diff --git a/zephcore/CMakeLists.txt b/zephcore/CMakeLists.txt
index 3ea2eca..95b74ab 100644
--- a/zephcore/CMakeLists.txt
+++ b/zephcore/CMakeLists.txt
@@ -551,6 +551,16 @@ elseif(CONFIG_ZEPHCORE_ROLE_ROOM_SERVER)
     helpers/TransportKeyStore.cpp
     helpers/CommonCLI.cpp
   )
+  # Headless build (e.g. native-Linux SBC): no display/buttons/buzzer, so the
+  # real ui_* implementation (helpers/ui-button or helpers/ui-joystick) isn't
+  # compiled. main_room_server.cpp calls ui_* unconditionally — pull in the weak
+  # no-op stubs to satisfy the link. Mirrors the repeater/observer branches.
+  if(NOT CONFIG_ZEPHCORE_UI_DESIGN_BUTTON AND NOT CONFIG_ZEPHCORE_UI_DESIGN_JOYSTICK)
+    target_sources(app PRIVATE helpers/ui/ui_headless_stubs.c)
+    target_include_directories(app PRIVATE
+      ${CMAKE_CURRENT_SOURCE_DIR}/helpers/ui
+    )
+  endif()
   # Shared USBD CDC ACM init + 1200-baud DFU + DTR event/callback module.
   if(NOT CONFIG_CDC_ACM_SERIAL_INITIALIZE_AT_BOOT AND (CONFIG_USB_CDC_ACM OR CONFIG_USBD_CDC_ACM_CLASS))
     target_sources(app PRIVATE
diff --git a/zephcore/adapters/usb/ZephyrCompanionUSB.cpp b/zephcore/adapters/usb/ZephyrCompanionUSB.cpp
index f535b6b..e96dcdf 100644
--- a/zephcore/adapters/usb/ZephyrCompanionUSB.cpp
+++ b/zephcore/adapters/usb/ZephyrCompanionUSB.cpp
@@ -221,7 +221,13 @@ void zephcore_usb_companion_init(struct k_event *mesh_events,
 
 	s_rx_work = rx_work;
 
-#if DT_HAS_COMPAT_STATUS_OKAY(zephyr_cdc_acm_uart)
+	/* The cdc_acm_uart DT node may be present without the class driver compiled
+	 * (shared esp32s3_usb_otg.dtsi exposes the node unconditionally; the class is
+	 * only enabled with esp32s3_usb.conf). Gate the device-get on the class too,
+	 * else DEVICE_DT_GET_ONE references an undefined device ordinal on, e.g., a
+	 * debug ESP32-S3 companion (CONFIG_LOG=y) built without esp32s3_usb.conf. */
+#if DT_HAS_COMPAT_STATUS_OKAY(zephyr_cdc_acm_uart) && \
+	(IS_ENABLED(CONFIG_USB_CDC_ACM) || IS_ENABLED(CONFIG_USBD_CDC_ACM_CLASS))
 	usb_dev = DEVICE_DT_GET_ONE(zephyr_cdc_acm_uart);
 	if (device_is_ready(usb_dev)) {
 		LOG_INF("USB CDC device ready: %s", usb_dev->name);
diff --git a/zephcore/app/main_observer.cpp b/zephcore/app/main_observer.cpp
index d3288a1..b36c034 100644
--- a/zephcore/app/main_observer.cpp
+++ b/zephcore/app/main_observer.cpp
@@ -20,6 +20,14 @@
 
 #define CLI_REPLY_SIZE 256
 
+/* The CDC-ACM device object exists only when the class driver is compiled.
+ * A board overlay may expose a cdc_acm_uart DT node unconditionally (the shared
+ * esp32s3_usb_otg.dtsi does), so gate the device-get on the Kconfig class, not
+ * on DT_HAS_COMPAT_STATUS_OKAY alone, or DEVICE_DT_GET_ONE references an
+ * undefined device ordinal (e.g. an ESP32-S3 observer without esp32s3_usb.conf). */
+#define ZEPHCORE_USB_STACK \
+	(IS_ENABLED(CONFIG_USB_CDC_ACM) || IS_ENABLED(CONFIG_USBD_CDC_ACM_CLASS))
+
 LOG_MODULE_REGISTER(zephcore_observer_main, CONFIG_ZEPHCORE_MAIN_LOG_LEVEL);
 
