diff --git a/zephcore/boards/example_board/README.md b/zephcore/boards/example_board/README.md
index 28cacbc..cb97721 100644
--- a/zephcore/boards/example_board/README.md
+++ b/zephcore/boards/example_board/README.md
@@ -27,7 +27,7 @@ SWD flash: `west flash` (requires J-Link, pyocd, or nrfjprog connected).
 | Board               | Build string                                              | Flash           |
 |----------------------|-----------------------------------------------------------|-----------------|
 | XIAO ESP32-C3        | `west build -b xiao_esp32c3 zephcore`                   | `west flash`    |
-| XIAO ESP32-C6        | `west build -b xiao_esp32c6 zephcore`                   | `west flash`    |
+| XIAO ESP32-C6        | `west build -b xiao_esp32c6/esp32c6/hpcore zephcore`    | `west flash`    |
 | LilyGo TLoRa C6      | `west build -b lilygo_tlora_c6/esp32c6/hpcore zephcore` | `west flash`    |
 | XIAO ESP32-S3        | `west build -b xiao_esp32s3/esp32s3/procpu zephcore`     | `west flash`    |
 | Station G2           | `west build -b station_g2/esp32s3/procpu zephcore`       | `west flash`    |
diff --git a/zephcore/boards/supported_boards.md b/zephcore/boards/supported_boards.md
index dd1fa99..5ab08ca 100644
--- a/zephcore/boards/supported_boards.md
+++ b/zephcore/boards/supported_boards.md
@@ -21,7 +21,7 @@ promicro_lr2021
 
 ```
 xiao_esp32c3
-xiao_esp32c6
+xiao_esp32c6/esp32c6/hpcore
 xiao_esp32s3/esp32s3/procpu
 lilygo_tlora_c6/esp32c6/hpcore
 station_g2/esp32s3/procpu
diff --git a/zephcore/sysbuild/CMakeLists.txt b/zephcore/sysbuild/CMakeLists.txt
index fd45423..96678a4 100644
--- a/zephcore/sysbuild/CMakeLists.txt
+++ b/zephcore/sysbuild/CMakeLists.txt
@@ -2,5 +2,52 @@
 # so the BOARD_ROOT set there never fires. Set it here, before find_package(Sysbuild).
 set(BOARD_ROOT "${APP_DIR}" CACHE STRING "" FORCE)
 
+# Auto-propagate flash size from board.conf to MCUboot.
+#
+# MCUboot is built as a separate image and does NOT inherit the app's board.conf
+# or esp32_common.conf. Boards with non-default flash sizes set
+# CONFIG_ESPTOOLPY_FLASHSIZE_<N>MB=y in their board.conf, but without this
+# block MCUboot builds with whatever its defconfig/Kconfig default is.
+# A mismatch between MCUboot's image header flash size and the actual hardware
+# causes the ROM bootloader to misconfigure the SPI flash controller, making
+# MCUboot hang at "spi_flash: detected chip:".
+#
+# We set mcuboot_EXTRA_CONF_FILE BEFORE find_package(Sysbuild) so it takes
+# priority — sysbuild_extensions.cmake uses CACHE INTERNAL (no FORCE) which
+# only writes if the variable is not already set.
+#
+# Static conf files (mcuboot_flashsize_*.conf) are used instead of generating
+# a temp file to avoid Windows line-ending issues with cmake file(WRITE).
+string(REPLACE "/" ";" _board_parts "${BOARD}")
+list(GET _board_parts 0 _board_base)
+
+file(GLOB_RECURSE _board_conf_candidates
+    "${APP_DIR}/boards/*/${_board_base}/board.conf")
+if(NOT _board_conf_candidates AND
+   EXISTS "${APP_DIR}/boards/${_board_base}/board.conf")
+    set(_board_conf_candidates "${APP_DIR}/boards/${_board_base}/board.conf")
+endif()
+
+if(_board_conf_candidates)
+    list(GET _board_conf_candidates 0 _board_conf)
+    file(READ "${_board_conf}" _board_conf_content)
+
+    set(_flashsize_fragment "")
+    foreach(_size 16MB 8MB 32MB 64MB 128MB)
+        if(_board_conf_content MATCHES "CONFIG_ESPTOOLPY_FLASHSIZE_${_size}=y")
+            string(TOLOWER "${_size}" _size_lower)
+            set(_flashsize_fragment
+                "${CMAKE_CURRENT_SOURCE_DIR}/mcuboot_flashsize_${_size_lower}.conf")
+            break()
+        endif()
+    endforeach()
+
+    if(_flashsize_fragment AND EXISTS "${_flashsize_fragment}")
+        set(mcuboot_EXTRA_CONF_FILE
+            "${CMAKE_CURRENT_SOURCE_DIR}/mcuboot.conf;${_flashsize_fragment}"
+            CACHE INTERNAL "MCUboot conf: global + flash size auto-propagated from board.conf")
+    endif()
+endif()
+
 find_package(Sysbuild REQUIRED HINTS $ENV{ZEPHYR_BASE})
 project(sysbuild LANGUAGES)
diff --git a/zephcore/sysbuild/mcuboot.conf b/zephcore/sysbuild/mcuboot.conf
index b1ed31d..db1ab4c 100644
--- a/zephcore/sysbuild/mcuboot.conf
+++ b/zephcore/sysbuild/mcuboot.conf
@@ -28,5 +28,11 @@ CONFIG_SPI=n
 # unavailable in MCUboot's minimal kernel.
 CONFIG_POWER_DOMAIN=n
 
+# Flash size baseline — Kconfig default is 2MB, but all zephcore boards use at
+# least 4MB. Boards with larger flash (8MB, 16MB) override this via the
+# auto-propagation logic in sysbuild/CMakeLists.txt which appends
+# mcuboot_flashsize_<n>mb.conf when the app's board.conf sets a larger size.
+CONFIG_ESPTOOLPY_FLASHSIZE_4MB=y
+
 # ESP32 HAL's mcuboot_config.h is patched (patches/modules/hal/espressif/...)
 # to respect Zephyr Kconfig: mbedTLS detection, AUTO sector count, slot0 validation.
diff --git a/zephcore/sysbuild/mcuboot_flashsize_16mb.conf b/zephcore/sysbuild/mcuboot_flashsize_16mb.conf
new file mode 100644
index 0000000..dc395a7
--- /dev/null
+++ b/zephcore/sysbuild/mcuboot_flashsize_16mb.conf
@@ -0,0 +1 @@
+CONFIG_ESPTOOLPY_FLASHSIZE_16MB=y
diff --git a/zephcore/sysbuild/mcuboot_flashsize_8mb.conf b/zephcore/sysbuild/mcuboot_flashsize_8mb.conf
new file mode 100644
index 0000000..88b7b69
--- /dev/null
+++ b/zephcore/sysbuild/mcuboot_flashsize_8mb.conf
@@ -0,0 +1 @@
+CONFIG_ESPTOOLPY_FLASHSIZE_8MB=y