diff --git a/frontend-s2s/.env.example b/frontend-s2s/.env.example
index dedf168..6acf1c4 100644
--- a/frontend-s2s/.env.example
+++ b/frontend-s2s/.env.example
@@ -1,4 +1,6 @@
-# Existing s2s-client FastAPI server. Used only by Vite's server-side proxy.
-S2S_BACKEND_URL=http://127.0.0.1:8420
-# Comma-separated hostnames, only needed when accessing Vite through a remote host.
+# FastAPI target when running npm run dev independently. start_ui.sh sets this
+# to its own backend using S2S_BACKEND_PORT (default 8421).
+S2S_BACKEND_URL=http://127.0.0.1:8421
+# Comma-separated hostnames for remote access, without scheme or port.
+# With start_ui.sh this can also be set in the repository root .env.
 S2S_DEV_ALLOWED_HOSTS=
diff --git a/frontend-s2s/src/test/backend-proxy.test.ts b/frontend-s2s/src/test/backend-proxy.test.ts
index 4ab4f36..29ed1b0 100644
--- a/frontend-s2s/src/test/backend-proxy.test.ts
+++ b/frontend-s2s/src/test/backend-proxy.test.ts
@@ -4,9 +4,9 @@ import { createBackendProxy } from "../../config/backend-proxy";
 
 describe("development backend proxy", () => {
   it("keeps the public host and original paths for the FastAPI runtime URL", () => {
-    const proxy = createBackendProxy("http://127.0.0.1:8420");
-    expect(proxy["/api"]).toEqual({ target: "http://127.0.0.1:8420", changeOrigin: false });
-    expect(proxy["/v1/realtime"]).toEqual({ target: "http://127.0.0.1:8420", changeOrigin: false, ws: true });
+    const proxy = createBackendProxy("http://127.0.0.1:8421");
+    expect(proxy["/api"]).toEqual({ target: "http://127.0.0.1:8421", changeOrigin: false });
+    expect(proxy["/v1/realtime"]).toEqual({ target: "http://127.0.0.1:8421", changeOrigin: false, ws: true });
     expect(proxy["/openapi.json"]).toEqual(proxy["/api"]);
     expect(proxy["/oauth"]).toEqual(proxy["/api"]);
   });
diff --git a/frontend-s2s/vite.config.ts b/frontend-s2s/vite.config.ts
index bb92ec4..73ab647 100644
--- a/frontend-s2s/vite.config.ts
+++ b/frontend-s2s/vite.config.ts
@@ -14,13 +14,14 @@ export default defineConfig(({ mode }) => {
     resolve: { alias: { "@": fileURLToPath(new URL("./src", import.meta.url)) } },
     server: {
       host: "127.0.0.1",
-      port: 5173,
+      port: 8420,
       strictPort: true,
+      ws: { path: "/__vite_hmr" },
       allowedHosts: (env.S2S_DEV_ALLOWED_HOSTS ?? "")
         .split(",")
         .map((host) => host.trim())
         .filter(Boolean),
-      proxy: createBackendProxy(env.S2S_BACKEND_URL ?? "http://127.0.0.1:8420"),
+      proxy: createBackendProxy(env.S2S_BACKEND_URL ?? "http://127.0.0.1:8421"),
     },
     build: { target: "es2022" },
   };
diff --git a/start_ui.sh b/start_ui.sh
index 12a57f0..3e18dbd 100755
--- a/start_ui.sh
+++ b/start_ui.sh
@@ -4,16 +4,45 @@ set -euo pipefail
 ROOT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
 VENV_DIR="$ROOT_DIR/.venv"
 REQ_FILE="$ROOT_DIR/s2s-client/requirements.txt"
-UI_PORT="${UI_PORT:-8420}"
+FRONTEND_DIR="$ROOT_DIR/frontend-s2s"
 if [ -f "$ROOT_DIR/.env" ]; then
   set -a
   source "$ROOT_DIR/.env"
   set +a
 fi
+UI_PORT="${UI_PORT:-8420}"
+UI_HOST="${UI_HOST:-0.0.0.0}"
+S2S_BACKEND_PORT="${S2S_BACKEND_PORT:-8421}"
 S2S_PROXY_TARGET="${S2S_PROXY_TARGET:-ws://ai-server:12345/v1/realtime}"
 
-if ! command -v python3 >/dev/null 2>&1; then
-  echo "Error: python3 was not found."
+if (( BASH_VERSINFO[0] < 4 || (BASH_VERSINFO[0] == 4 && BASH_VERSINFO[1] < 4) )); then
+  echo "Error: start_ui.sh requires Bash 4.4 or newer."
+  exit 1
+fi
+
+for required_command in python3 node npm setsid curl; do
+  if ! command -v "$required_command" >/dev/null 2>&1; then
+    echo "Error: $required_command was not found."
+    exit 1
+  fi
+done
+
+if ! node -e 'const [major, minor] = process.versions.node.split(".").map(Number); process.exit((major === 22 && minor >= 18) || major >= 24 ? 0 : 1)'; then
+  echo "Error: frontend-s2s requires Node.js 22.18+ on the 22.x line, or Node.js 24+."
+  exit 1
+fi
+
+for port_name in UI_PORT S2S_BACKEND_PORT; do
+  port_value="${!port_name}"
+  if [[ ! "$port_value" =~ ^[0-9]{1,5}$ ]] || (( 10#$port_value < 1 || 10#$port_value > 65535 )); then
+    echo "Error: $port_name must be a port between 1 and 65535."
+    exit 1
+  fi
+  # Normalize leading zeroes before passing ports to Node and Uvicorn.
+  printf -v "$port_name" '%d' "$((10#$port_value))"
+done
+if [ "$UI_PORT" = "$S2S_BACKEND_PORT" ]; then
+  echo "Error: UI_PORT and S2S_BACKEND_PORT must be different."
   exit 1
 fi
 
