diff --git a/core/front/tiny_heap_v2.h b/core/front/tiny_heap_v2.h
index 0a6fa563..a694dec4 100644
--- a/core/front/tiny_heap_v2.h
+++ b/core/front/tiny_heap_v2.h
@@ -46,28 +46,31 @@ extern __thread TinyHeapV2Mag g_tiny_heap_v2_mag[TINY_NUM_CLASSES];
 extern __thread TinyHeapV2Stats g_tiny_heap_v2_stats[TINY_NUM_CLASSES];
 
 // Enable flag (cached)
+// ENV: HAKMEM_TINY_HEAP_V2
+// - 0: Disable TinyHeapV2 (use existing front only)
+// - 1 (default): Enable TinyHeapV2 (Mode 0 Stealing, +18% @ 32B)
 static inline int tiny_heap_v2_enabled(void) {
     static int g_enable = -1;
-    static int g_first_call = 1;
     if (__builtin_expect(g_enable == -1, 0)) {
         const char* e = getenv("HAKMEM_TINY_HEAP_V2");
-        g_enable = (e && *e && *e != '0') ? 1 : 0;
+        if (e && *e) {
+            g_enable = (*e != '0') ? 1 : 0;
+        } else {
+            g_enable = 1;  // Default: ON (Phase 13-B decision)
+        }
+#if !HAKMEM_BUILD_RELEASE
         fprintf(stderr, "[HeapV2-INIT] tiny_heap_v2_enabled() called: ENV='%s' → %d\n",
                 e ? e : "(null)", g_enable);
         fflush(stderr);
-    }
-    if (g_first_call && g_enable) {
-        fprintf(stderr, "[HeapV2-FIRST] Returning enabled=%d\n", g_enable);
-        fflush(stderr);
-        g_first_call = 0;
+#endif
     }
     return g_enable;
 }
 
 // Class-specific enable mask (cached)
 // ENV: HAKMEM_TINY_HEAP_V2_CLASS_MASK (bitmask: bit 0=C0, bit 1=C1, bit 2=C2, bit 3=C3)
-// Default: 0xF (all classes C0-C3 enabled)
-// Example: 0x2 = C1 only, 0x8 = C3 only, 0x6 = C1+C2
+// Default: 0xE (C1-C3 only, skip C0 8B due to -5% regression)
+// Example: 0x2 = C1 only, 0x8 = C3 only, 0x6 = C1+C2, 0xF = all C0-C3
 static inline int tiny_heap_v2_class_enabled(int class_idx) {
     static int g_class_mask = -1;
     if (__builtin_expect(g_class_mask == -1, 0)) {
@@ -78,7 +81,7 @@ static inline int tiny_heap_v2_class_enabled(int class_idx) {
             long val = strtol(e, &endptr, 0);  // 0 = auto-detect base (0x for hex, else decimal)
             g_class_mask = (int)val;
         } else {
-            g_class_mask = 0xF;  // Default: C0-C3 all enabled
+            g_class_mask = 0xE;  // Default: C1-C3 (16/32/64B), skip C0 8B (-5% regression)
         }
     }