diff --git a/core/box/pool_mid_inuse_deferred_env_box.h b/core/box/pool_mid_inuse_deferred_env_box.h
new file mode 100644
index 00000000..a6d0d979
--- /dev/null
+++ b/core/box/pool_mid_inuse_deferred_env_box.h
@@ -0,0 +1,25 @@
+// pool_mid_inuse_deferred_env_box.h — Box: Deferred inuse_dec ENV gate
+// HAKMEM_POOL_MID_INUSE_DEFERRED=1 (default 0)
+//
+// Purpose: ENV-based runtime toggle for deferred inuse_dec optimization
+// Pattern: Static cached ENV check with __builtin_expect
+// Phase: POOL-MID-DN-BATCH Step 1
+
+#ifndef POOL_MID_INUSE_DEFERRED_ENV_BOX_H
+#define POOL_MID_INUSE_DEFERRED_ENV_BOX_H
+
+#include <stdlib.h>
+
+// Check if deferred inuse_dec is enabled via ENV
+// Default: 0 (disabled) for safety
+// Set HAKMEM_POOL_MID_INUSE_DEFERRED=1 to enable
+static inline int hak_pool_mid_inuse_deferred_enabled(void) {
+    static int g = -1;
+    if (__builtin_expect(g == -1, 0)) {
+        const char* e = getenv("HAKMEM_POOL_MID_INUSE_DEFERRED");
+        g = (e && *e == '1') ? 1 : 0;
+    }
+    return g;
+}
+
+#endif  // POOL_MID_INUSE_DEFERRED_ENV_BOX_H