diff --git a/core/hakmem_build_flags.h b/core/hakmem_build_flags.h
index 8a414861..c4e5a0f2 100644
--- a/core/hakmem_build_flags.h
+++ b/core/hakmem_build_flags.h
@@ -91,6 +91,15 @@
 #  define HAKMEM_TINY_INLINE_SLL 0
 #endif
 
+// Phase 1A3: Always-inline tiny_region_id_write_header()
+// Default: OFF (HAKMEM_TINY_HEADER_WRITE_ALWAYS_INLINE=0) - enable after A/B validation
+// Purpose: Force inline expansion of header write to reduce alloc path overhead
+// Expected impact: +0.5-2% on Mixed workloads
+// Build: make EXTRA_CFLAGS=-DHAKMEM_TINY_HEADER_WRITE_ALWAYS_INLINE=1 [target]
+#ifndef HAKMEM_TINY_HEADER_WRITE_ALWAYS_INLINE
+#  define HAKMEM_TINY_HEADER_WRITE_ALWAYS_INLINE 0
+#endif
+
 // Phase 7 Task 3: Pre-warm TLS cache at init
 // Default: OFF (enable after implementation)
 // Build: make PREWARM_TLS=1 or make phase7
diff --git a/core/tiny_region_id.h b/core/tiny_region_id.h
index 4735654e..f60a8fdc 100644
--- a/core/tiny_region_id.h
+++ b/core/tiny_region_id.h
@@ -212,6 +212,9 @@ static inline int tiny_header_mode(void)
 // Write class_idx to header (called after allocation)
 // Input: base (block start from SuperSlab)
 // Returns: user pointer (base + 1, skipping header)
+#if HAKMEM_TINY_HEADER_WRITE_ALWAYS_INLINE
+__attribute__((always_inline))
+#endif
 static inline void* tiny_region_id_write_header(void* base, int class_idx) {
     if (!base) return base;
 
@@ -439,6 +442,9 @@ static inline size_t tiny_region_id_user_size(size_t alloc_size) {
 #else // !HAKMEM_TINY_HEADER_CLASSIDX
 
 // Disabled: No-op implementations
+#if HAKMEM_TINY_HEADER_WRITE_ALWAYS_INLINE
+__attribute__((always_inline))
+#endif
 static inline void* tiny_region_id_write_header(void* ptr, int class_idx) {
     (void)class_idx;
     return ptr;