 #include <app/RepeaterDataStore.h>
@@ -301,10 +309,11 @@ int main(void)
 	}
 
 	/* Initialize USB serial for CLI */
-#if DT_HAS_COMPAT_STATUS_OKAY(zephyr_cdc_acm_uart)
+#if ZEPHCORE_USB_STACK && DT_HAS_COMPAT_STATUS_OKAY(zephyr_cdc_acm_uart)
 	usb_dev = DEVICE_DT_GET_ONE(zephyr_cdc_acm_uart);
 #else
-	/* ESP32 native USB or other console UART */
+	/* ESP32 native USB / other console UART, or a cdc_acm DT node present
+	 * without the class driver compiled — fall back to the chosen console. */
 	usb_dev = DEVICE_DT_GET(DT_CHOSEN(zephyr_console));
 #endif
 	if (device_is_ready(usb_dev)) {
diff --git a/zephcore/src/main_repeater.cpp b/zephcore/src/main_repeater.cpp
index 2478240..cf16f55 100644
--- a/zephcore/src/main_repeater.cpp
+++ b/zephcore/src/main_repeater.cpp
@@ -33,8 +33,18 @@ extern "C" void bt_ctlr_assert_handle(char *file, uint32_t line)
 }
 #endif
 
-/* USB CDC ACM init + 1200-baud DFU + DTR callbacks (shared with companion) */
-#if !IS_ENABLED(CONFIG_CDC_ACM_SERIAL_INITIALIZE_AT_BOOT)
+/* USB CDC ACM init + 1200-baud DFU + DTR callbacks (shared with companion).
+ *
+ * Gate on the CDC-ACM class driver, NOT on DT_HAS_COMPAT_STATUS_OKAY alone: a
+ * board overlay may expose a cdc_acm_uart DT node unconditionally (the shared
+ * esp32s3_usb_otg.dtsi does), so the node can be present while the class driver
+ * — and therefore zephcore_usbd_* and the device object — is not compiled
+ * (e.g. an ESP32-S3 repeater built without esp32s3_usb.conf). This mirrors the
+ * repeater CMake condition that compiles ZephyrUSBCDC.cpp. */
+#define ZEPHCORE_USB_STACK \
+	(IS_ENABLED(CONFIG_USB_CDC_ACM) || IS_ENABLED(CONFIG_USBD_CDC_ACM_CLASS))
+
+#if ZEPHCORE_USB_STACK && !IS_ENABLED(CONFIG_CDC_ACM_SERIAL_INITIALIZE_AT_BOOT)
 #include <ZephyrUSBCDC.h>
 #endif
 
@@ -388,10 +398,9 @@ int main(void)
 	 * host to open the port (DTR asserted) — event-driven via the usbd
 	 * message callback.  Unplugged → 2 s timeout, no banner; attached →
 	 * banner reaches the user the moment the port opens. */
-#if !IS_ENABLED(CONFIG_CDC_ACM_SERIAL_INITIALIZE_AT_BOOT) && DT_HAS_COMPAT_STATUS_OKAY(zephyr_cdc_acm_uart)
+#if ZEPHCORE_USB_STACK && DT_HAS_COMPAT_STATUS_OKAY(zephyr_cdc_acm_uart) && \
+	!IS_ENABLED(CONFIG_CDC_ACM_SERIAL_INITIALIZE_AT_BOOT)
 	zephcore_usbd_init();
-#endif
-#if DT_HAS_COMPAT_STATUS_OKAY(zephyr_cdc_acm_uart)
 	zephcore_usbd_wait_dtr(2000);
 #endif
 	LOG_INF("=== ZephCore Repeater starting ===");
@@ -521,10 +530,12 @@ int main(void)
 
