diff --git a/artiq-fast/wfvm-legacy.nix b/artiq-fast/wfvm-legacy.nix
new file mode 100644
index 0000000..a520492
--- /dev/null
+++ b/artiq-fast/wfvm-legacy.nix
@@ -0,0 +1,16 @@
+{ pkgs }:
+
+let
+  # Pin nixpkgs to avoid frequent resource-intensive Windows reinstallations on Hydra.
+  wfvm-pkgs = pkgs.fetchFromGitHub {
+    owner = "NixOS";
+    repo = "nixpkgs";
+    rev = "f8248ab6d9e69ea9c07950d73d48807ec595e923";
+    sha256 = "009i9j6mbq6i481088jllblgdnci105b2q4mscprdawg3knlyahk";
+  };
+  wfvm = pkgs.fetchgit {
+    url = "https://git.m-labs.hk/M-Labs/wfvm.git";
+    rev = "4b497938ffd9fcddf84a3dbe2f01524395292adb";
+    sha256 = "0m3kdbbcskqc1lf8b5f7ccbll9b7vkl4r00kbyx3yjb2rs6cqvil";
+  };
+in import "${wfvm}/wfvm" { pkgs = (import wfvm-pkgs {}); }
diff --git a/artiq-fast/windows/run-test.nix b/artiq-fast/windows/run-test.nix
index 5fc86f4..ef27500 100644
--- a/artiq-fast/windows/run-test.nix
+++ b/artiq-fast/windows/run-test.nix
@@ -10,11 +10,11 @@ let
       inherit port;
     }) tcpPorts;
 
-  wfvm = import ../wfvm.nix { inherit pkgs; };
+  artiq6 = pkgs.lib.strings.versionAtLeast artiqpkgs.artiq.version "6.0";
+  wfvm = import (if artiq6 then ../wfvm.nix else ../wfvm-legacy.nix) { inherit pkgs; };
   conda-deps = {
     name = "conda-deps";
     script = let
-      artiq6 = pkgs.lib.strings.versionAtLeast artiqpkgs.artiq.version "6.0";
       qt-asyncio-package = if artiq6 then artiqpkgs.conda-qasync else artiqpkgs.conda-quamash;
       conda-deps-noarch = import (if artiq6 then ./conda_noarch_packages.nix else ./conda_noarch_packages-legacy.nix) { inherit pkgs; };
       conda-deps-win-64 = import (if artiq6 then ./conda_win-64_packages.nix else ./conda_win-64_packages-legacy.nix) { inherit pkgs; };