diff --git a/My project/Assets/FlowScope/Runtime/Config/FileConfigSource.cs b/My project/Assets/FlowScope/Runtime/Config/FileConfigSource.cs
new file mode 100644
index 0000000..be05e9a
--- /dev/null
+++ b/My project/Assets/FlowScope/Runtime/Config/FileConfigSource.cs	
@@ -0,0 +1,31 @@
+using System;
+using System.IO;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace FlowScope.Config
+{
+    public sealed class FileConfigSource : IConfigSource
+    {
+        private readonly string _rootPath;
+
+        public FileConfigSource(string rootPath, string name = "File")
+        {
+            _rootPath = string.IsNullOrWhiteSpace(rootPath)
+                ? throw new ArgumentException("Config root path is required.", nameof(rootPath))
+                : rootPath;
+            Name = string.IsNullOrWhiteSpace(name)
+                ? throw new ArgumentException("Config source name is required.", nameof(name))
+                : name;
+        }
+
+        public string Name { get; }
+
+        public Task<string> LoadTextAsync(string fileName, CancellationToken cancellationToken)
+        {
+            cancellationToken.ThrowIfCancellationRequested();
+            var path = Path.Combine(_rootPath, fileName);
+            return Task.Run(() => File.ReadAllText(path), cancellationToken);
+        }
+    }
+}
diff --git a/My project/Assets/FlowScope/Runtime/Config/FileConfigSource.cs.meta b/My project/Assets/FlowScope/Runtime/Config/FileConfigSource.cs.meta
new file mode 100644
index 0000000..1fbea3e
--- /dev/null
+++ b/My project/Assets/FlowScope/Runtime/Config/FileConfigSource.cs.meta	
@@ -0,0 +1,11 @@
+fileFormatVersion: 2
+guid: 8d50cd860ba849a1aa7647deba4e71db
+MonoImporter:
+  externalObjects: {}
+  serializedVersion: 2
+  defaultReferences: []
+  executionOrder: 0
+  icon: {instanceID: 0}
+  userData: 
+  assetBundleName: 
+  assetBundleVariant: 
diff --git a/My project/Assets/FlowScope/Runtime/Config/InMemoryConfigSource.cs b/My project/Assets/FlowScope/Runtime/Config/InMemoryConfigSource.cs
new file mode 100644
index 0000000..55ad6a2
--- /dev/null
+++ b/My project/Assets/FlowScope/Runtime/Config/InMemoryConfigSource.cs	
@@ -0,0 +1,37 @@
+using System;
+using System.Collections.Generic;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace FlowScope.Config
+{
+    public sealed class InMemoryConfigSource : IConfigSource
+    {
+        private readonly Dictionary<string, string> _files;
+
+        public InMemoryConfigSource(
+            IReadOnlyDictionary<string, string> files,
+            string name = "InMemory")
+        {
+            _files = files != null
+                ? new Dictionary<string, string>(files)
+                : throw new ArgumentNullException(nameof(files));
+            Name = string.IsNullOrWhiteSpace(name)
+                ? throw new ArgumentException("Config source name is required.", nameof(name))
+                : name;
+        }
+
+        public string Name { get; }
+
+        public Task<string> LoadTextAsync(string fileName, CancellationToken cancellationToken)
+        {
+            cancellationToken.ThrowIfCancellationRequested();
+            if (!_files.TryGetValue(fileName, out var text))
+            {
+                throw new InvalidOperationException($"Config file is missing: {fileName}");
+            }
+
+            return Task.FromResult(text);
+        }
+    }
+}
diff --git a/My project/Assets/FlowScope/Runtime/Config/InMemoryConfigSource.cs.meta b/My project/Assets/FlowScope/Runtime/Config/InMemoryConfigSource.cs.meta
new file mode 100644
index 0000000..dcdce36
--- /dev/null
+++ b/My project/Assets/FlowScope/Runtime/Config/InMemoryConfigSource.cs.meta	
@@ -0,0 +1,11 @@
+fileFormatVersion: 2
+guid: 607d9b20e7f04565929ea5876e5c25e1
+MonoImporter:
+  externalObjects: {}
+  serializedVersion: 2
+  defaultReferences: []
+  executionOrder: 0
+  icon: {instanceID: 0}
+  userData: 
+  assetBundleName: 
+  assetBundleVariant: 
diff --git a/My project/Assets/FlowScope/Runtime/Config/JsonConfigParser.cs b/My project/Assets/FlowScope/Runtime/Config/JsonConfigParser.cs
new file mode 100644
index 0000000..2cb4263
--- /dev/null
+++ b/My project/Assets/FlowScope/Runtime/Config/JsonConfigParser.cs	
@@ -0,0 +1,16 @@
+using System.Collections.Generic;
+using FlowScope.Data;
+
+namespace FlowScope.Config
+{
+    public sealed class JsonConfigParser : IConfigParser
+    {
+        public string Format => "json";
+
+        public IReadOnlyList<T> ParseRows<T>(string fileName, string text)
+            where T : class, IConfigRow
+        {
+            return ReactivePropertyJsonConverter.DeserializeArray<T>(text);
+        }
+    }
+}
diff --git a/My project/Assets/FlowScope/Runtime/Config/JsonConfigParser.cs.meta b/My project/Assets/FlowScope/Runtime/Config/JsonConfigParser.cs.meta
new file mode 100644
index 0000000..cf41d48
--- /dev/null
+++ b/My project/Assets/FlowScope/Runtime/Config/JsonConfigParser.cs.meta	
@@ -0,0 +1,11 @@
+fileFormatVersion: 2
+guid: f60f8a8e66394f26a20ed6f5ef314c63
+MonoImporter:
+  externalObjects: {}
+  serializedVersion: 2
+  defaultReferences: []
+  executionOrder: 0
+  icon: {instanceID: 0}
+  userData: 
+  assetBundleName: 
+  assetBundleVariant: 
diff --git a/My project/Assets/FlowScope/Runtime/Config/JsonConfigProvider.cs b/My project/Assets/FlowScope/Runtime/Config/JsonConfigProvider.cs
index 17ac4de..1e495b4 100644
--- a/My project/Assets/FlowScope/Runtime/Config/JsonConfigProvider.cs	
+++ b/My project/Assets/FlowScope/Runtime/Config/JsonConfigProvider.cs	
@@ -1,8 +1,8 @@
 using System;
 using System.Collections.Generic;
+using System.Reflection;
 using System.Threading;
 using System.Threading.Tasks;
-using FlowScope.Data;
 
 namespace FlowScope.Config
 {
@@ -19,7 +19,19 @@ namespace FlowScope.Config
         public static ConfigMapping Mapping<T>(string fileName, Func<CancellationToken, Task<string>> loadTextAsync)
             where T : class, IConfigRow
         {
-            return new ConfigMapping(typeof(T), fileName, loadTextAsync);
+            return Mapping<T>(
+                fileName,
+                new DelegateConfigSource(fileName, loadTextAsync),
+                new JsonConfigParser());
+        }
+
+        public static ConfigMapping Mapping<T>(
+            string fileName,
+            IConfigSource source,
+            IConfigParser parser)
+            where T : class, IConfigRow
+        {
+            return new ConfigMapping(typeof(T), fileName, source, parser);
         }
 
         public async Task LoadAllAsync(CancellationToken cancellationToken)
@@ -33,7 +45,7 @@ namespace FlowScope.Config
                 string json;
                 try
                 {
-                    json = await mapping.LoadTextAsync(cancellationToken);
+                    json = await mapping.Source.LoadTextAsync(mapping.FileName, cancellationToken);
                 }
                 catch (OperationCanceledException)
                 {
@@ -41,7 +53,9 @@ namespace FlowScope.Config
                 }
                 catch (Exception exception)
                 {
-                    throw new InvalidOperationException($"Failed to load config file '{mapping.FileName}'.", exception);
+                    throw new InvalidOperationException(
+                        $"Failed to load config file '{mapping.FileName}' from source '{mapping.Source.Name}'.",
+                        exception);
                 }
 
                 try
@@ -84,42 +98,42 @@ namespace FlowScope.Config
 
         public sealed class ConfigMapping
         {
