diff --git a/CURRENT_TASK.md b/CURRENT_TASK.md
index 779d8474..63783d36 100644
--- a/CURRENT_TASK.md
+++ b/CURRENT_TASK.md
@@ -12,27 +12,35 @@ Updated: 2025‑09‑24
 ### ✅ **重要な発見：既存システムが完全実装済み！**
 - **環境変数制御**: `NYASH_USE_PLUGIN_BUILTINS=1` + `NYASH_PLUGIN_OVERRIDE_TYPES="StringBox,IntegerBox"`
 - **実装箇所**: `src/box_factory/mod.rs:119-143`に完全な優先度制御システム
-- **結論**: 新規実装不要、既存機能の活用が鍵
+- **課題発見**: 予約型保護（src/box_factory/mod.rs:73-86）がプラグイン登録を阻止
+- **解決策**: 環境変数で予約型保護を条件付き解除
+
+### 🎯 **最終目標**
+**3層構造→2層構造への完全移行**
+```
+現状: コアBox（nyrt） + プラグインBox + ユーザーBox
+最終: プラグインBox（デフォルト） + ユーザーBox
+```
 
 ### 実装フェーズ計画
-#### Phase A: プラグイン版動作確認（1週目）
-- [ ] 環境変数制御のドキュメント作成
-- [ ] プラグイン版StringBox/IntegerBox/ArrayBox/MapBoxの動作テスト
-- [ ] パフォーマンス測定（FFIオーバーヘッド確認）
+#### Phase A: 予約型保護解除（1週目）
+- [ ] `src/box_factory/mod.rs`の`is_reserved_type()`修正
+- [ ] 環境変数で条件付き保護解除実装
+- [ ] プラグイン版StringBox/IntegerBox動作確認
 
 #### Phase B: MIRビルダー統一（2週目）
 - [ ] `src/mir/builder.rs`の特別扱い削除（行407-424）
 - [ ] `src/mir/builder/utils.rs`の型推論削除（行134-156）
 - [ ] すべてのBoxを`MirType::Box(name)`として統一
 
-#### Phase C: nyrt実装削除（3週目）
-- [ ] `crates/nyrt/src/lib.rs`からコアBox関数削除（約300行）
-- [ ] `crates/nyrt/src/plugin/array.rs`削除（143行）
-- [ ] `crates/nyrt/src/plugin/string.rs`削除（173行）
+#### Phase C: 完全統一（3週目）
+- [ ] 予約型保護の完全削除
+- [ ] nyrt実装削除（約600行）
+- [ ] デフォルト動作をプラグインBox化
+- [ ] 環境変数を廃止（プラグインがデフォルト）
 
 #### Phase D: Nyashコード化（将来）
 - [ ] `apps/lib/core_boxes/`にNyash実装作成
-- [ ] StringBox, IntegerBox, ArrayBox, MapBoxのNyash版実装
 - [ ] 静的リンクによる性能最適化
 
 ### ✅ **MIR Call命令統一実装完了済み**（2025-09-24）
diff --git a/docs/development/roadmap/phases/phase-15/phase-15.5-core-box-unification.md b/docs/development/roadmap/phases/phase-15/phase-15.5-core-box-unification.md
index 81a009b4..6cecb69d 100644
--- a/docs/development/roadmap/phases/phase-15/phase-15.5-core-box-unification.md
+++ b/docs/development/roadmap/phases/phase-15/phase-15.5-core-box-unification.md
@@ -11,12 +11,14 @@
 - **特別扱い削除**: MIRビルダー/バックエンドから約100行
 - **全体寄与**: Phase 15目標（80k→20k）の約1%
 
-## 📊 現状の3層構造（削除対象）
+## 📊 構造変更の全体像
+
+### 現状（3層構造）
 ```
-1. コアBox（nyrt内蔵）← 削除対象
+1. コアBox（nyrt内蔵）← 完全削除
    - StringBox, IntegerBox, BoolBox
    - ArrayBox, MapBox
-   - 特別な最適化パス
+   - 特別な最適化パス・予約型保護
 
 2. プラグインBox（.so/.dll）
    - FileBox, NetBox, ConsoleBox等
@@ -26,22 +28,36 @@
    - アプリケーション固有Box
 ```
 
