diff --git a/ports/cortex_r5/ac5/inc/tx_port.h b/ports/cortex_r5/ac5/inc/tx_port.h
index 3fa52578..d1c69fba 100644
--- a/ports/cortex_r5/ac5/inc/tx_port.h
+++ b/ports/cortex_r5/ac5/inc/tx_port.h
@@ -9,6 +9,8 @@
  * SPDX-License-Identifier: MIT
  **************************************************************************/
 
+// Some portions generated by Copilot (Opus 5).
+
 
 /**************************************************************************/
 /**************************************************************************/
@@ -183,7 +185,16 @@ typedef unsigned short                          USHORT;
 
 #define TX_THREAD_EXTENSION_0
 #define TX_THREAD_EXTENSION_1
-#define TX_THREAD_EXTENSION_2
+
+/* TX_THREAD_EXTENSION_2 carries the per-thread VFP enable flag that the lazy
+   floating-point save and restore in tx_thread_schedule, tx_thread_system_return
+   and tx_thread_context_restore reads and writes as [thread, #144].  Without it
+   a TX_ENABLE_VFP_SUPPORT build has no such member and those accesses land on
+   tx_thread_filex_ptr instead.  Defined unconditionally, matching the Armv7-A
+   ports: the offset is hard-coded in assembly, so a conditional member would
+   move every following field and be correct in only one configuration.  */
+
+#define TX_THREAD_EXTENSION_2                   ULONG       tx_thread_vfp_enable;
 #define TX_THREAD_EXTENSION_3
 
 
diff --git a/ports/cortex_r5/ac5/src/tx_port_offset_check.c b/ports/cortex_r5/ac5/src/tx_port_offset_check.c
new file mode 100644
index 00000000..11eaa253
--- /dev/null
+++ b/ports/cortex_r5/ac5/src/tx_port_offset_check.c
@@ -0,0 +1,87 @@
+/***************************************************************************
+ * Copyright (c) 2026 Eclipse ThreadX contributors
+ *
+ * This program and the accompanying materials are made available under the
+ * terms of the MIT License which is available at
+ * https://opensource.org/licenses/MIT.
+ *
+ * AI Disclosure: This file was largely AI-generated by Copilot (Opus 5).
+ * The AI-generated portions may be considered public domain (CC0-1.0)
+ * and not subject to the project's licence. The human contributor has
+ * reviewed and verified that the code is correct.
+ *
+ * SPDX-License-Identifier: MIT and CC0-1.0
+ **************************************************************************/
+
+/**************************************************************************/
+/**************************************************************************/
+/**                                                                       */
+/** ThreadX Component                                                     */
+/**                                                                       */
+/**   Port Specific                                                       */
+/**                                                                       */
+/**************************************************************************/
+/**************************************************************************/
+
+/**************************************************************************/
+/*                                                                        */
+/*  PORT SPECIFIC C INFORMATION                            RELEASE        */
+/*                                                                        */
+/*    tx_port_offset_check.c                           Cortex-R5/AC5      */
+/*                                                                        */
+/*  DESCRIPTION                                                           */
+/*                                                                        */
+/*    Compile-time verification of the TX_THREAD structure offsets that    */
+/*    this port's assembly reaches by hard-coded displacement.  The file   */
+/*    emits no code; it exists purely so that a layout change becomes a    */
+/*    build failure.                                                      */
+/*                                                                        */
+/*    Why this is needed: tx_thread_schedule.s,                            */
+/*    tx_thread_system_return.s and tx_thread_context_restore.s read and   */
+/*    write the per-thread VFP enable flag as [thread, #144].  Nothing in  */
+/*    the toolchain connects that literal to the C structure, so adding a  */
+/*    member, enabling an option that adds one, or reordering the port     */
+/*    extensions would silently retarget those accesses at an unrelated    */
+/*    field -- corrupting thread state in a way that is extremely hard to    */
+/*    diagnose from the symptom.  The offset was measured at 144 for the   */
+/*    default build and is asserted here.                                  */
+/*                                                                        */
+/*    NOTE: this port has no CMake build of its own, so this file takes     */
+/*    effect only in builds that compile everything under src/.  It is       */
+/*    still worthwhile: it emits no code, costs nothing, and protects every  */
+/*    build that does pick it up.                                          */
+/*                                                                        */
+/*    A negative array dimension is used rather than _Static_assert        */
+/*    because this project targets C99, where _Static_assert does not      */
+/*    exist.  If an assertion below fails, the compiler reports a negative  */
+/*    or zero-sized array for the named typedef.                           */
+/*                                                                        */
+/**************************************************************************/
+
+#include "tx_api.h"
+#include <stddef.h>
+
+/* Offset of the VFP enable flag as encoded in this port's assembly.
+   Overridable only so that the assertion itself can be tested.  */
+
+#ifndef TX_PORT_VFP_ENABLE_OFFSET
+#define TX_PORT_VFP_ENABLE_OFFSET   144
+#endif
+
+typedef char
+tx_port_assert_vfp_enable_offset_is_144[
+    (offsetof(TX_THREAD, tx_thread_vfp_enable) == TX_PORT_VFP_ENABLE_OFFSET)
+        ? 1 : -1];
+
+/* The stack pointer is read as [thread, #8] by tx_thread_schedule.S and
+   written by tx_thread_system_return.s, and the run counter as
+   [thread, #4].  Assert those too, since they are on the context-switch
+   critical path.  */
+
+typedef char
+tx_port_assert_stack_ptr_offset_is_8[
+    (offsetof(TX_THREAD, tx_thread_stack_ptr) == 8) ? 1 : -1];
+
+typedef char
+tx_port_assert_run_count_offset_is_4[
+    (offsetof(TX_THREAD, tx_thread_run_count) == 4) ? 1 : -1];