diff --git a/zephcore/adapters/ble/ZephyrBLE.cpp b/zephcore/adapters/ble/ZephyrBLE.cpp
index 8818730..bc7ae69 100644
--- a/zephcore/adapters/ble/ZephyrBLE.cpp
+++ b/zephcore/adapters/ble/ZephyrBLE.cpp
@@ -26,11 +26,6 @@ LOG_MODULE_REGISTER(zephcore_ble, CONFIG_ZEPHCORE_BLE_LOG_LEVEL);
 
 #include "ZephyrBLE.h"
 
-/* MAX_FRAME_SIZE from CompanionMesh.h — keep in sync */
-#ifndef MAX_FRAME_SIZE
-#define MAX_FRAME_SIZE 172
-#endif
-
 /* ========== Constants ========== */
 
 #define DEVICE_NAME_MAX 29
diff --git a/zephcore/adapters/ble/ZephyrBLE.h b/zephcore/adapters/ble/ZephyrBLE.h
index b1df197..fdc5507 100644
--- a/zephcore/adapters/ble/ZephyrBLE.h
+++ b/zephcore/adapters/ble/ZephyrBLE.h
@@ -9,6 +9,10 @@
 #include <stdint.h>
 #include <stddef.h>
 
+/* Maximum companion transport frame size.
+ * +4 over the base 172 to accommodate transport codes (region scoping). */
+#define MAX_FRAME_SIZE  176
+
 #ifdef __cplusplus
 extern "C" {
 #endif
diff --git a/zephcore/adapters/transport/LinuxTCPTransport.c b/zephcore/adapters/transport/LinuxTCPTransport.c
index 14efa2b..745592c 100644
--- a/zephcore/adapters/transport/LinuxTCPTransport.c
+++ b/zephcore/adapters/transport/LinuxTCPTransport.c
@@ -41,13 +41,6 @@
 
 LOG_MODULE_REGISTER(linux_tcp_transport, LOG_LEVEL_INF);
 
-/* Keep in sync with MAX_FRAME_SIZE in CompanionMesh.h. ZephyrBLE.cpp
- * uses the same constant; we mirror it here so a single transport build
- * doesn't have to pull the C++ header. */
-#ifndef MAX_FRAME_SIZE
-#define MAX_FRAME_SIZE 172
-#endif
-
 #define FRAME_QUEUE_SIZE CONFIG_ZEPHCORE_BLE_QUEUE_SIZE
 #define LISTEN_PORT      CONFIG_ZEPHCORE_LINUX_TCP_PORT
 #define LISTEN_BACKLOG   1
diff --git a/zephcore/app/CompanionMesh.h b/zephcore/app/CompanionMesh.h
index 43281de..dcd9f94 100644
--- a/zephcore/app/CompanionMesh.h
+++ b/zephcore/app/CompanionMesh.h
@@ -36,8 +36,10 @@
 #define AUTO_ADD_ROOM_SERVER       (1 << 3)
 #define AUTO_ADD_SENSOR            (1 << 4)
 
-/* Max BLE frame size — must match NUS MTU negotiated size */
-#define MAX_FRAME_SIZE 172
+/* Canonical definition is in ZephyrBLE.h; guard here for TUs that don't include it */
+#ifndef MAX_FRAME_SIZE
+#define MAX_FRAME_SIZE  176
+#endif
 
 /* 1 header + 32 pubkey + 1 type + 1 flags + 1 path_len + 64 path + 32 name + 4*4 fields = 148 */
 #define CONTACT_FRAME_SIZE 148