diff --git a/pkgs/servers/home-assistant/default.nix b/pkgs/servers/home-assistant/default.nix
index 9cd83d69eec3..866bbe25a866 100644
--- a/pkgs/servers/home-assistant/default.nix
+++ b/pkgs/servers/home-assistant/default.nix
@@ -743,7 +743,7 @@ in with py.pkgs; buildPythonApplication rec {
 
   pytestFlagsArray = [
     # parallelize test run
-    "--numprocesses auto"
+    "--numprocesses $NIX_BUILD_CORES"
     # assign tests grouped by file to workers
     "--dist loadfile"
     # retry racy tests that end in "RuntimeError: Event loop is closed"