diff --git a/core/box/pool_mid_inuse_deferred_stats_box.h b/core/box/pool_mid_inuse_deferred_stats_box.h
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+++ b/core/box/pool_mid_inuse_deferred_stats_box.h
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+// pool_mid_inuse_deferred_stats_box.h — Box: Deferred inuse_dec Statistics
+//
+// Purpose: Track deferred inuse_dec performance counters
+// Pattern: Atomic counters with destructor dump
+// Phase: POOL-MID-DN-BATCH Step 5
+//
+// Counters:
+// - mid_inuse_deferred_hit: Total deferred decrements (hot path)
+// - drain_calls: Number of drain operations (cold path)
+// - pages_drained: Total pages processed in drain
+// - empty_transitions: Pages that triggered DONTNEED
+
+#ifndef POOL_MID_INUSE_DEFERRED_STATS_BOX_H
+#define POOL_MID_INUSE_DEFERRED_STATS_BOX_H
+
+#include <stdint.h>
+#include <stdatomic.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+// Statistics structure
+typedef struct {
+    _Atomic uint64_t mid_inuse_deferred_hit;      // Total deferred decrements
+    _Atomic uint64_t drain_calls;                  // Number of drain calls
+    _Atomic uint64_t pages_drained;                // Total pages drained
+    _Atomic uint64_t empty_transitions;            // Pages that went to 0
+} MidInuseDeferredStats;
+
+// Global stats instance
+static MidInuseDeferredStats g_mid_inuse_deferred_stats;
+
+// Stats increment macros (inline for hot path)
+#define MID_INUSE_DEFERRED_STAT_INC(field) \
+    atomic_fetch_add_explicit(&g_mid_inuse_deferred_stats.field, 1, memory_order_relaxed)
+
+#define MID_INUSE_DEFERRED_STAT_ADD(field, n) \
+    atomic_fetch_add_explicit(&g_mid_inuse_deferred_stats.field, (n), memory_order_relaxed)
+
+// Dump stats on exit (if ENV var set)
+static void mid_inuse_deferred_stats_dump(void) {
+    // Only dump if deferred is enabled
+    const char* e = getenv("HAKMEM_POOL_MID_INUSE_DEFERRED");
+    if (!e || *e != '1') return;
+
+    uint64_t hits = atomic_load_explicit(&g_mid_inuse_deferred_stats.mid_inuse_deferred_hit, memory_order_relaxed);
+    uint64_t drains = atomic_load_explicit(&g_mid_inuse_deferred_stats.drain_calls, memory_order_relaxed);
+    uint64_t pages = atomic_load_explicit(&g_mid_inuse_deferred_stats.pages_drained, memory_order_relaxed);
+    uint64_t empties = atomic_load_explicit(&g_mid_inuse_deferred_stats.empty_transitions, memory_order_relaxed);
+
+    if (hits > 0 || drains > 0) {
+        fprintf(stderr, "\n=== Mid Inuse Deferred Stats ===\n");
+        fprintf(stderr, "Deferred hits:      %lu\n", hits);
+        fprintf(stderr, "Drain calls:        %lu\n", drains);
+        fprintf(stderr, "Pages drained:      %lu\n", pages);
+        fprintf(stderr, "Empty transitions:  %lu\n", empties);
+        if (drains > 0) {
+            fprintf(stderr, "Avg pages/drain:    %.2f\n", (double)pages / (double)drains);
+        }
+        fprintf(stderr, "================================\n");
+    }
+}
+
+// Register destructor to dump stats on exit
+static void mid_inuse_deferred_stats_init(void) __attribute__((constructor));
+static void mid_inuse_deferred_stats_init(void) {
+    atexit(mid_inuse_deferred_stats_dump);
+}
+
+#endif  // POOL_MID_INUSE_DEFERRED_STATS_BOX_H