diff --git a/README.md b/README.md
index fc379b2..2332845 100644
--- a/README.md
+++ b/README.md
@@ -90,6 +90,20 @@ These are system-owned executables from Nix, not unmanaged `npm -g`, `pip instal
 
 ## Build and apply
 
+### One-command sync on this laptop
+
+Save your work, then run from any directory (it requests sudo when needed):
+
+```sh
+/etc/nix/switch-system.sh               # apply now and make it the boot default
+/etc/nix/switch-system.sh dry-activate  # build and preview changes without applying
+/etc/nix/switch-system.sh boot          # stage for the next boot instead
+```
+
+This builds the current **`/etc/nix#nixos`** checkout and activates that exact output, including `dev`'s Home Manager configuration. It shares the automatic updater's lock, stops on build failure, keeps recovery generations and never reboots. It does **not** pull remote Git changes or update `flake.lock`: “latest” here means the files currently checked out in `/etc/nix`. New source files must be added to Git to be included. Open a new terminal afterward for shell environment changes; some desktop changes require a fresh login. `--help` lists the modes, including temporary `test` activation.
+
+### Detailed validation and activation
+
 On the **physical laptop**, from an administrator shell:
 
 ```sh
diff --git a/switch-system.sh b/switch-system.sh
new file mode 100755
index 0000000..83b808d
--- /dev/null
+++ b/switch-system.sh
@@ -0,0 +1,44 @@
+#!/usr/bin/env bash
+# Apply this laptop's checkout, never the separate EC2 "dev" configuration.
+set -euo pipefail
+
+usage() {
+  printf 'Usage: %s [switch|dry-activate|boot|test]\n' "$0"
+  printf '%s\n' \
+    'Build /etc/nix#nixos using flake.lock; default: switch now and save for boot.' \
+    'dry-activate previews changes; boot stages them; test applies temporarily.' \
+    'Does not pull Git, update package pins, delete generations, or reboot.'
+}
+
+if [[ $# -eq 1 && ($1 == --help || $1 == -h) ]]; then
+  usage
+  exit 0
+fi
+if (($# > 1)); then
+  usage >&2
+  exit 2
+fi
+action=${1:-switch}
+case "$action" in
+  switch | dry-activate | boot | test) ;;
+  *)
+    usage >&2
+    exit 2
+    ;;
+esac
+
+if ((EUID != 0)); then
+  exec sudo -- "$(readlink -f -- "${BASH_SOURCE[0]}")" "$@"
+fi
+
+# Share the existing updater's lock so it cannot change the checkout/profile
+# underneath this build and activation.
+mkdir -p /var/cache/nixos-update
+exec 9>/var/cache/nixos-update/lock
+flock 9
+
+cd /etc/nix
+printf 'Building the current /etc/nix checkout for nixos (%s).\n' "$action"
+built=$(nix build .#nixosConfigurations.nixos.config.system.build.toplevel \
+  --no-update-lock-file --no-link --print-out-paths)
+exec "$built/sw/bin/nixos-rebuild" "$action" --no-reexec --store-path "$built"