-            private readonly Func<string, object> _parse;
+            private readonly MethodInfo _parseRowsMethod;
 
-            internal ConfigMapping(Type rowType, string fileName, Func<CancellationToken, Task<string>> loadTextAsync)
+            internal ConfigMapping(
+                Type rowType,
+                string fileName,
+                IConfigSource source,
+                IConfigParser parser)
             {
                 RowType = rowType ?? throw new ArgumentNullException(nameof(rowType));
                 FileName = string.IsNullOrWhiteSpace(fileName)
                     ? throw new ArgumentException("Config file name is required.", nameof(fileName))
                     : fileName;
-                LoadTextAsync = loadTextAsync ?? throw new ArgumentNullException(nameof(loadTextAsync));
-                _parse = CreateParser(rowType);
+                Source = source ?? throw new ArgumentNullException(nameof(source));
+                Parser = parser ?? throw new ArgumentNullException(nameof(parser));
+                _parseRowsMethod = typeof(IConfigParser)
+                    .GetMethod(nameof(IConfigParser.ParseRows))
+                    .MakeGenericMethod(rowType);
             }
 
             public Type RowType { get; }
             public string FileName { get; }
-            public Func<CancellationToken, Task<string>> LoadTextAsync { get; }
+            public IConfigSource Source { get; }
+            public IConfigParser Parser { get; }
 
-            internal object Parse(string json) => _parse(json);
-
-            private static Func<string, object> CreateParser(Type rowType)
+            internal object Parse(string json)
             {
-                var method = typeof(ConfigMapping)
-                    .GetMethod(nameof(ParseRows), System.Reflection.BindingFlags.Static | System.Reflection.BindingFlags.NonPublic)
-                    .MakeGenericMethod(rowType);
-
-                return json => method.Invoke(null, new object[] { json });
-            }
-
-            private static object ParseRows<T>(string json) where T : class, IConfigRow
-            {
-                var rows = ReactivePropertyJsonConverter.DeserializeArray<T>(json);
-                var byId = new Dictionary<int, T>();
-                foreach (var row in rows)
+                var rows = (System.Collections.IEnumerable)_parseRowsMethod.Invoke(
+                    Parser,
+                    new object[] { FileName, json });
+                var byIdType = typeof(Dictionary<,>).MakeGenericType(typeof(int), RowType);
+                var byId = (System.Collections.IDictionary)Activator.CreateInstance(byIdType);
+                foreach (IConfigRow row in rows)
                 {
-                    if (byId.ContainsKey(row.Id))
+                    if (byId.Contains(row.Id))
                     {
-                        throw new InvalidOperationException($"Duplicate config id {row.Id} for {typeof(T).Name}.");
+                        throw new InvalidOperationException($"Duplicate config id {row.Id} for {RowType.Name}.");
                     }
 
                     byId.Add(row.Id, row);
@@ -128,5 +142,25 @@ namespace FlowScope.Config
                 return byId;
             }
         }
+
+        private sealed class DelegateConfigSource : IConfigSource
+        {
+            private readonly Func<CancellationToken, Task<string>> _loadTextAsync;
+
+            public DelegateConfigSource(
+                string fileName,
+                Func<CancellationToken, Task<string>> loadTextAsync)
+            {
+                Name = fileName;
+                _loadTextAsync = loadTextAsync ?? throw new ArgumentNullException(nameof(loadTextAsync));
+            }
+
