diff --git a/skills/datastore/scripts/garmin_sync.py b/skills/datastore/scripts/garmin_sync.py
index 14dd094..8508758 100755
--- a/skills/datastore/scripts/garmin_sync.py
+++ b/skills/datastore/scripts/garmin_sync.py
@@ -428,9 +428,11 @@ def sync_date_range(
             try:
                 sync_payload = {
                     "date": d,
+                    "daily": [summary] if summary else [],
+                    "sleep": [sleep] if sleep else [],
+                    "activities": activities or [],
                     "daily_summary": summary,
                     "sleep_data": sleep,
-                    "activities": activities,
                 }
                 sync_url = f"{target_backend}/garmin/sync"
                 req = urllib.request.Request(
@@ -440,9 +442,27 @@ def sync_date_range(
                     method="POST",
                 )
                 with urllib.request.urlopen(req, timeout=30) as resp:
-                    if resp.status == 200:
-                        logger.info(f"Successfully posted Garmin sync for {d} to backend")
+                    resp_body = {}
+                    try:
+                        resp_body = json.loads(resp.read().decode("utf-8"))
+                    except Exception:
+                        pass
+
+                    expected_records = (1 if summary else 0) + (1 if sleep else 0) + (len(activities) if activities else 0)
+                    synced_records = (
+                        resp_body.get("synced_daily", 0)
+                        + resp_body.get("synced_sleep", 0)
+                        + resp_body.get("synced_activities", 0)
+                    )
+
+                    if resp.status == 200 and (expected_records == 0 or synced_records > 0):
+                        logger.info(f"Successfully posted Garmin sync for {d} to backend: {resp_body}")
                         synced_via_backend = True
+                    else:
+                        logger.warning(
+                            f"Backend returned HTTP {resp.status} for {d} but synced {synced_records} records "
+                            f"(expected {expected_records}): {resp_body}. Falling back to local writer."
+                        )
             except Exception as e:
                 logger.warning(f"Failed to sync via backend {target_backend} for {d}: {e}. Falling back to local writer if available.")