+### 最終形（2層構造）
+```
+1. プラグインBox（.so/.dll）← デフォルト動作
+   - StringBox, IntegerBox（元コア）
+   - FileBox, NetBox等（既存プラグイン）
+   - 統一されたFFI TypeBox v2インターフェース
+
+2. ユーザー定義Box（Nyashコード）
+   - アプリケーション固有Box
+   - 将来：StringBox等もNyashコード実装
+```
+
 ## 🚀 実装計画
 
-### Phase A: プラグイン版動作確認（1週目）
+### Phase A: 予約型保護解除（1週目）
 
-#### 1. 環境変数制御の実装
+#### 1. 予約型保護の条件付き解除
 ```rust
-// src/mir/builder/utils.rs に追加
-fn get_box_provider(box_type: &str) -> BoxProvider {
-    if env::var("NYASH_USE_PLUGIN_CORE_BOXES").is_ok() {
-        // プラグイン版を優先
-        if plugin_registry::exists(box_type) {
-            return BoxProvider::Plugin(box_type);
+// src/box_factory/mod.rs: is_reserved_type()修正
+fn is_reserved_type(name: &str) -> bool {
+    // 環境変数でプラグイン優先モード時は保護解除
+    if std::env::var("NYASH_USE_PLUGIN_BUILTINS").is_ok() {
+        if let Ok(types) = std::env::var("NYASH_PLUGIN_OVERRIDE_TYPES") {
+            if types.split(',').any(|t| t.trim() == name) {
+                return false;  // 予約型として扱わない
+            }
         }
     }
-    // nyrt内蔵にフォールバック
-    BoxProvider::Builtin(box_type)
+
+    matches!(name, "StringBox" | "IntegerBox" | ...)
 }
 ```
 
@@ -90,9 +106,21 @@ match class.as_str() {
 MirType::Box(class.to_string())
 ```
 
-### Phase C: nyrt実装削除（3週目）
+### Phase C: 完全統一（3週目）
 
-#### 削除対象
+#### 1. 予約型保護の完全削除
+```rust
+// src/box_factory/mod.rs: 予約型保護を完全削除
+// この関数を削除またはコメントアウト
+// fn is_reserved_type(name: &str) -> bool { ... }
+
+// 登録処理から予約型チェックを削除
+// if is_reserved_type(type_name) && !factory.is_builtin_factory() {
+//     continue; // この部分を削除
+// }
+```
+
+#### 2. nyrt実装削除
 1. **crates/nyrt/src/lib.rs**
    - StringBox関連: 約150行
    - IntegerBox関連: 約50行
@@ -106,6 +134,12 @@ MirType::Box(class.to_string())
 3. **src/backend/llvm/compiler/codegen/instructions/newbox.rs**
    - コアBox最適化パス削除
 
+#### 3. デフォルト動作の確立
+```bash
+# 環境変数なしでプラグインBox使用
+./target/release/nyash test.nyash  # StringBox = プラグイン版
+```
+
 ### Phase D: Nyashコード実装（将来）
 