+            public string Name { get; }
+
+            public Task<string> LoadTextAsync(string fileName, CancellationToken cancellationToken)
+            {
+                return _loadTextAsync(cancellationToken);
+            }
+        }
     }
 }
diff --git a/My project/Assets/FlowScope/Runtime/Save/JsonSaveSerializer.cs b/My project/Assets/FlowScope/Runtime/Save/JsonSaveSerializer.cs
index 5c44877..ff99832 100644
--- a/My project/Assets/FlowScope/Runtime/Save/JsonSaveSerializer.cs	
+++ b/My project/Assets/FlowScope/Runtime/Save/JsonSaveSerializer.cs	
@@ -12,6 +12,22 @@ namespace FlowScope.Save
 
     public sealed class JsonSaveSerializer : ISaveSerializer
     {
+        private readonly int _currentVersion;
+        private readonly SaveMigrationRegistry _migrations;
+
+        public JsonSaveSerializer(
+            int currentVersion = 1,
+            SaveMigrationRegistry migrations = null)
+        {
+            if (currentVersion < 1)
+            {
+                throw new ArgumentOutOfRangeException(nameof(currentVersion));
+            }
+
+            _currentVersion = currentVersion;
+            _migrations = migrations ?? new SaveMigrationRegistry();
+        }
+
         public byte[] Serialize<T>(T data)
         {
             return Encoding.UTF8.GetBytes(SerializeToString(data));
@@ -29,19 +45,75 @@ namespace FlowScope.Save
 
         public string SerializeToString<T>(T data)
         {
-            return ReactivePropertyJsonConverter.SerializeObject(data);
+            var payload = ReactivePropertyJsonConverter.SerializeObject(data);
+            if (_currentVersion <= 1)
+            {
+                return payload;
+            }
+
+            return ReactivePropertyJsonConverter.SerializeObject(new SaveEnvelope
+            {
+                Version = _currentVersion,
+                Payload = payload
+            });
         }
 
         public T DeserializeFromString<T>(string json)
         {
             var instance = (T)Activator.CreateInstance(typeof(T), true);
-            PopulateFromString(json, instance);
+            PopulateFromString(ResolvePayload<T>(json), instance);
             return instance;
         }
 
         public void PopulateFromString<T>(string json, T target)
         {
-            ReactivePropertyJsonConverter.PopulateObject(json, target);
+            ReactivePropertyJsonConverter.PopulateObject(ResolvePayload<T>(json), target);
+        }
+
+        private string ResolvePayload<T>(string json)
+        {
+            if (!TryReadEnvelope(json, out var envelope))
+            {
+                return _currentVersion > 1
+                    ? _migrations.Migrate(GetMigrationKey<T>(), 1, _currentVersion, json)
+                    : json;
+            }
+
+            if (envelope.Version == _currentVersion)
+            {
+                return envelope.Payload;
+            }
+
+            if (envelope.Version > _currentVersion)
+            {
+                throw new InvalidOperationException(
+                    $"Save version {envelope.Version} is newer than current version {_currentVersion}.");
+            }
+
+            return _migrations.Migrate(
+                GetMigrationKey<T>(),
+                envelope.Version,
+                _currentVersion,
+                envelope.Payload);
+        }
+
+        private static bool TryReadEnvelope(string json, out SaveEnvelope envelope)
+        {
+            envelope = null;
+            try
+            {
+                envelope = ReactivePropertyJsonConverter.DeserializeObject<SaveEnvelope>(json);
+                return envelope.Version > 0 && envelope.Payload != null;
+            }
+            catch
+            {
+                return false;
+            }
+        }
+
