diff --git a/.devcontainer/setup-zephyr.sh b/.devcontainer/setup-zephyr.sh
new file mode 100755
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--- /dev/null
+++ b/.devcontainer/setup-zephyr.sh
@@ -0,0 +1,98 @@
+#!/usr/bin/env bash
+#
+# Installs the Nordic nRF Connect SDK (NCS) + Zephyr toolchain *alongside* the
+# existing PlatformIO setup, so MeshCore can be ported to the Zephyr-based
+# Seeed XIAO nRF54L15 without disturbing the ESP32/nRF52/RP2040/STM32 builds.
+#
+# This is intentionally idempotent: re-running it skips work that's already done.
+# It is HEAVY (multi-GB download + Zephyr SDK). Expect the first run to take a while.
+#
+# Usage:
+#   bash .devcontainer/setup-zephyr.sh
+#
+# Override defaults via env, e.g.:
+#   NCS_REV=v3.0.0 NCS_WORKSPACE=$HOME/ncs bash .devcontainer/setup-zephyr.sh
+#
+set -euo pipefail
+
+# --- Tunables ----------------------------------------------------------------
+# Pick the latest NCS release that ships the Seeed XIAO nRF54L15 board
+NCS_REV="${NCS_REV:-v2.9.0}"
+NCS_WORKSPACE="${NCS_WORKSPACE:-$HOME/ncs}"
+VENV_DIR="${VENV_DIR:-$HOME/.zephyr-venv}"
+# -----------------------------------------------------------------------------
+
+echo ">> NCS revision : ${NCS_REV}"
+echo ">> Workspace    : ${NCS_WORKSPACE}"
+echo ">> Python venv  : ${VENV_DIR}"
+
+# 1) Isolated Python venv for west (don't pollute the PlatformIO/pipx env).
+if [ ! -d "${VENV_DIR}" ]; then
+  echo ">> Creating Python venv for west..."
+  python3 -m venv "${VENV_DIR}"
+fi
+# shellcheck disable=SC1091
+source "${VENV_DIR}/bin/activate"
+pip install --upgrade pip wheel >/dev/null
+pip install --upgrade west >/dev/null
+
+# 2) Initialise the NCS workspace (skip if already initialised).
+if [ ! -d "${NCS_WORKSPACE}/.west" ]; then
+  echo ">> west init NCS workspace (${NCS_REV})..."
+  west init -m https://github.com/nrfconnect/sdk-nrf --mr "${NCS_REV}" "${NCS_WORKSPACE}"
+else
+  echo ">> NCS workspace already initialised, skipping west init."
+fi
+
+cd "${NCS_WORKSPACE}"
+echo ">> west update (this is the slow part)..."
+west update
+west zephyr-export
+
+# 3) Zephyr's Python requirements.
+echo ">> Installing Zephyr Python requirements..."
+pip install -r zephyr/scripts/requirements.txt >/dev/null
+
+# 4) Zephyr SDK (toolchain). `west sdk install` auto-selects the version that
+#    matches this Zephyr tree, so we don't hardcode (and mis-pin) an SDK version.
+echo ">> Installing the matching Zephyr SDK (compilers)..."
+west sdk install || {
+  echo "!! 'west sdk install' failed. On older west you may need to install the"
+  echo "!! Zephyr SDK manually: https://docs.zephyrproject.org/latest/develop/getting_started/index.html"
+}
+
+# 5) Sanity build: confirm the XIAO nRF54L15 target actually compiles here.
+echo ">> Smoke-building Zephyr 'hello_world' for seeed/xiao_nrf54l15..."
+if west build -p always -b seeed/xiao_nrf54l15 zephyr/samples/hello_world -d /tmp/xiao_hello; then
+  echo ">> OK: native Zephyr toolchain can build for the XIAO nRF54L15."
+else
+  echo "!! Zephyr build for seeed/xiao_nrf54l15 FAILED."
+  echo "!! Likely cause: NCS_REV=${NCS_REV} predates XIAO nRF54L15 board support."
+  echo "!! Re-run with a newer release, e.g.  NCS_REV=v3.0.0 bash .devcontainer/setup-zephyr.sh"
+fi
+
+cat <<EOF
+
+============================================================================
+Zephyr / NCS toolchain ready (workspace: ${NCS_WORKSPACE}).
+
+To use it in a shell:
+    source ${VENV_DIR}/bin/activate
+    source ${NCS_WORKSPACE}/zephyr/zephyr-env.sh
+
+PlatformIO is untouched and still drives the existing ESP32/nRF52/etc. variants.
+
+--- Arduino-Zephyr core (the recommended on-ramp; see tools/lr2021_nrf54l15_smoke) ---
+The nRF54L15 Arduino core is itself Zephyr-based and reuses this NCS install.
+Install arduino-cli + the core, then build the smoke test:
+
+    # arduino-cli (user space)
+    curl -fsSL https://raw.githubusercontent.com/arduino/arduino-cli/master/install.sh | sh
+
+    # TODO: add the nRF54L15 Arduino core's Boards Manager URL, then:
+    #   arduino-cli config add board_manager.additional_urls <CORE_INDEX_JSON_URL>
+    #   arduino-cli core update-index
+    #   arduino-cli core install <core:nrf54l>
+    # Ref: https://blog.adafruit.com/2026/02/17/an-arduino-core-for-nrf54l15-zephyr-based/
+============================================================================
+EOF