diff --git a/core/hakmem_build_flags.h b/core/hakmem_build_flags.h
index f019b86c..4b0b1f83 100644
--- a/core/hakmem_build_flags.h
+++ b/core/hakmem_build_flags.h
@@ -45,6 +45,13 @@
 #  define HAKMEM_TINY_PHASE6_BOX_REFACTOR 1
 #endif
 
+// SuperSlab backend toggle (compile-time)
+// Default: 1 (ON) - SuperSlab is the core architecture.
+// Set to 0 only for legacy/compat testing.
+#ifndef HAKMEM_TINY_USE_SUPERSLAB
+#  define HAKMEM_TINY_USE_SUPERSLAB 1
+#endif
+
 // ------------------------------------------------------------
 // Phase 7: Region-ID Direct Lookup (Header-based optimization)
 // ------------------------------------------------------------
diff --git a/core/hakmem_tiny_superslab_internal.h b/core/hakmem_tiny_superslab_internal.h
index 5923a726..5c4381a8 100644
--- a/core/hakmem_tiny_superslab_internal.h
+++ b/core/hakmem_tiny_superslab_internal.h
@@ -6,6 +6,7 @@
 #ifndef HAKMEM_TINY_SUPERSLAB_INTERNAL_H
 #define HAKMEM_TINY_SUPERSLAB_INTERNAL_H
 
+#include "hakmem_build_flags.h"  // CRITICAL: Ensure HAKMEM_TINY_USE_SUPERSLAB is defined
 #include "hakmem_tiny_superslab.h"
 #include "box/ss_hot_cold_box.h"
 #include "box/ss_allocation_box.h"  // CRITICAL: For superslab_allocate() declaration (fixes implicit int assumption)