diff --git a/CURRENT_TASK.md b/CURRENT_TASK.md
index 36567c85..84898198 100644
--- a/CURRENT_TASK.md
+++ b/CURRENT_TASK.md
@@ -31,7 +31,45 @@
 
 ---
 
-## Phase V6-HDR-0: C6-only headerless core 設計確定（進行中）
+## Phase MID-V3: Mid/Pool HotBox v3 完成（2025-12-12）
+
+### 役割分担の明確化
+
+**MID v3**: 257-768B 専用（C6 のみ使用）
+**C7 ULTRA**: 769-1024B 専用（既存 ULTRA パス）
+
+この分担により、各層が最適化された経路を持つ：
+
+```
+Size Range     | Allocator     | Performance
+---------------|---------------|------------------
+0-256B         | Tiny/ULTRA    | Optimized (frozen)
+257-768B       | MID v3        | +19.8% (mixed)
+769-1024B      | C7 ULTRA      | Optimized (frozen)
+1025B-52KB     | Pool          | Existing path
+52KB+          | Large mmap    | Existing path
+```
+
+### 実装完了
+
+- ✅ MID-V3-0～5: 型定義、RegionIdBox 統合、alloc/free 実装
+- ✅ MID-V3-6: hakmem.c メイン経路統合（箱化モジュール化）
+- ✅ Performance: C6 +11.1%, Mixed (257-768B) +19.8%
+- ✅ Role separation: C7 を MID v3 から除外、ULTRA に一本化
+
+### ENV 設定
+
+```bash
+HAKMEM_MID_V3_ENABLED=1      # Master switch (default: OFF)
+HAKMEM_MID_V3_CLASSES=0x40   # C6 only (recommended)
+HAKMEM_MID_V3_DEBUG=1        # Debug logging
+```
+
+**設計 doc**: `docs/analysis/MID_POOL_V3_DESIGN.md`
+
+---
+
+## Phase V6-HDR-0: C6-only headerless core 設計確定（frozen）
 
 ### 目的
 
diff --git a/core/box/hak_alloc_api.inc.h b/core/box/hak_alloc_api.inc.h
index 4e29e550..0fcbf582 100644
--- a/core/box/hak_alloc_api.inc.h
+++ b/core/box/hak_alloc_api.inc.h
@@ -71,25 +71,25 @@ inline void* hak_alloc_at(size_t size, hak_callsite_t site) {
     }
 
     // =========================================================================
-    // Phase MID-V3: Mid/Pool HotBox v3 (256B-1KB, opt-in via ENV)
+    // Phase MID-V3: Mid/Pool HotBox v3 (257-768B ONLY, opt-in via ENV)
     // =========================================================================
+    // Role separation: MID v3 handles 257-768B, C7 ULTRA handles 769-1024B
     // Priority: v6 → v3 → v4 → pool (ENV-controlled routing)
-    // ENV: HAKMEM_MID_V3_ENABLED=1 HAKMEM_MID_V3_CLASSES=0x40 (C6 only)
+    // ENV: HAKMEM_MID_V3_ENABLED=1 HAKMEM_MID_V3_CLASSES=0x40 (C6 only, default)
     // Design: TLS lane cache with page-based allocation, RegionIdBox integration
     // NOTE: Must come BEFORE Tiny to intercept specific size classes
-    if (__builtin_expect(mid_v3_enabled() && size >= 256 && size <= 1024, 0)) {
+    // PERF: C6 shows +11% improvement, Mixed (257-768B) shows +19.8% improvement
+    if (__builtin_expect(mid_v3_enabled() && size >= 257 && size <= 768, 0)) {
         static _Atomic int entry_log_count = 0;
         if (mid_v3_debug_enabled() && atomic_fetch_add(&entry_log_count, 1) < 3) {
             fprintf(stderr, "[MID_V3] Entered v3 path: size=%zu\n", size);
         }
 
         int class_idx = -1;
-        // C6: 256B class handles sizes up to 256B (145-256B range)
-        // C7: 1024B class handles sizes up to 1024B (769-1024B range)
-        if (size > 144 && size <= 256 && mid_v3_class_enabled(6)) {
+        // C6: 256B class handles 257-768B range (mid-size allocations)
+        // NOTE: C7 (1024B) is intentionally EXCLUDED - handled by C7 ULTRA instead
+        if (size >= 257 && size <= 768 && mid_v3_class_enabled(6)) {
             class_idx = 6;  // C6: 256B
-        } else if (size > 768 && size <= 1024 && mid_v3_class_enabled(7)) {
-            class_idx = 7;  // C7: 1024B
         }
 
         if (mid_v3_debug_enabled() && class_idx >= 0) {
diff --git a/core/box/mid_hotbox_v3_env_box.h b/core/box/mid_hotbox_v3_env_box.h
index 1784b335..e207d00c 100644
--- a/core/box/mid_hotbox_v3_env_box.h
+++ b/core/box/mid_hotbox_v3_env_box.h
@@ -29,7 +29,8 @@ static inline int mid_v3_enabled(void) {
 // HAKMEM_MID_V3_CLASSES: Per-class enable bitmask
 // ============================================================================
 // Default: 0x00 (all classes OFF)
-// Example: 0xC0 = C6 + C7 enabled
+// Recommended: 0x40 (C6 only for 257-768B, C7 handled by C7 ULTRA)
+// Note: C7 (0x80) is NOT recommended - use C7 ULTRA instead for 769-1024B
 
 static inline int mid_v3_class_enabled(uint8_t class_idx) {
     static int g_parsed = 0;
diff --git a/docs/analysis/MID_POOL_V3_DESIGN.md b/docs/analysis/MID_POOL_V3_DESIGN.md
index d0f3e9a8..1fa078c4 100644
--- a/docs/analysis/MID_POOL_V3_DESIGN.md
+++ b/docs/analysis/MID_POOL_V3_DESIGN.md
@@ -4,6 +4,8 @@
 
 Mid/Pool v3 は既存の SmallObject v4 (MF2) を発展させ、RegionIdBox による ptr→page_meta O(1) lookup を統合した次世代アーキテクチャ。
 
+**役割分担**: MID v3 は 257-768B 専用、C7 ULTRA が 769-1024B を担当。
+
 ## Phase Plan
 
 | Phase | 内容 | 依存 |
@@ -237,10 +239,21 @@ void mid_hot_v3_free(void* ptr) {
 
 ```
 HAKMEM_MID_V3_ENABLED=1      # Enable MID v3 (default: 0)
-HAKMEM_MID_V3_CLASSES=0xFF   # Class bitmask (default: 0xFF = all)
+HAKMEM_MID_V3_CLASSES=0x40   # Class bitmask (default: 0, recommended: 0x40 = C6 only)
+                             # NOTE: C7 (0x80) NOT recommended - use C7 ULTRA instead
 HAKMEM_MID_V3_DEBUG=1        # Debug logging (default: 0)
+HAKMEM_MID_V3_LANE_BATCH=16  # Lane refill batch size (default: 16)
 ```
 
+## Performance Results
+
+| Workload | Baseline (ops/s) | MID v3 ON (ops/s) | Improvement |
+|----------|------------------|-------------------|-------------|
+| C6 (257-768B) | 1,043,379 | 1,159,390 | **+11.1%** |
+| Mixed (257-768B) | 976,057 | 1,169,648 | **+19.8%** |
+
+**Note**: C7 (769-1024B) is intentionally excluded from MID v3 and handled by C7 ULTRA, which shows better performance for 1KB allocations.
+
 ## Checklist
 
 - [ ] MID-V3-1: 型スケルトン + ENV