diff --git a/docs/development/current/main/01-JoinIR-Selfhost-INDEX.md b/docs/development/current/main/01-JoinIR-Selfhost-INDEX.md
index f3c4e0fa..c872dd12 100644
--- a/docs/development/current/main/01-JoinIR-Selfhost-INDEX.md
+++ b/docs/development/current/main/01-JoinIR-Selfhost-INDEX.md
@@ -42,7 +42,14 @@ JoinIR の箱構造と責務、ループ/if の lowering パターンを把握
    - `docs/development/current/main/phase81-pattern2-exitline-contract.md`（promoted carriers の ExitLine 契約検証）
 5. Boxification feedback（Phase 78–85 の振り返りと Phase 86 推奨）
    - `docs/development/current/main/phase78-85-boxification-feedback.md`
-4. 代表的な Phase 文書（現役ラインとの接点だけ絞ったもの）
+6. Phase 86: Carrier Init Builder + Error Tags ✅
+   - **Status**: COMPLETE (2025-12-13)
+   - **Modules**:
+     - `src/mir/builder/control_flow/joinir/merge/carrier_init_builder.rs` (+8 tests)
+     - `src/mir/join_ir/lowering/error_tags.rs` (+5 tests)
+   - **Achievements**: SSOT 確立（CarrierInit → ValueId 生成統一、エラータグ中央化、DebugOutputBox 完全移行）
+   - **Impact**: 987/987 tests PASS, +13 unit tests, Single Responsibility validated
+7. 代表的な Phase 文書（現役ラインとの接点だけ絞ったもの）
    - `docs/development/current/main/phase33-16-INDEX.md`
    - `docs/development/current/main/phase33-17-joinir-modularization-analysis.md`
    - `docs/development/current/main/phase183-selfhost-depth2-joinir-status.md`
diff --git a/docs/development/current/main/10-Now.md b/docs/development/current/main/10-Now.md
index 3d9e2beb..42798ba5 100644
--- a/docs/development/current/main/10-Now.md
+++ b/docs/development/current/main/10-Now.md
@@ -17,8 +17,24 @@
 - 残課題（JoinIR ライン）：
   - JoinIR→MIR merge の一般化（複雑な Select/PHI パターンの統合）
   - JsonParserBox など実アプリ側での長期運用テスト
-  - Boxification（箱化）の振り返りと、次の小粒リファクタ（Phase 86 推奨）:
-    - `docs/development/current/main/phase78-85-boxification-feedback.md`
+
+### Phase 86: Carrier Init Builder + Error Tags (2025-12-13) ✅
+
+**Status**: COMPLETE - SSOT 確立完了
+
+**Achievements**:
+- Carrier Initialization Builder: CarrierInit → ValueId 生成を単一関数化 (carrier_init_builder.rs, +8 tests)
+- Error Message Centralization: JoinIR エラータグを SSOT 化 (error_tags.rs, +5 tests)
+- DebugOutputBox Migration: 全 JOINIR_DEBUG 使用箇所を DebugOutputBox に統一
+
+**Impact**:
+- Tests: 987/987 PASS (+13 unit tests from Phase 86)
+- SSOT modules: carrier_init_builder, error_tags (一貫性向上)
+- Code quality: Single Responsibility, Testability First principles validated
+
+**Next**: Phase 87 (LLVM exe line SSOT)
+
+**Reference**: phase78-85-boxification-feedback.md, carrier_init_builder.rs, error_tags.rs
 
 ### Scope / BindingId（dev-only の段階移行ライン）
 
diff --git a/docs/development/current/main/joinir-architecture-overview.md b/docs/development/current/main/joinir-architecture-overview.md
index c2e5e795..8082c7be 100644
--- a/docs/development/current/main/joinir-architecture-overview.md
+++ b/docs/development/current/main/joinir-architecture-overview.md
@@ -683,6 +683,20 @@ Local Region (1000..=LOCAL_MAX):
   - **Phase 200-A**: 既存実装に委譲する skeleton。
   - **Phase 200-B**: CapturedEnv の変数を condition_bindings に追加する実装予定。
 
+- **Phase 86 SSOT Modules (2025-12-13) ✅**
+
+  - **carrier_init_builder.rs**: CarrierInit → ValueId 生成の SSOT
+    - Location: `src/mir/builder/control_flow/joinir/merge/carrier_init_builder.rs`
+    - Function: `init_value(builder, init, host_id, name, debug) -> ValueId`
+    - Purpose: FromHost/BoolConst/LoopLocalZero の統一変換（34+ 行の重複 match 削減）
+    - Tests: 8 unit tests
+
+  - **error_tags.rs**: JoinIR エラーメッセージ整形の SSOT
+    - Location: `src/mir/join_ir/lowering/error_tags.rs`
+    - Functions: `freeze()`, `exit_line_contract()`, `ownership_relay_unsupported()`, `pattern_detection_failed()`, `lowering_error()`
+    - Purpose: 一貫したエラータグフォーマット（typo 防止、重複文字列削減）
+    - Tests: 5 unit tests
+
 ### 2.4 expr result ライン（式としての戻り値）
 
 - **exit_phi_builder**