diff --git a/artiq-fast.nix b/artiq-fast.nix
index 1256e4b..5ea4562 100644
--- a/artiq-fast.nix
+++ b/artiq-fast.nix
@@ -59,8 +59,8 @@ in
     extended-tests = pkgs.stdenv.mkDerivation {
       name = "extended-tests";
 
-      # requires patched Nix
-      __networked = true;
+      __networked = true;  # compatibility with old patched Nix
+      __impure = true;     # Nix 2.8+
 
       buildInputs = [
         (pkgs.python3.withPackages(ps: [ ps.paramiko artiqpkgs.artiq artiqpkgs.artiq-board-kc705-nist_clock ]))
diff --git a/artiq-zynq.nix b/artiq-zynq.nix
index 1f720d9..c778ed3 100644
--- a/artiq-zynq.nix
+++ b/artiq-zynq.nix
@@ -21,8 +21,8 @@ in
     zc706-hitl-tests = pkgs.lib.hydraJob (pkgs.stdenv.mkDerivation {
       name = "zc706-hitl-tests";
 
-      # requires patched Nix
-      __networked = true;
+      __networked = true;  # compatibility with old patched Nix
+      __impure = true;     # Nix 2.8+
 
       buildInputs = [
         pkgs.netcat pkgs.openssh pkgs.rsync artiq-fast.artiq artiq-fast.artiq-netboot