@@ -29,17 +58,105 @@ if [ ! -f "$REQ_FILE" ]; then
   exit 1
 fi
 
-if ! python -c "import fastapi, uvicorn, httpx, huggingface_hub, dotenv" >/dev/null 2>&1; then
+if ! python -c "import fastapi, uvicorn, httpx, huggingface_hub, dotenv, websockets" >/dev/null 2>&1; then
   echo "Installing dependencies from s2s-client/requirements.txt..."
   python -m pip install -r "$REQ_FILE"
 fi
 
+# Keep restarts offline when the installed frontend manifests have not changed.
+if [ ! -f "$FRONTEND_DIR/node_modules/vite/bin/vite.js" ] || \
+  ! cmp -s "$FRONTEND_DIR/package-lock.json" "$FRONTEND_DIR/node_modules/.s2s-package-lock.json" || \
+  ! cmp -s "$FRONTEND_DIR/package.json" "$FRONTEND_DIR/node_modules/.s2s-package.json"; then
+  echo "Installing frontend dependencies from frontend-s2s/package-lock.json..."
+  npm --prefix "$FRONTEND_DIR" ci --include=dev
+  cp "$FRONTEND_DIR/package-lock.json" "$FRONTEND_DIR/node_modules/.s2s-package-lock.json"
+  cp "$FRONTEND_DIR/package.json" "$FRONTEND_DIR/node_modules/.s2s-package.json"
+fi
+
+# The combined launcher always proxies to the backend it starts. An alternate
+# S2S_BACKEND_URL is only used when running Vite independently.
+export S2S_BACKEND_URL="http://127.0.0.1:$S2S_BACKEND_PORT"
 export S2S_PROXY_TARGET
 
-echo "💻 Starting the S2S client UI (port $UI_PORT)..."
+child_pids=()
+cleanup() {
+  local exit_status=$?
+  trap - EXIT
+  trap '' INT TERM HUP
+  for child_pid in "${child_pids[@]}"; do
+    kill -TERM -- "-$child_pid" 2>/dev/null || true
+  done
+  # Stop whole process groups, including Uvicorn's reload worker. Bound the
+  # graceful shutdown so a stuck connection cannot leave ports occupied.
+  for ((attempt = 0; attempt < 20; attempt++)); do
+    local alive=false
+    for child_pid in "${child_pids[@]}"; do
+      if kill -0 -- "-$child_pid" 2>/dev/null; then
+        alive=true
+      fi
+    done
+    if [ "$alive" = false ]; then
+      break
+    fi
+    sleep 0.05
+  done
+  for child_pid in "${child_pids[@]}"; do
+    kill -KILL -- "-$child_pid" 2>/dev/null || true
+    wait "$child_pid" 2>/dev/null || true
+  done
+  exit "$exit_status"
+}
+trap cleanup EXIT
+trap 'exit 130' INT
+trap 'exit 143' TERM
+trap 'exit 129' HUP
+
+echo "💻 Starting the React UI at http://$UI_HOST:$UI_PORT (Vite hot reload)..."
+echo "🔧 FastAPI backend: $S2S_BACKEND_URL (Python reload enabled)"
 echo "🔌 Realtime proxy target: $S2S_PROXY_TARGET"
-echo " Press Ctrl+C in this terminal to stop it."
+echo " Press Ctrl+C in this terminal to stop both servers."
 echo "----------------------------------------------------"
 
 cd "$ROOT_DIR"
-PYTHONPATH=s2s-client python -m uvicorn server:app --host 0.0.0.0 --port "$UI_PORT" --reload
+setsid "$VENV_DIR/bin/python" -m uvicorn server:app \
+  --app-dir "$ROOT_DIR/s2s-client" \
+  --host 127.0.0.1 --port "$S2S_BACKEND_PORT" \
+  --reload --reload-dir "$ROOT_DIR/s2s-client" &
+backend_pid=$!
+child_pids+=("$backend_pid")
+
+backend_ready=false
+startup_deadline=$((SECONDS + 30))
+while (( SECONDS < startup_deadline )); do
+  if ! kill -0 "$backend_pid" 2>/dev/null; then
+    echo "Error: FastAPI exited before it was ready."
+    exit 1
+  fi
+  if curl --noproxy '*' --silent --fail --max-time 1 \
+    --output /dev/null "$S2S_BACKEND_URL/api/config"; then
+    backend_ready=true
+    break
+  fi
+  sleep 0.2
+done
+if [ "$backend_ready" = false ]; then
+  echo "Error: FastAPI did not become ready within 30 seconds."
+  exit 1
+fi
+
+(
+  cd "$FRONTEND_DIR"
+  exec setsid node node_modules/vite/bin/vite.js --host "$UI_HOST" --port "$UI_PORT" --strictPort
+) &
+frontend_pid=$!
+child_pids+=("$frontend_pid")
+
+# PM2 supervises this script. If either server exits, stop its sibling and fail
+# the launcher so PM2 can restart the complete pair instead of a partial UI.
+exit_status=0
+wait -n "$backend_pid" "$frontend_pid" || exit_status=$?
+echo "Error: a UI server exited; stopping both servers."
+if [ "$exit_status" -eq 0 ]; then
+  exit_status=1
+fi
+exit "$exit_status"