diff --git a/core/tiny_region_id.h b/core/tiny_region_id.h
index 444275ec..f7e4e18a 100644
--- a/core/tiny_region_id.h
+++ b/core/tiny_region_id.h
@@ -232,9 +232,21 @@ static inline void* tiny_region_id_write_header(void* base, int class_idx) {
         }
     } while (0);
 
-    *header_ptr = HEADER_MAGIC | (class_idx & HEADER_CLASS_MASK);
-    PTR_TRACK_HEADER_WRITE(base, HEADER_MAGIC | (class_idx & HEADER_CLASS_MASK));
-    void* user = header_ptr + 1;  // skip header for user pointer
+    // P3: Skip header write when class_map is active (default)
+    // class_map provides class_idx lookup, so header byte is no longer needed
+    // ENV: HAKMEM_TINY_WRITE_HEADER=1 to force header write (legacy mode)
+    // Memory layout preserved: user = base + 1 (1B unused when skipped)
+    static int g_write_header = -1;
+    if (__builtin_expect(g_write_header == -1, 0)) {
+        const char* e = getenv("HAKMEM_TINY_WRITE_HEADER");
+        g_write_header = (e && *e && *e != '0') ? 1 : 0;
+    }
+    if (__builtin_expect(g_write_header, 0)) {
+        // Legacy mode: write header for debugging or compatibility
+        *header_ptr = HEADER_MAGIC | (class_idx & HEADER_CLASS_MASK);
+        PTR_TRACK_HEADER_WRITE(base, HEADER_MAGIC | (class_idx & HEADER_CLASS_MASK));
+    }
+    void* user = header_ptr + 1;  // skip header for user pointer (layout preserved)
     PTR_TRACK_MALLOC(base, 0, class_idx);  // Track at BASE (where header is)
 
     // ========== ALLOCATION LOGGING (Debug builds only) ==========