diff --git a/homeassistant/components/google_assistant/__init__.py b/homeassistant/components/google_assistant/__init__.py
index 8d4ac9f01c9d8a..6f725d8948ff59 100644
--- a/homeassistant/components/google_assistant/__init__.py
+++ b/homeassistant/components/google_assistant/__init__.py
@@ -32,8 +32,6 @@
 
 DEPENDENCIES = ['http']
 
-DEFAULT_AGENT_USER_ID = 'home-assistant'
-
 ENTITY_SCHEMA = vol.Schema({
     vol.Optional(CONF_NAME): cv.string,
     vol.Optional(CONF_EXPOSE): cv.boolean,
@@ -67,10 +65,12 @@ async def request_sync_service_handler(call: ServiceCall):
         websession = async_get_clientsession(hass)
         try:
             with async_timeout.timeout(5, loop=hass.loop):
+                agent_user_id = call.data.get('agent_user_id') or \
+                                call.context.user_id
                 res = await websession.post(
                     REQUEST_SYNC_BASE_URL,
                     params={'key': api_key},
-                    json={'agent_user_id': call.context.user_id})
+                    json={'agent_user_id': agent_user_id})
                 _LOGGER.info("Submitted request_sync request to Google")
                 res.raise_for_status()
         except aiohttp.ClientResponseError:
diff --git a/homeassistant/components/google_assistant/services.yaml b/homeassistant/components/google_assistant/services.yaml
index 6019b75bd9830a..7d3af71ac2bbab 100644
--- a/homeassistant/components/google_assistant/services.yaml
+++ b/homeassistant/components/google_assistant/services.yaml
@@ -1,2 +1,5 @@
 request_sync:
-  description: Send a request_sync command to Google.
\ No newline at end of file
+  description: Send a request_sync command to Google.
+  fields:
+    agent_user_id:
+      description: Optional. Only needed for automations. Specific Home Assistant user id to sync with Google Assistant. Do not need when you call this service through Home Assistant front end or API. Used in automation script or other place where context.user_id is missing.