 ```nyash
diff --git a/nyash.toml b/nyash.toml
index 7b4d539c..3dc86e9a 100644
--- a/nyash.toml
+++ b/nyash.toml
@@ -333,3 +333,45 @@ singleton = false
 birth = { method_id = 0 }
 compile = { method_id = 1 }
 fini = { method_id = 4294967295 }
+# StringBox Plugin (Core Box replacement)
+[libraries."libnyash_string_plugin.so"]
+boxes = ["StringBox"]
+path = "target/release/libnyash_string_plugin.so"
+
+[libraries."libnyash_string_plugin.so".StringBox]
+type_id = 10
+abi_version = 1
+singleton = false
+
+[libraries."libnyash_string_plugin.so".StringBox.methods]
+birth = { method_id = 0 }
+get = { method_id = 1 }
+set = { method_id = 2 }
+concat = { method_id = 3 }
+length = { method_id = 4 }
+substring = { method_id = 5 }
+charCodeAt = { method_id = 6 }
+indexOf = { method_id = 7 }
+lastIndexOf = { method_id = 8 }
+replace = { method_id = 9 }
+split = { method_id = 10 }
+trim = { method_id = 11 }
+toUpper = { method_id = 12 }
+toLower = { method_id = 13 }
+fini = { method_id = 4294967295 }
+
+# IntegerBox Plugin (Core Box replacement)
+[libraries."libnyash_integer_plugin.so"]
+boxes = ["IntegerBox"]
+path = "target/release/libnyash_integer_plugin.so"
+
+[libraries."libnyash_integer_plugin.so".IntegerBox]
+type_id = 12
+abi_version = 1
+singleton = false
+
+[libraries."libnyash_integer_plugin.so".IntegerBox.methods]
+birth = { method_id = 0 }
+get = { method_id = 1 }
+set = { method_id = 2 }
+fini = { method_id = 4294967295 }
\ No newline at end of file
diff --git a/nyash.toml.backup2 b/nyash.toml.backup2
new file mode 100644
index 00000000..7b4d539c
--- /dev/null
+++ b/nyash.toml.backup2
@@ -0,0 +1,335 @@
+[env]
+# Put environment defaults used by tools or examples if needed
+# Enable using resolver by default (can be disabled with NYASH_SKIP_TOML_ENV=1)
+NYASH_ENABLE_USING = "1"
+# Enable dev sugar preexpand for @ local alias (line-head) during parsing
+NYASH_DEV_AT_LOCAL = "1"
+
+[using]
+paths = ["apps", "lib", "."]
+
+[modules]
+# Map logical namespaces to Nyash source paths (consumed by runner)
+selfhost.compiler.debug = "apps/selfhost/compiler/boxes/debug_box.nyash"
+selfhost.compiler.parser = "apps/selfhost/compiler/boxes/parser_box.nyash"
+selfhost.compiler.emitter = "apps/selfhost/compiler/boxes/emitter_box.nyash"
+selfhost.compiler.mir = "apps/selfhost/compiler/boxes/mir_emitter_box.nyash"
+selfhost.vm.json_cur = "apps/selfhost/vm/boxes/json_cur.nyash"
+selfhost.vm.json = "apps/selfhost/common/json_adapter.nyash"
+selfhost.vm.core = "apps/selfhost/vm/boxes/mini_vm_core.nyash"
+selfhost.vm.scan = "apps/selfhost/common/mini_vm_scan.nyash"
+selfhost.vm.binop = "apps/selfhost/common/mini_vm_binop.nyash"
+selfhost.vm.compare = "apps/selfhost/common/mini_vm_compare.nyash"
+selfhost.vm.prints = "apps/selfhost/vm/boxes/mini_vm_prints.nyash"
+selfhost.vm.seam = "apps/selfhost/vm/boxes/seam_inspector.nyash"
+
+# Temporary alias keys (migration aid; keys kept stable)
+selfhost.common.json = "apps/selfhost/common/json_adapter.nyash"
+selfhost.common.scan = "apps/selfhost/common/mini_vm_scan.nyash"
+selfhost.common.binop = "apps/selfhost/common/mini_vm_binop.nyash"
+selfhost.common.compare = "apps/selfhost/common/mini_vm_compare.nyash"
