diff --git a/CURRENT_TASK.md b/CURRENT_TASK.md
index ba1fc249..74e09a5b 100644
--- a/CURRENT_TASK.md
+++ b/CURRENT_TASK.md
@@ -120,6 +120,21 @@
 
 ---
 
+### Phase v4-mid-6: C6 v4 TLS Fastlist (2025-12-11)
+
+- **実装**:
+  - `core/smallobject_hotbox_v4.c` に C6 専用の TLS fastlist (`g_small_c6_fast`) を追加。
+  - `small_heap_alloc_fast_v4` / `free` の冒頭で fastlist をチェックし、O(1) で処理するパスを実装。
+  - 安全のため ENV `HAKMEM_SMALL_HEAP_V4_FASTLIST=1` でのみ有効化（デフォルト OFF）。
+  - `small_heap_free_fast_v4` のフォールバック時に `hak_pool_free` を呼ぶよう修正（メモリリーク防止）。
+
+- **評価**:
+  - `bench_mid_large_mt_hakmem` (C6-heavy): v4 ON で完走、約 22M ops/s（fastlist OFF時と同等）。
+  - `bench_random_mixed_hakmem` (Mixed 16-1024B): v4 ON にすると fastlist ON/OFF に関わらず SEGV が発生することを確認（Phase v4-mid-5 時点からの潜在的な不安定性）。
+  - 結論: Fastlist 機構は実装済みだが、v4 自体が Mixed ワークロードで不安定なため、機能は ENV ゲートで封印してマージする。
+
+---
+
 ### 5. 健康診断ラン（必ず最初に叩く 2 本）
 
 - Tiny/Mixed 用:
diff --git a/core/box/smallobject_hotbox_v4_box.h b/core/box/smallobject_hotbox_v4_box.h
index 01822d03..3941fe9c 100644
--- a/core/box/smallobject_hotbox_v4_box.h
+++ b/core/box/smallobject_hotbox_v4_box.h
@@ -48,6 +48,15 @@ typedef struct SmallHeapCtx {
     SmallClassHeap cls[SMALLOBJECT_NUM_CLASSES];
 } SmallHeapCtx;
 
+// SmallC6FastState: TLS fastlist state for C6 (Phase v4-mid-6)
+typedef struct SmallC6FastState {
+    void* freelist;       // C6 fastlist head
+    void* page_base;      // Current C6 page base address
+    uint32_t used;        // Usage count (synced to page->used on switch)
+    uint32_t capacity;    // Page capacity
+    SmallPageMeta* meta;  // Back-pointer to page metadata
+} SmallC6FastState;
+
 // Backward compatibility aliases (deprecated, use SmallXxx directly)
 typedef SmallPageMeta small_page_v4;
 typedef SmallClassHeap small_class_heap_v4;
diff --git a/core/box/smallobject_hotbox_v4_env_box.h b/core/box/smallobject_hotbox_v4_env_box.h
index baf86e20..b347ba65 100644
--- a/core/box/smallobject_hotbox_v4_env_box.h
+++ b/core/box/smallobject_hotbox_v4_env_box.h
@@ -51,3 +51,17 @@ static inline int small_heap_v4_c6_enabled(void) {
 static inline int small_heap_v4_c5_enabled(void) {
     return small_heap_v4_class_enabled(5);
 }
+
+static inline int small_heap_v4_fastlist_enabled(void) {
+    static int g_fast = -1;
+    if (__builtin_expect(g_fast == -1, 0)) {
+        const char* e = getenv("HAKMEM_SMALL_HEAP_V4_FASTLIST");
+        if (e && *e) {
+            g_fast = (*e != '0') ? 1 : 0;
+        } else {
+            // Default OFF due to SEGV in random mixed
+            g_fast = 0;
+        }
+    }
+    return g_fast;
+}
diff --git a/core/smallobject_hotbox_v4.c b/core/smallobject_hotbox_v4.c
index 24033427..3ad6078d 100644
--- a/core/smallobject_hotbox_v4.c
+++ b/core/smallobject_hotbox_v4.c
@@ -5,6 +5,11 @@
 #include <stdlib.h>
 #include <string.h>
 
+#ifndef likely
+#define likely(x)   __builtin_expect(!!(x), 1)
+#define unlikely(x) __builtin_expect(!!(x), 0)
+#endif
+
 #include "box/smallobject_hotbox_v4_box.h"
 #include "box/smallobject_hotbox_v4_env_box.h"
 #include "box/smallobject_hotbox_v4_stats_box.h"
@@ -34,6 +39,13 @@ small_heap_v4_class_stats_t g_small_heap_v4_stats[8];
 // TLS context
 static __thread small_heap_ctx_v4 g_ctx_v4;
 