 	/* USB CDC was initialized earlier (right after clearBootloaderMagic).
 	 * Just acquire the device handle for the CLI's UART IRQ binding below. */
-#if DT_HAS_COMPAT_STATUS_OKAY(zephyr_cdc_acm_uart)
+#if ZEPHCORE_USB_STACK && DT_HAS_COMPAT_STATUS_OKAY(zephyr_cdc_acm_uart)
 	usb_dev = DEVICE_DT_GET_ONE(zephyr_cdc_acm_uart);
 #else
-	/* No CDC ACM (e.g. ESP32 usb_serial) — use chosen console UART */
+	/* No CDC ACM class driver (e.g. ESP32 usb_serial, or an ESP32-S3 repeater
+	 * built without esp32s3_usb.conf where the cdc_acm DT node exists but the
+	 * class isn't compiled) — use the chosen console UART. */
 	usb_dev = DEVICE_DT_GET(DT_CHOSEN(zephyr_console));
 #endif
 	if (device_is_ready(usb_dev)) {
diff --git a/zephcore/src/main_room_server.cpp b/zephcore/src/main_room_server.cpp
index 714161a..b4d90c6 100644
--- a/zephcore/src/main_room_server.cpp
+++ b/zephcore/src/main_room_server.cpp
@@ -33,8 +33,18 @@ extern "C" void bt_ctlr_assert_handle(char *file, uint32_t line)
 }
 #endif
 
-/* USB CDC ACM init + 1200-baud DFU + DTR callbacks (shared with companion) */
-#if !IS_ENABLED(CONFIG_CDC_ACM_SERIAL_INITIALIZE_AT_BOOT)
+/* USB CDC ACM init + 1200-baud DFU + DTR callbacks (shared with companion).
+ *
+ * Gate on the CDC-ACM class driver, NOT on DT_HAS_COMPAT_STATUS_OKAY alone: a
+ * board overlay may expose a cdc_acm_uart DT node unconditionally (the shared
+ * esp32s3_usb_otg.dtsi does), so the node can be present while the class driver
+ * — and therefore zephcore_usbd_* and the device object — is not compiled
+ * (e.g. an ESP32-S3 build without esp32s3_usb.conf). This mirrors the CMake
+ * condition that compiles ZephyrUSBCDC.cpp. */
+#define ZEPHCORE_USB_STACK \
+	(IS_ENABLED(CONFIG_USB_CDC_ACM) || IS_ENABLED(CONFIG_USBD_CDC_ACM_CLASS))
+
+#if ZEPHCORE_USB_STACK && !IS_ENABLED(CONFIG_CDC_ACM_SERIAL_INITIALIZE_AT_BOOT)
 #include <ZephyrUSBCDC.h>
 #endif
 
@@ -406,10 +416,9 @@ int main(void)
 	 * host to open the port (DTR asserted) — event-driven via the usbd
 	 * message callback.  Unplugged → 2 s timeout, no banner; attached →
 	 * banner reaches the user the moment the port opens. */
-#if !IS_ENABLED(CONFIG_CDC_ACM_SERIAL_INITIALIZE_AT_BOOT) && DT_HAS_COMPAT_STATUS_OKAY(zephyr_cdc_acm_uart)
+#if ZEPHCORE_USB_STACK && DT_HAS_COMPAT_STATUS_OKAY(zephyr_cdc_acm_uart) && \
+	!IS_ENABLED(CONFIG_CDC_ACM_SERIAL_INITIALIZE_AT_BOOT)
 	zephcore_usbd_init();
-#endif
-#if DT_HAS_COMPAT_STATUS_OKAY(zephyr_cdc_acm_uart)
 	zephcore_usbd_wait_dtr(2000);
 #endif
 	LOG_INF("=== ZephCore Room Server starting ===");
@@ -539,10 +548,12 @@ int main(void)
 
 	/* USB CDC was initialized earlier (right after clearBootloaderMagic).
 	 * Just acquire the device handle for the CLI's UART IRQ binding below. */
-#if DT_HAS_COMPAT_STATUS_OKAY(zephyr_cdc_acm_uart)
+#if ZEPHCORE_USB_STACK && DT_HAS_COMPAT_STATUS_OKAY(zephyr_cdc_acm_uart)
 	usb_dev = DEVICE_DT_GET_ONE(zephyr_cdc_acm_uart);
 #else
-	/* No CDC ACM (e.g. ESP32 usb_serial) — use chosen console UART */
+	/* No CDC ACM class driver (e.g. ESP32 usb_serial, or an ESP32-S3 build
+	 * without esp32s3_usb.conf where the cdc_acm DT node exists but the class
+	 * isn't compiled) — use the chosen console UART. */
 	usb_dev = DEVICE_DT_GET(DT_CHOSEN(zephyr_console));
 #endif
 	if (device_is_ready(usb_dev)) {