+
+# v2 Plugin libraries (loader reads these for TypeBox ABI)
+[libraries]
+[libraries."libnyash_filebox_plugin.so"]
+boxes = ["FileBox"]
+path = "plugins/nyash-filebox-plugin/target/release/libnyash_filebox_plugin.so"
+
+[libraries."libnyash_filebox_plugin.so".FileBox]
+type_id = 6
+abi_version = 1
+singleton = false
+
+[libraries."libnyash_filebox_plugin.so".FileBox.methods]
+birth = { method_id = 0 }
+open = { method_id = 1 }
+read = { method_id = 2 }
+write = { method_id = 3 }
+close = { method_id = 4 }
+exists = { method_id = 5 }
+copyFrom = { method_id = 7 }
+cloneSelf = { method_id = 8 }
+fini = { method_id = 4294967295 }
+
+[libraries."libnyash_path_plugin.so"]
+boxes = ["PathBox"]
+path = "plugins/nyash-path-plugin/target/release/libnyash_path_plugin.so"
+
+[libraries."libnyash_path_plugin.so".PathBox]
+type_id = 55
+abi_version = 1
+singleton = false
+
+[libraries."libnyash_path_plugin.so".PathBox.methods]
+birth = { method_id = 0 }
+join = { method_id = 1 }
+dirname = { method_id = 2 }
+basename = { method_id = 3 }
+extname = { method_id = 4 }
+isAbs = { method_id = 5 }
+normalize = { method_id = 6 }
+fini = { method_id = 4294967295 }
+
+[libraries."libnyash_math_plugin.so"]
+boxes = ["MathBox", "TimeBox"]
+path = "plugins/nyash-math-plugin/target/release/libnyash_math_plugin.so"
+
+[libraries."libnyash_math_plugin.so".MathBox]
+type_id = 50
+abi_version = 1
+singleton = false
+
+[libraries."libnyash_math_plugin.so".MathBox.methods]
+birth = { method_id = 0 }
+sqrt = { method_id = 1 }
+sin = { method_id = 2 }
+cos = { method_id = 3 }
+round = { method_id = 4 }
+fini = { method_id = 4294967295 }
+
+[libraries."libnyash_math_plugin.so".TimeBox]
+type_id = 51
+abi_version = 1
+singleton = false
+
+[libraries."libnyash_math_plugin.so".TimeBox.methods]
+birth = { method_id = 0 }
+now = { method_id = 1 }
+fini = { method_id = 4294967295 }
+
+[libraries."libnyash_regex_plugin.so"]
+boxes = ["RegexBox"]
+path = "plugins/nyash-regex-plugin/target/release/libnyash_regex_plugin.so"
+
+[libraries."libnyash_regex_plugin.so".RegexBox]
+type_id = 52
+abi_version = 1
+singleton = false
+
+[libraries."libnyash_regex_plugin.so".RegexBox.methods]
+birth = { method_id = 0 }
+compile = { method_id = 1 }
+isMatch = { method_id = 2 }
+find = { method_id = 3 }
+replaceAll = { method_id = 4 }
+split = { method_id = 5 }
+fini = { method_id = 4294967295 }
+
+[libraries."libnyash_net_plugin.so"]
+boxes = ["ClientBox", "ResponseBox", "RequestBox", "ServerBox", "SockServerBox", "SockClientBox", "SockConnBox"]
+path = "plugins/nyash-net-plugin/target/release/libnyash_net_plugin.so"
+
+[libraries."libnyash_net_plugin.so".ClientBox]
+type_id = 23
+abi_version = 1
+singleton = false
+
+[libraries."libnyash_net_plugin.so".ClientBox.methods]
+birth = { method_id = 0 }
+get = { method_id = 1 }
+post = { method_id = 2 }
+fini = { method_id = 4294967295 }
+
+[libraries."libnyash_net_plugin.so".ResponseBox]
+type_id = 22
+abi_version = 1
+singleton = false
+
+[libraries."libnyash_net_plugin.so".ResponseBox.methods]
+birth = { method_id = 0 }
+setStatus = { method_id = 1 }
+setHeader = { method_id = 2 }