+        private static string GetMigrationKey<T>()
+        {
+            return typeof(T).FullName;
         }
     }
 }
diff --git a/My project/Assets/FlowScope/Runtime/Save/SaveEnvelope.cs b/My project/Assets/FlowScope/Runtime/Save/SaveEnvelope.cs
new file mode 100644
index 0000000..f2013f0
--- /dev/null
+++ b/My project/Assets/FlowScope/Runtime/Save/SaveEnvelope.cs	
@@ -0,0 +1,8 @@
+namespace FlowScope.Save
+{
+    internal sealed class SaveEnvelope
+    {
+        public int Version { get; set; }
+        public string Payload { get; set; }
+    }
+}
diff --git a/My project/Assets/FlowScope/Runtime/Save/SaveEnvelope.cs.meta b/My project/Assets/FlowScope/Runtime/Save/SaveEnvelope.cs.meta
new file mode 100644
index 0000000..11acd8a
--- /dev/null
+++ b/My project/Assets/FlowScope/Runtime/Save/SaveEnvelope.cs.meta	
@@ -0,0 +1,11 @@
+fileFormatVersion: 2
+guid: 67ada4b73d47420db150f34ab12a236a
+MonoImporter:
+  externalObjects: {}
+  serializedVersion: 2
+  defaultReferences: []
+  executionOrder: 0
+  icon: {instanceID: 0}
+  userData: 
+  assetBundleName: 
+  assetBundleVariant: 
diff --git a/My project/Assets/FlowScope/Runtime/Save/SaveMigrationRegistry.cs b/My project/Assets/FlowScope/Runtime/Save/SaveMigrationRegistry.cs
new file mode 100644
index 0000000..4ee5fa6
--- /dev/null
+++ b/My project/Assets/FlowScope/Runtime/Save/SaveMigrationRegistry.cs	
@@ -0,0 +1,74 @@
+using System;
+using System.Collections.Generic;
+
+namespace FlowScope.Save
+{
+    public sealed class SaveMigrationRegistry
+    {
+        private readonly Dictionary<MigrationKey, ISaveMigration> _migrations = new();
+
+        public void Register(ISaveMigration migration)
+        {
+            if (migration == null)
+            {
+                throw new ArgumentNullException(nameof(migration));
+            }
+
+            _migrations[new MigrationKey(migration.Key, migration.FromVersion)] = migration;
+        }
+
+        public string Migrate(
+            string key,
+            int fromVersion,
+            int currentVersion,
+            string json)
+        {
+            var current = fromVersion;
+            var payload = json;
+            while (current < currentVersion)
+            {
+                if (!_migrations.TryGetValue(new MigrationKey(key, current), out var migration) ||
+                    migration.ToVersion != current + 1)
+                {
+                    throw new InvalidOperationException(
+                        $"Missing save migration for key '{key}' from version {current}.");
+                }
+
+                payload = migration.Migrate(payload);
+                current = migration.ToVersion;
+            }
+
+            return payload;
+        }
+
+        private readonly struct MigrationKey : IEquatable<MigrationKey>
+        {
+            private readonly string _key;
+            private readonly int _fromVersion;
+
+            public MigrationKey(string key, int fromVersion)
+            {
+                _key = key ?? string.Empty;
+                _fromVersion = fromVersion;
+            }
+
+            public bool Equals(MigrationKey other)
+            {
+                return _key == other._key && _fromVersion == other._fromVersion;
+            }
+
+            public override bool Equals(object obj)
+            {
+                return obj is MigrationKey other && Equals(other);
+            }
+
+            public override int GetHashCode()
+            {
+                unchecked
+                {
+                    return ((_key != null ? _key.GetHashCode() : 0) * 397) ^ _fromVersion;
+                }
+            }
+        }
+    }
+}
diff --git a/My project/Assets/FlowScope/Runtime/Save/SaveMigrationRegistry.cs.meta b/My project/Assets/FlowScope/Runtime/Save/SaveMigrationRegistry.cs.meta
