }
+        }
+
+        return warnings;
+    }
+
+    private static void CollectFallbacks(
+        object? value,
+        ISet<object> visited,
+        ICollection<IFallback> fallbacks)
+    {
+        if (value is null or string || !visited.Add(value))
+            return;
+
+        if (value is IFallback fallback)
+        {
+            fallbacks.Add(fallback);
+            return;
+        }
+
+        if (value is EngineObject engineObject)
+        {
+            foreach (IProperty property in engineObject.Properties)
+            {
+                CollectFallbacks(property.CurrentValue, visited, fallbacks);
+                if (property.Animation is IKeyFrameAnimation animation)
+                {
+                    foreach (IKeyFrame keyFrame in animation.KeyFrames)
+                    {
+                        CollectFallbacks(keyFrame.Value, visited, fallbacks);
+                    }
+                }
+            }
+        }
+
+        if (value is IEnumerable enumerable)
+        {
+            foreach (object? item in enumerable)
+            {
+                CollectFallbacks(item, visited, fallbacks);
+            }
+        }
+    }
+
     [McpServerTool(Name = "create_project")]
     [Description("Creates and saves a new Beutl .bep project with one scene, then makes it the active editing session. In the in-app host the project opens in the Beutl editor (single open project, LiveEditor session); in the stdio host it becomes a file-backed session. Paths without an extension are saved as .bep; .beutl is reserved for project packages. The output path is restricted to BEUTL_WORKSPACE.")]
     public ValueTask<ToolResult<CreateProjectResponse>> CreateProject(

src/Beutl.Core/CoreObject.cs

@@ -74,6 +74,10 @@ public string Name
 
     public Uri? Uri { get; set; }
 
+    // Non-null while this object stands in for a file the serializer must not regenerate:
+    // StoreToUri skips the source location and copies the raw text verbatim to any new one.
+    internal SuppressedStorageSource? SuppressedStorageSource { get; set; }
+
     private Dictionary<int, IEntry> Values => _values ??= [];
 
     private Dictionary<int, string> Errors => _errors ??= [];

src/Beutl.Core/Serialization/CoreSerializer.cs

@@ -67,6 +67,12 @@ public static object DeserializeFromJsonObject(JsonObject json, Type baseType, C
 
         try
         {
+            if (!baseType.IsAssignableFrom(actualType))
+            {
+                throw new InvalidCastException(
+                    $"Discriminator type '{actualType}' is not assignable to the expected type '{baseType}'.");
+            }
+
             var obj = Activator.CreateInstance(actualType) as ICoreSerializable
                       ?? throw new InvalidOperationException($"Could not create instance of type {actualType.FullName}.");
 
@@ -83,6 +89,7 @@ public static object DeserializeFromJsonObject(JsonObject json, Type baseType, C
             if (obj is IFallback fallbackObj)
             {
                 fallbackObj.Reason = FallbackReason.TypeNotFound;
+                DeserializationIncidents.RecordFallback();
             }
 
             return obj;
@@ -158,7 +165,10 @@ public static object RestoreFromUri(Uri uri, Type type)
         // 互換性処理
         // 1.x で作成されたファイルでは一部のオブジェクトに $type が付与されないため、
         // 期待される型に基づいてディスクリミネータを補完する。
-        if (!node.TryGetDiscriminator(out Type? _))
+        // Presence is checked on the property key alone: a present-but-unparsable or non-string
+        // discriminator must fail as an unknown type, not silently deserialize as the legacy
+        // default and overwrite the original data on the next save.
+        if (!jsonObject.ContainsKey("$type") && !jsonObject.ContainsKey("@type"))
         {
             if (type == typeof(ProjectItem))
             {
@@ -176,6 +186,14 @@ public static object RestoreFromUri(Uri uri, Type type)
             throw new InvalidOperationException("Discriminator not found in JSON object.");
         }
 
+        if (!type.IsAssignableFrom(actualType))
+        {
+            // Reject before instantiating: deserializing the declared type first would run its own
+            // load side effects (e.g. a Scene declared in a .belm globs and reopens element files).
+            throw new InvalidCastException(
+                $"Discriminator type '{actualType}' is not assignable to the expected type '{type}'.");
+        }
+
         try
         {
             var obj = Activator.CreateInstance(actualType) as ICoreSerializable
@@ -192,6 +210,7 @@ public static object RestoreFromUri(Uri uri, Type type)
             if (obj is IFallback fallbackObj)
             {
                 fallbackObj.Reason = FallbackReason.TypeNotFound;
+                DeserializationIncidents.RecordFallback();
             }
 