+write = { method_id = 3 }
+readBody = { method_id = 4 }
+getStatus = { method_id = 5 }
+getHeader = { method_id = 6 }
+fini = { method_id = 4294967295 }
+
+[libraries."libnyash_net_plugin.so".RequestBox]
+type_id = 21
+abi_version = 1
+singleton = false
+
+[libraries."libnyash_net_plugin.so".RequestBox.methods]
+birth = { method_id = 0 }
+path = { method_id = 1 }
+readBody = { method_id = 2 }
+respond = { method_id = 3 }
+fini = { method_id = 4294967295 }
+
+[libraries."libnyash_net_plugin.so".ServerBox]
+type_id = 20
+abi_version = 1
+singleton = false
+
+[libraries."libnyash_net_plugin.so".ServerBox.methods]
+birth = { method_id = 0 }
+start = { method_id = 1 }
+stop = { method_id = 2 }
+accept = { method_id = 3 }
+fini = { method_id = 4294967295 }
+
+[libraries."libnyash_net_plugin.so".SockServerBox]
+type_id = 30
+abi_version = 1
+singleton = false
+
+[libraries."libnyash_net_plugin.so".SockServerBox.methods]
+birth = { method_id = 0 }
+start = { method_id = 1 }
+stop = { method_id = 2 }
+accept = { method_id = 3 }
+acceptTimeout = { method_id = 4 }
+fini = { method_id = 4294967295 }
+
+[libraries."libnyash_net_plugin.so".SockClientBox]
+type_id = 32
+abi_version = 1
+singleton = false
+
+[libraries."libnyash_net_plugin.so".SockClientBox.methods]
+birth = { method_id = 0 }
+connect = { method_id = 1 }
+fini = { method_id = 4294967295 }
+
+[libraries."libnyash_net_plugin.so".SockConnBox]
+type_id = 31
+abi_version = 1
+singleton = false
+
+[libraries."libnyash_net_plugin.so".SockConnBox.methods]
+birth = { method_id = 0 }
+send = { method_id = 1 }
+recv = { method_id = 2 }
+close = { method_id = 3 }
+recvTimeout = { method_id = 4 }
+fini = { method_id = 4294967295 }
+
+[libraries."libnyash_encoding_plugin.so"]
+boxes = ["EncodingBox"]
+path = "plugins/nyash-encoding-plugin/target/release/libnyash_encoding_plugin.so"
+
+[libraries."libnyash_encoding_plugin.so".EncodingBox]
+type_id = 53
+abi_version = 1
+singleton = false
+
+[libraries."libnyash_encoding_plugin.so".EncodingBox.methods]
+birth = { method_id = 0 }
+toUtf8Bytes = { method_id = 1 }
+fromUtf8Bytes = { method_id = 2 }
+base64Encode = { method_id = 3 }
+base64Decode = { method_id = 4 }
+hexEncode = { method_id = 5 }
+hexDecode = { method_id = 6 }
+fini = { method_id = 4294967295 }
+
+[libraries."libnyash_json_plugin.so"]
+boxes = ["JsonDocBox", "JsonNodeBox"]
+path = "plugins/nyash-json-plugin/target/release/libnyash_json_plugin.so"
+
+[libraries."libnyash_json_plugin.so".JsonDocBox]
+type_id = 70
+abi_version = 1
+singleton = false
+
+[libraries."libnyash_json_plugin.so".JsonDocBox.methods]
+birth = { method_id = 0 }
+parse = { method_id = 1 }
+root = { method_id = 2 }
+error = { method_id = 3 }
+fini = { method_id = 4294967295 }
+
+[libraries."libnyash_json_plugin.so".JsonNodeBox]
+type_id = 71
+abi_version = 1
+singleton = false
+
+[libraries."libnyash_json_plugin.so".JsonNodeBox.methods]
+birth = { method_id = 0 }
+kind = { method_id = 1 }
+get = { method_id = 2 }
+size = { method_id = 3 }
+at = { method_id = 4 }
+str = { method_id = 5 }
+int = { method_id = 6 }
+bool = { method_id = 7 }
+fini = { method_id = 4294967295 }
