diff --git a/backend/routes.py b/backend/routes.py
index 291f370..5d6f34b 100644
--- a/backend/routes.py
+++ b/backend/routes.py
@@ -267,7 +267,8 @@ async def series4_heartbeat(request: Request, db: Session = Depends(get_db)):
     """
     payload = await request.json()
 
-    source = payload.get("source", "series4_emitter")
+    # Accept source_id (new standard field) with fallback to legacy "source" key
+    source = payload.get("source_id") or payload.get("source", "series4_emitter")
     units = payload.get("units", [])
     version = payload.get("version")
     log_tail = payload.get("log_tail")