diff --git a/action.yml b/action.yml
index d5a5793..5f18615 100644
--- a/action.yml
+++ b/action.yml
@@ -111,6 +111,45 @@ runs:
 
         echo "Testbox initialized: testbox_id=${TESTBOX_ID}"
 
+    - name: Start heartbeat
+      if: steps.metadata.outputs.available == 'true' && inputs.testbox_id
+      shell: bash
+      run: |
+        STATE=/tmp/.testbox
+        if [ ! -f "$STATE/testbox_id" ]; then
+          echo "Warning: state directory not initialized, skipping heartbeat"
+          exit 0
+        fi
+
+        # Launch a background heartbeat loop that POSTs to the backend
+        # every 30 seconds. The backend uses this to detect dead VMs
+        # whose phone-home (completed) never arrived.
+        #
+        # nohup + fd redirects + disown fully detaches the process
+        # from this step's shell. Without this, GitHub Actions may
+        # wait on or SIGTERM child processes between composite action
+        # steps, killing the heartbeat before run-testbox starts.
+        #
+        # The state files are read inside the loop so the script is
+        # self-contained and does not depend on shell variable scope.
+        nohup bash -c 'STATE=/tmp/.testbox
+        API_URL=$(cat "$STATE/api_url")
+        AUTH_TOKEN=$(cat "$STATE/auth_token")
+        TESTBOX_ID=$(cat "$STATE/testbox_id")
+        INSTALLATION_MODEL_ID=$(cat "$STATE/installation_model_id")
+        while true; do
+          sleep 30
+          curl -s -f --max-time 10 -X POST "${API_URL}/api/testbox/heartbeat" \
+            -H "Content-Type: application/json" \
+            -H "Authorization: Bearer ${AUTH_TOKEN}" \
+            -d "{\"testbox_id\":\"${TESTBOX_ID}\",\"installation_model_id\":${INSTALLATION_MODEL_ID}}" \
+            >/dev/null 2>&1 || true
+        done' </dev/null >/dev/null 2>&1 &
+        HEARTBEAT_PID=$!
+        echo "$HEARTBEAT_PID" > "$STATE/heartbeat_pid"
+        disown "$HEARTBEAT_PID"
+        echo "Heartbeat started (PID $HEARTBEAT_PID, every 30s, disowned)"
+
     - name: Install SSH public key
       if: steps.metadata.outputs.available == 'true' && inputs.testbox_id
       shell: bash