new file mode 100644
index 0000000..e74593f
--- /dev/null
+++ b/My project/Assets/FlowScope/Runtime/Save/SaveMigrationRegistry.cs.meta	
@@ -0,0 +1,11 @@
+fileFormatVersion: 2
+guid: 3f2c3dd694c94ed59d08bcdf88d830a7
+MonoImporter:
+  externalObjects: {}
+  serializedVersion: 2
+  defaultReferences: []
+  executionOrder: 0
+  icon: {instanceID: 0}
+  userData: 
+  assetBundleName: 
+  assetBundleVariant: 
diff --git a/My project/Assets/FlowScope/Tests/EditMode/Config/ConfigExtensionTests.cs b/My project/Assets/FlowScope/Tests/EditMode/Config/ConfigExtensionTests.cs
new file mode 100644
index 0000000..37b25d5
--- /dev/null
+++ b/My project/Assets/FlowScope/Tests/EditMode/Config/ConfigExtensionTests.cs	
@@ -0,0 +1,91 @@
+using System;
+using System.Collections.Generic;
+using System.Threading;
+using System.Threading.Tasks;
+using FlowScope.Config;
+using NUnit.Framework;
+
+namespace FlowScope.Tests.EditMode.Config
+{
+    public sealed class ConfigExtensionTests
+    {
+        [Test]
+        public void LoadAllAsync_UsesConfiguredSourceAndParser()
+        {
+            var source = new InMemoryConfigSource(
+                new Dictionary<string, string>
+                {
+                    ["player.json"] = "[{\"id\":7,\"name\":\"Starter\"}]"
+                });
+            var provider = new JsonConfigProvider(new[]
+            {
+                JsonConfigProvider.Mapping<PlayerConfig>("player.json", source, new JsonConfigParser())
+            });
+
+            provider.LoadAllAsync(CancellationToken.None).GetAwaiter().GetResult();
+
+            Assert.That(provider.Get<PlayerConfig>(7).Name, Is.EqualTo("Starter"));
+        }
+
+        [Test]
+        public void LoadAllAsync_WhenSourceFails_IncludesSourceAndFileName()
+        {
+            var source = new InMemoryConfigSource(new Dictionary<string, string>(), "MemoryA");
+            var provider = new JsonConfigProvider(new[]
+            {
+                JsonConfigProvider.Mapping<PlayerConfig>("missing.json", source, new JsonConfigParser())
+            });
+
+            var exception = ThrowsAsync<InvalidOperationException>(
+                () => provider.LoadAllAsync(CancellationToken.None));
+
+            Assert.That(exception.Message, Does.Contain("MemoryA"));
+            Assert.That(exception.Message, Does.Contain("missing.json"));
+        }
+
+        [Test]
+        public void LoadAllAsync_WhenParserFindsDuplicateId_IncludesFileName()
+        {
+            var source = new InMemoryConfigSource(
+                new Dictionary<string, string>
+                {
+                    ["player.json"] = "[{\"id\":7,\"name\":\"A\"},{\"id\":7,\"name\":\"B\"}]"
+                });
+            var provider = new JsonConfigProvider(new[]
+            {
+                JsonConfigProvider.Mapping<PlayerConfig>("player.json", source, new JsonConfigParser())
+            });
+
+            var exception = ThrowsAsync<InvalidOperationException>(
+                () => provider.LoadAllAsync(CancellationToken.None));
+
+            Assert.That(exception.Message, Does.Contain("player.json"));
+            Assert.That(exception.Message, Does.Contain("7"));
+        }
+
+        private static TException ThrowsAsync<TException>(Func<Task> action)
+            where TException : Exception
+        {
+            try
+            {
+                action().GetAwaiter().GetResult();
+            }
+            catch (TException exception)
+            {
+                return exception;
+            }
+
+            Assert.Fail($"Expected {typeof(TException).Name}.");
+            return null;
+        }
+
+        private sealed class PlayerConfig : IConfigRow
+        {
+            public int id;
+            public string name;