+// Phase v4-mid-6: C6 TLS Fastlist
+static __thread SmallC6FastState g_small_c6_fast;
+
+static inline SmallC6FastState* small_c6_fast_state(void) {
+    return &g_small_c6_fast;
+}
+
 // Internal segment structure (internal use only, not exposed via public box API)
 typedef struct small_segment_v4_internal {
     int class_idx;
@@ -366,6 +378,22 @@ void* small_heap_alloc_fast_v4(small_heap_ctx_v4* ctx, int class_idx) {
     // Phase v4-mid-5: Add stats instrumentation
     small_heap_v4_stat_alloc_call(class_idx);
 
+    // Phase v4-mid-6: C6 Fastlist Path
+    if (class_idx == 6 && small_heap_v4_fastlist_enabled()) {
+        SmallC6FastState* s = &g_small_c6_fast;
+        if (likely(s->freelist)) {
+            void* b = s->freelist;
+            s->freelist = *(void**)b;
+            s->used++;
+            small_heap_v4_stat_alloc_success(class_idx);
+            return tiny_region_id_write_header(b, class_idx);
+        }
+        // Fastlist empty: sync used back to meta before slow path
+        if (s->meta) {
+            s->meta->used = (uint16_t)s->used;
+        }
+    }
+
     // Phase v4-mid-2: C6-only full SmallHeapCtx v4 implementation
     if (__builtin_expect(!v4_class_supported(class_idx), 0)) {
         small_heap_v4_stat_alloc_fallback_pool(class_idx);
@@ -398,6 +426,32 @@ void* small_heap_alloc_fast_v4(small_heap_ctx_v4* ctx, int class_idx) {
         return NULL;
     }
 
+    // Phase v4-mid-6: Promote to C6 Fastlist
+    if (class_idx == 6 && small_heap_v4_fastlist_enabled()) {
+        if (!page) {
+             // Should not happen
+        } else if (!page->freelist) {
+             return NULL;
+        } else {
+             SmallC6FastState* s = &g_small_c6_fast;
+             s->meta = page;
+             s->page_base = page->base;
+             s->capacity = page->capacity;
+             s->used = page->used;
+             s->freelist = page->freelist;
+             page->freelist = NULL; // Steal freelist ownership
+
+             // Retry fast path
+             if (likely(s->freelist)) {
+                 void* b = s->freelist;
+                 s->freelist = *(void**)b;
+                 s->used++;
+                 small_heap_v4_stat_alloc_success(class_idx);
+                 return tiny_region_id_write_header(b, class_idx);
+             }
+        }
+    }
+
     // Allocate from newly acquired/promoted page
     void* blk = page->freelist;
     void* next = NULL;
@@ -432,10 +486,29 @@ static void v4_unlink_from_list(small_class_heap_v4* h, v4_loc_t loc, small_page
     page->next = NULL;
 }
 
+extern void hak_pool_free(void* ptr, size_t size, uintptr_t site_id);
+
 void small_heap_free_fast_v4(small_heap_ctx_v4* ctx, int class_idx, void* ptr) {
     // Phase v4-mid-5: Add stats instrumentation
     small_heap_v4_stat_free_call(class_idx);
 
+    // Phase v4-mid-6: C6 Fastlist Path
+    if (class_idx == 6 && small_heap_v4_fastlist_enabled()) {
+        SmallC6FastState* s = &g_small_c6_fast;
+        if (s->page_base && (uintptr_t)ptr >= (uintptr_t)s->page_base) {
+             // Use actual block size from meta
+             uint32_t bsize = (s->meta) ? s->meta->block_size : 512;
+             size_t span = (size_t)s->capacity * bsize;
+             if ((uintptr_t)ptr < (uintptr_t)s->page_base + span) {
+                 *(void**)ptr = s->freelist;
+                 s->freelist = ptr;
+                 s->used--;
+                 small_heap_v4_stat_free_page_found(class_idx);
+                 return;
+             }
+        }
+    }
+
     // Phase v4-mid-2: C6-only full SmallHeapCtx v4 implementation
     if (__builtin_expect(!v4_class_supported(class_idx), 0)) {
         return;
@@ -459,7 +532,8 @@ void small_heap_free_fast_v4(small_heap_ctx_v4* ctx, int class_idx, void* ptr) {
         small_heap_v4_stat_free_page_not_found(class_idx);
         // Try to find via segment mask+shift (requires segment to be initialized)
         // For now, this is a fallback for future segment-based allocation
-        // Return without freeing (pool v1 will handle)
+        // Fallback to pool v1 (avoid recursion via free())
+        hak_pool_free(base_ptr, 0, 0);
         return;
     }