diff --git a/core/box/tiny_free_route_cache_env_box.h b/core/box/tiny_free_route_cache_env_box.h
new file mode 100644
index 00000000..35309008
--- /dev/null
+++ b/core/box/tiny_free_route_cache_env_box.h
@@ -0,0 +1,32 @@
+// tiny_free_route_cache_env_box.h - Phase 3 D1: Free Path Route Cache
+// 役割:
+//   - ENV gate for HAKMEM_FREE_STATIC_ROUTE (default OFF)
+//   - Enable cached route lookup in free path to eliminate tiny_route_for_class() overhead
+//
+// Target: 4.39% self + 24.78% children in tiny_route_for_class() during free
+// Expected gain: +1-2% in Mixed workload
+//
+#pragma once
+
+#include <stdlib.h>
+#include <stdio.h>
+
+static int g_free_static_route_enabled = -1;  // -1 = uninitialized
+
+// tiny_free_static_route_enabled() - Check if free path route cache is enabled
+// Returns: 1 if enabled, 0 if disabled (default)
+// ENV: HAKMEM_FREE_STATIC_ROUTE=0/1
+static inline int tiny_free_static_route_enabled(void) {
+    if (__builtin_expect(g_free_static_route_enabled >= 0, 1)) {
+        return g_free_static_route_enabled;
+    }
+
+    const char* e = getenv("HAKMEM_FREE_STATIC_ROUTE");
+    int enabled = (e && *e && *e != '0') ? 1 : 0;
+    g_free_static_route_enabled = enabled;
+
+    if (enabled) {
+        fprintf(stderr, "[FREE_STATIC_ROUTE] enabled (Phase 3 D1)\n");
+    }
+    return enabled;
+}
diff --git a/core/front/malloc_tiny_fast.h b/core/front/malloc_tiny_fast.h
index c3922bd9..50bef2ae 100644
--- a/core/front/malloc_tiny_fast.h
+++ b/core/front/malloc_tiny_fast.h
@@ -68,6 +68,7 @@
 #include "../box/free_tiny_fast_hotcold_env_box.h" // Phase FREE-TINY-FAST-HOTCOLD-OPT-1: ENV control
 #include "../box/free_tiny_fast_hotcold_stats_box.h" // Phase FREE-TINY-FAST-HOTCOLD-OPT-1: Stats
 #include "../box/tiny_metadata_cache_hot_box.h" // Phase 3 C2: Policy hot cache (metadata cache optimization)
+#include "../box/tiny_free_route_cache_env_box.h" // Phase 3 D1: Free path route cache
 
 // Helper: current thread id (low 32 bits) for owner check
 #ifndef TINY_SELF_U32_LOCAL_DEFINED
@@ -369,7 +370,19 @@ static int free_tiny_fast_cold(void* ptr, void* base, int class_idx)
 {
     FREE_TINY_FAST_HOTCOLD_STAT_INC(cold_hit);
 
-    tiny_route_kind_t route = tiny_route_for_class((uint8_t)class_idx);
+    // Phase 3 D1: Free path route cache (eliminate tiny_route_for_class overhead)
+    tiny_route_kind_t route;
+    if (__builtin_expect(tiny_free_static_route_enabled(), 0)) {
+        // Use cached route (bypasses tiny_route_for_class())
+        route = g_tiny_route_class[(unsigned)class_idx & 7u];
+        if (__builtin_expect(route == TINY_ROUTE_LEGACY && !g_tiny_route_snapshot_done, 0)) {
+            // Fallback if uninitialized
+            route = tiny_route_for_class((uint8_t)class_idx);
+        }
+    } else {
+        // Standard path
+        route = tiny_route_for_class((uint8_t)class_idx);
+    }
     const int use_tiny_heap = tiny_route_is_heap_kind(route);
     const TinyFrontV3Snapshot* front_snap =
         __builtin_expect(tiny_front_v3_enabled(), 0) ? tiny_front_v3_snapshot_get() : NULL;
@@ -763,7 +776,19 @@ static inline int free_tiny_fast(void* ptr) {
 
 legacy_fallback:
     // LEGACY fallback path
-    tiny_route_kind_t route = tiny_route_for_class((uint8_t)class_idx);
+    // Phase 3 D1: Free path route cache (eliminate tiny_route_for_class overhead)
+    tiny_route_kind_t route;
+    if (__builtin_expect(tiny_free_static_route_enabled(), 0)) {
+        // Use cached route (bypasses tiny_route_for_class())
+        route = g_tiny_route_class[(unsigned)class_idx & 7u];
+        if (__builtin_expect(route == TINY_ROUTE_LEGACY && !g_tiny_route_snapshot_done, 0)) {
+            // Fallback if uninitialized
+            route = tiny_route_for_class((uint8_t)class_idx);
+        }
+    } else {
+        // Standard path
+        route = tiny_route_for_class((uint8_t)class_idx);
+    }
     const int use_tiny_heap = tiny_route_is_heap_kind(route);
     const TinyFrontV3Snapshot* front_snap =
         __builtin_expect(tiny_front_v3_enabled(), 0) ? tiny_front_v3_snapshot_get() : NULL;
diff --git a/docs/analysis/PHASE3_D1_FREE_ROUTE_CACHE_1_DESIGN.md b/docs/analysis/PHASE3_D1_FREE_ROUTE_CACHE_1_DESIGN.md
new file mode 100644
index 00000000..a895cc2a
--- /dev/null
+++ b/docs/analysis/PHASE3_D1_FREE_ROUTE_CACHE_1_DESIGN.md
@@ -0,0 +1,46 @@
+# Phase 3 D1: Free Path Route Cache 設計メモ
+
+## 目的
+Free path の `tiny_route_for_class()` コストを削減（4.39% self + 24.78% children）
+
+## 観察
+- free() → tiny_free_fast() → tiny_route_for_class() → g_tiny_route_snapshot_done check
+- Route determination が free path の支配的なボトルネック
+- Phase 3 C3 (Static routing for alloc) と同じアプローチを free に適用
+
+## 実装アプローチ
+
+### L0: Env（戻せる）
+- `HAKMEM_FREE_STATIC_ROUTE=0/1` (default: 0, OFF)
+
+### L1: IntegrationBox（境界: 1箇所）
+
+`tiny_route_env_box.h` に既存する `g_tiny_route` を free path で活用：
+- `tiny_route_for_class()` を呼ばずに直接 route を決定
+- Cache invalidate: policy version change on sync
+
+### 実装指示
+
+**File 1**: `core/box/tiny_free_route_cache_env_box.h` (新規)
+- Inline function: `tiny_free_static_route_enabled()`
+  - Check `HAKMEM_FREE_STATIC_ROUTE` ENV
+  - Lazy init with -1 sentinel
+  - Return cached value
+
+**File 2**: Modify `core/box/tiny_route_env_box.h` (既存)
+- Add: `SmallRouteKind tiny_route_get_kind(int class_idx)` if not exist
+- Use existing `g_tiny_route.route_kind[class]` cache
+
+**File 3**: Modify `core/front/tiny_legacy_fallback_box.h` (既存)
+- In `tiny_legacy_fallback_free_base()` function
+- Check: `if (tiny_free_static_route_enabled())` before calling `tiny_route_for_class()`
+- Fallback: call `tiny_route_for_class()` if disabled
+
+## A/B テスト
+
+- Mixed (10-run): HAKMEM_FREE_STATIC_ROUTE=0 vs =1
+- GO: +1.0%+, NO-GO: -1.0%-
+
+## 期待
+- tiny_route_for_class() call 削減 → L1 cache pressure 低下
+- +1-2% gain in free path