+
+            public int Id => id;
+            public string Name => name;
+        }
+    }
+}
diff --git a/My project/Assets/FlowScope/Tests/EditMode/Config/ConfigExtensionTests.cs.meta b/My project/Assets/FlowScope/Tests/EditMode/Config/ConfigExtensionTests.cs.meta
new file mode 100644
index 0000000..06f2a40
--- /dev/null
+++ b/My project/Assets/FlowScope/Tests/EditMode/Config/ConfigExtensionTests.cs.meta	
@@ -0,0 +1,11 @@
+fileFormatVersion: 2
+guid: 58935522df66446e93252f079376b603
+MonoImporter:
+  externalObjects: {}
+  serializedVersion: 2
+  defaultReferences: []
+  executionOrder: 0
+  icon: {instanceID: 0}
+  userData: 
+  assetBundleName: 
+  assetBundleVariant: 
diff --git a/My project/Assets/FlowScope/Tests/EditMode/Save/SaveMigrationTests.cs b/My project/Assets/FlowScope/Tests/EditMode/Save/SaveMigrationTests.cs
new file mode 100644
index 0000000..5d153cc
--- /dev/null
+++ b/My project/Assets/FlowScope/Tests/EditMode/Save/SaveMigrationTests.cs	
@@ -0,0 +1,69 @@
+using System;
+using FlowScope.Save;
+using NUnit.Framework;
+using R3;
+
+namespace FlowScope.Tests.EditMode.Save
+{
+    public sealed class SaveMigrationTests
+    {
+        [Test]
+        public void Deserialize_WhenEnvelopeVersionIsOld_AppliesMigrationChain()
+        {
+            var registry = new SaveMigrationRegistry();
+            registry.Register(new RenameCoinsToGoldMigration());
+            var serializer = new JsonSaveSerializer(currentVersion: 2, migrations: registry);
+            var oldPayload = "{\"Coins\":13}";
+            var envelope = "{\"Version\":1,\"Payload\":\"{\\\"Coins\\\":13}\"}";
+
+            var loaded = serializer.DeserializeFromString<PlayerSaveV2>(envelope);
+
+            Assert.That(oldPayload, Does.Contain("Coins"));
+            Assert.That(loaded.Gold.Value, Is.EqualTo(13));
+        }
+
+        [Test]
+        public void Deserialize_WhenMigrationIsMissing_ThrowsWithKeyAndVersion()
+        {
+            var serializer = new JsonSaveSerializer(currentVersion: 2, migrations: new SaveMigrationRegistry());
+            var envelope = "{\"Version\":1,\"Payload\":\"{\\\"Coins\\\":13}\"}";
+
+            var exception = Assert.Throws<InvalidOperationException>(
+                () => serializer.DeserializeFromString<PlayerSaveV2>(envelope));
+
+            Assert.That(exception.Message, Does.Contain(typeof(PlayerSaveV2).FullName));
+            Assert.That(exception.Message, Does.Contain("1"));
+        }
+
+        [Test]
+        public void Serialize_WritesEnvelopeWithCurrentVersion()
+        {
+            var serializer = new JsonSaveSerializer(currentVersion: 2);
+            var data = new PlayerSaveV2();
+            data.Gold.Value = 21;
+
+            var json = serializer.SerializeToString(data);
+
+            Assert.That(json, Does.Contain("\"Version\":2"));
+            Assert.That(json, Does.Contain("\"Payload\""));
+            Assert.That(json, Does.Contain("Gold"));
+        }
+
+        private sealed class RenameCoinsToGoldMigration : ISaveMigration
+        {
+            public string Key => typeof(PlayerSaveV2).FullName;
+            public int FromVersion => 1;
+            public int ToVersion => 2;
+
+            public string Migrate(string json)
+            {
+                return json.Replace("Coins", "Gold");
+            }
+        }
+
+        private sealed class PlayerSaveV2
+        {
+            public ReactiveProperty<int> Gold { get; set; } = new();
+        }
+    }
+}
diff --git a/My project/Assets/FlowScope/Tests/EditMode/Save/SaveMigrationTests.cs.meta b/My project/Assets/FlowScope/Tests/EditMode/Save/SaveMigrationTests.cs.meta
new file mode 100644
index 0000000..8443cd1
--- /dev/null
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