             return obj;
@@ -227,6 +246,60 @@ public static void PopulateFromUri(ICoreSerializable obj, Type type, Uri uri)
     public static void StoreToUri<T>(T obj, Uri uri, CoreSerializationMode? mode = null)
         where T : ICoreSerializable
     {
+        if (obj is CoreObject { SuppressedStorageSource: { } suppressed } suppressedObj)
+        {
+            if (uri == suppressed.SourceUri || uri.Scheme != "file")
+            {
+                return;
+            }
+
+            // Rehomed (save-as): the retained bytes move verbatim so the new project copy keeps the
+            // element. The suppression record is never mutated — the source location stays
+            // skip-protected even if a failed multi-file save rolls Uri back afterwards.
+            string rehomedPath = uri.LocalPath;
+            string? rehomedDirectory = Path.GetDirectoryName(rehomedPath);
+            if (rehomedDirectory != null)
+            {
+                Directory.CreateDirectory(rehomedDirectory);
+            }
+
+            FileStream stream;
+            try
+            {
+                stream = new FileStream(rehomedPath, FileMode.CreateNew, FileAccess.Write);
+            }
+            catch (IOException) when (File.Exists(rehomedPath))
+            {
+                // An existing file may hold a manual repair of the recovered content (or an earlier
+                // verbatim copy); never overwrite it.
+                suppressedObj.Uri = uri;
+                return;
+            }
+
+            try
+            {
+                using (stream)
+                {
+                    stream.Write(suppressed.RawBytes);
+                }
+            }
+            catch
+            {
+                try
+                {
+                    File.Delete(rehomedPath);
+                }
+                catch
+                {
+                }
+
+                throw;
+            }
+
+            suppressedObj.Uri = uri;
+            return;
+        }
+
         if (uri.Scheme == "file")
         {
             if (obj is CoreObject coreObj)

src/Beutl.Core/Serialization/DeserializationIncidents.cs

@@ -0,0 +1,16 @@
+namespace Beutl.Serialization;
+
+/// <summary>
+/// Thread-local tally of fallback substitutions, letting a caller detect fallbacks created in
+/// positions a hierarchical traversal of the deserialized result cannot reach (e.g. plain
+/// property values such as keyframe values).
+/// </summary>
+internal static class DeserializationIncidents
+{
+    [ThreadStatic]
+    private static int t_fallbackCount;
+
+    internal static int FallbackCount => t_fallbackCount;
+
+    internal static void RecordFallback() => t_fallbackCount++;
+}

src/Beutl.Core/Serialization/FallbackDeserializationHelper.cs

@@ -29,6 +29,7 @@ internal static class FallbackDeserializationHelper
         fallback.Reason = FallbackReason.DeserializationFailed;
         fallback.ErrorMessage = exception?.Message;
 
+        DeserializationIncidents.RecordFallback();
         return fallback;
     }
 }

src/Beutl.Core/Serialization/JsonSerializationContext.Deserialize.cs

@@ -88,6 +88,12 @@ private static bool TryDeserializeCoreSerializable(
 
         try
         {
+            if (!baseType.IsAssignableFrom(actualType))
+            {
+                throw new InvalidCastException(
+                    $"Discriminator type '{actualType}' is not assignable to the expected type '{baseType}'.");
+            }
+
             var instance = Activator.CreateInstance(actualType) as ICoreSerializable
                            ?? throw new InvalidOperationException(
                                $"Could not create instance of type {actualType.FullName}.");
@@ -110,6 +116,7 @@ private static bool TryDeserializeCoreSerializable(
             if (instance is IFallback fallbackObj)
             {
                 fallbackObj.Reason = FallbackReason.TypeNotFound;
+                DeserializationIncidents.RecordFallback();
             }
 
             result = instance;

src/Beutl.Core/Serialization/SuppressedStorageSource.cs

@@ -0,0 +1,8 @@
+namespace Beutl;
+
+/// <summary>
+/// The retained on-disk bytes of an object the serializer must not regenerate, together with the
+/// location those bytes came from. The source location is never rewritten; any other location
+/// receives a verbatim copy.
+/// </summary>
+internal sealed record SuppressedStorageSource(byte[] RawBytes, Uri SourceUri);

src/Beutl.Core/TypeFormat.cs

@@ -10,8 +10,20 @@ internal static class TypeFormat
         public static Type? ToType(string fullName)
         {
             fullName = fullName.Replace("Beutl.Embedding.FFmpeg", "Beutl.Extensions.FFmpeg");
-            List<Token> tokens = new TypeNameTokenizer(fullName).Tokenize();
-            return new TypeNameParser(tokens).Parse();
+            try
+            {
+                List<Token> tokens = new TypeNameTokenizer(fullName).Tokenize();
+                return new TypeNameParser(tokens).Parse();
+            }
+            // The tokenizer/parser index freely and throw on ill-formed names (e.g. "x" from a
+            // hand-edited file); every caller already treats null as "unknown type".
+            catch (Exception ex) when (ex is IndexOutOfRangeException
+                                           or ArgumentOutOfRangeException
+                                           or ArgumentException
+                                           or InvalidOperationException)
+            {
+                return null;
+            }
         }
 
         public static string ToString(Type type)
@@ -324,6 +336,11 @@ private static string TakeTypeNameTokens(Span<Token> tokens, out Span<Token> gen
                     type = _assembly?.GetType($"{_namespace ?? ""}.{typeName}{suffix}")!;
                 }
 
+                if (type == null)
+                {
+                    return null;
+                }
+
                 if (genericArgs.Length > 0)
                 {
                     type = type.MakeGenericType(genericArgs);

src/Beutl.Editor/AutoSaveService.cs

@@ -41,7 +41,7 @@ public void SaveObjects(IEnumerable<CoreObject> objectsToSave)
             {
                 if (obj is IHierarchical hierarchical && hierarchical.HierarchicalRoot == null)
                 {
-                    if (obj.Uri!.Scheme == "file")
+                    if (obj.SuppressedStorageSource is null && obj.Uri!.Scheme == "file")
                     {
                         var path = obj.Uri.LocalPath;
                         if (File.Exists(path))