+
+[libraries."libnyash_toml_plugin.so"]
+boxes = ["TOMLBox"]
+path = "plugins/nyash-toml-plugin/target/release/libnyash_toml_plugin.so"
+
+[libraries."libnyash_toml_plugin.so".TOMLBox]
+type_id = 54
+abi_version = 1
+singleton = false
+
+[libraries."libnyash_toml_plugin.so".TOMLBox.methods]
+birth = { method_id = 0 }
+parse = { method_id = 1 }
+get = { method_id = 2 }
+toJson = { method_id = 3 }
+fini = { method_id = 4294967295 }
+
+# Python (v2 TypeBox) plugins
+[libraries."libnyash_python_plugin.so"]
+boxes = ["PyRuntimeBox", "PyObjectBox"]
+path = "plugins/nyash-python-plugin/target/release/libnyash_python_plugin.so"
+
+[libraries."libnyash_python_plugin.so".PyRuntimeBox]
+type_id = 40
+abi_version = 1
+singleton = false
+
+[libraries."libnyash_python_plugin.so".PyRuntimeBox.methods]
+birth = { method_id = 0 }
+eval = { method_id = 1 }
+import = { method_id = 2 }
+evalR = { method_id = 11 }
+importR = { method_id = 12 }
+fini = { method_id = 4294967295 }
+
+[libraries."libnyash_python_plugin.so".PyObjectBox]
+type_id = 41
+abi_version = 1
+singleton = false
+
+[libraries."libnyash_python_plugin.so".PyObjectBox.methods]
+birth = { method_id = 0 }
+getattr = { method_id = 1 }
+call = { method_id = 2 }
+str = { method_id = 3 }
+callKw = { method_id = 5 }
+getattrR = { method_id = 11 }
+callR = { method_id = 12 }
+callKwR = { method_id = 15 }
+fini = { method_id = 4294967295 }
+
+[libraries."libnyash_python_parser_plugin.so"]
+boxes = ["PythonParserBox"]
+path = "plugins/nyash-python-parser-plugin/target/release/libnyash_python_parser_plugin.so"
+
+[libraries."libnyash_python_parser_plugin.so".PythonParserBox]
+type_id = 60
+abi_version = 1
+singleton = false
+
+[libraries."libnyash_python_parser_plugin.so".PythonParserBox.methods]
+birth = { method_id = 0 }
+parse = { method_id = 1 }
+fini = { method_id = 4294967295 }
+
+[libraries."libnyash_python_compiler_plugin.so"]
+boxes = ["PythonCompilerBox"]
+path = "plugins/nyash-python-compiler-plugin/target/release/libnyash_python_compiler_plugin.so"
+
+[libraries."libnyash_python_compiler_plugin.so".PythonCompilerBox]
+type_id = 61
+abi_version = 1
+singleton = false
+
+[libraries."libnyash_python_compiler_plugin.so".PythonCompilerBox.methods]
+birth = { method_id = 0 }
+compile = { method_id = 1 }
+fini = { method_id = 4294967295 }
diff --git a/src/box_factory/mod.rs b/src/box_factory/mod.rs
index ec7036f6..b9f9c571 100644
--- a/src/box_factory/mod.rs
+++ b/src/box_factory/mod.rs
@@ -71,6 +71,15 @@ impl UnifiedBoxRegistry {
         let mut cache = self.type_cache.write().unwrap();
         // Reserved core types that must remain builtin-owned
         fn is_reserved_type(name: &str) -> bool {
+            // Phase 15.5: 環境変数でプラグイン優先モード時は保護解除
+            if std::env::var("NYASH_USE_PLUGIN_BUILTINS").is_ok() {
+                if let Ok(types) = std::env::var("NYASH_PLUGIN_OVERRIDE_TYPES") {
+                    if types.split(',').any(|t| t.trim() == name) {
+                        return false;  // 予約型として扱わない
+                    }
+                }
+            }
+
             matches!(
                 name,
                 // Core value types